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Miami Pedestrian Bridge, Part XIV
50

Miami Pedestrian Bridge, Part XIV

Miami Pedestrian Bridge, Part XIV

(OP)

RE: Miami Pedestrian Bridge, Part XIV

Hokie,

The top chord and diagonals shrinks/creeps as well. So you have to look at the relative movement/shortening. Also, there is no significant restraint from the joints or top chord. If the bottom chord shrinks relative to the rest of the truss, it will just camber up. The creep and shrinkage does not create a significant shearing force at the end joints.

You can call it a truss or frame (although by far, the stiff load path is the truss). It doesn't mater. You don't have restraint from the top chord to act as enough of a strong back to create significant shears. Even if you had a full web, a girder just typically cambers with PT in the bottom flange and this structure has less internal restraint than a girder.

Even with a vierendeel, the truss will still cambers. The verticals may get high bending curvature if the chords are much stiffer in bending than the verticals and you get differential creep/shrinkage. But in our case, the diagonals are very stiff since they take load axially and the camber is only resisted by bending in the chords which are much less stiff than the truss configuration.

In a true truss, there are zero shear forces due to PT. This structure is much closer to a true truss than a vierendeel.

RE: Miami Pedestrian Bridge, Part XIV

I know that is your opinion, as stated previously. But I disagree, and believe that axial shortening of the bottom chord played a big role in the cracking, and ultimate failure, of the structure. Hopefully, someone else will comment about my last post in Thread XIII.

RE: Miami Pedestrian Bridge, Part XIV

Hokie, there’s nothing to resist the axial shortening. It’s a simply supported truss so the beam geometry can change near freely in response to differential shrinkage of the members.

The flexural strength and stiffness of members are no way near enough to resist the shrinkage to that degree.

What do you imagine is resisting the shrinkage force in the deck, sufficient to rupture the node?

RE: Miami Pedestrian Bridge, Part XIV

Those end diagonals are stiff, and shortening of the deck caused distress in the joint. That's two of you who disagree with my hypothesis. Fair enough, but I would still like to hear from others who have thoughtfully looked at the photos of cracking at those joints.

RE: Miami Pedestrian Bridge, Part XIV

And further to that, was axial shortening considered in the analysis load cases? I'm not going to try to figure that out, but does someone know?

RE: Miami Pedestrian Bridge, Part XIV

Hokie,

The stiffness of the diagonals is not relevant. Take them as being absolutely rigid. No mater how rigid they are, they will only bow the top chord (and truss). It is the flexibility of the chords relative to the truss that maters if differential shortening is going to cause shear stress at the joint.

There are many things that Figg did or didn't do that really surprises everyone. So I can't say that they did look at the long term response. I can say that any competent design team would look at both the short and long term behaviour which would include the creep, shinkage, and temperature changes.

The long term response is particularly important in this structure due to the tube stays. If the long term response wants to cambers up, there is big trouble with additional shear at the joints and compression on the diagonals. That is because unlike the truss chords bending stiffness, the stays are really stiff and even with a little camber, will produce large forces. They would produce enough force to keep the truss flat (or in the initial position). This is when the PT can actually stress the joints.

If I find the time, I can model it to get a more affirmative answer. I may approximate the section shapes just to get a feel for the long term response. Once those stays and back span are on, then the analysis gets to be more complex. Without the stays, it is not so bad to analyze by hand (at least with the simply supported case and only the gravity loads).

RE: Miami Pedestrian Bridge, Part XIV

hokie66 - I'd find it very hard to believe that any shortening of the deck or canopy would have no effect on the diagonals or their connections at the deck and canopy. From the report and info it's rather hard to say exactly which issue was the straw which caused the cracking to start, but what you're pointing out certainly wasn't helping the structure any.

There's been a bunch of comments and theory about the backspan/tower helping support the 11/12 node. I posted way back that the main span needed to support itself without anything else helping for any chance of the whole structure working long term and I still believe that. From the report and structure build info, it's been made clear errors were made so this wasn't the case, but then that was rather obvious by just looking at the pictures of the cracks.

RE: Miami Pedestrian Bridge, Part XIV

I remember seeing the detail of the faux stays at the top of the canopy, and it looked undeveloped. If these were just for aesthetics, I think they would have needed some type of slip joint.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Earth314159 (Structural)If I find the time, I can model it)

Will this data help?
From Bridge Factors Attachment 70 FIU Superstructure Longitudinal
FIGG LARSA Calc Pg 5, pdf file page 13

Sections
     Name    Section Area    Shear Area  Shear Area    Torsion   Inertia Izz  Inertia Iyy  Plastic
                 (ft²)       in yy (ft²)  in zz (ft²)   Constant  (ft^4)        (ft^4)      Modulus
                                                   (ft^4)                                   (ft³)    
 Truss_diagonal   3.5000      2.9167       2.9167         1.6798     0.8932       1.1667    0.0000 
 Truss_diagonal 2 6.1250      5.1042       5.1042         4.2955     1.5632       6.2526 
End Diagonal      5.2500      4.3750       4.3750         3.3994     1.3398       3.9375
Truss Top Chord  16.4607     16.4607      16.4607         5.3100    352.4200      2.6925
-16ft wide
Truss Bottom     45.6753     45.6753       45.6753        36.1683      2,995.0267  12.4210 
Chord-31ft 8in_wide 
Concrete Truss_diagonal Rectangle 1.7500 2.0000
Concrete Truss_diagonal 2 Rectangle 1.7500 3.5000
End Truss Diag 2 is 3.5X21" - do not know where that is used

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance)

Will this data help?

Yes it will help. Thank you.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Earth314159 (Structural)3 Nov 19 18:07
Quote (Vance)
Will this data help?)

I am not sure I can sort out the joints and member assignments - they are much less evident - self generated, and so forth.

RE: Miami Pedestrian Bridge, Part XIV

Horizontal shear in a rectangular beam (and more)
If we consider a rectangular solid beam placed horizontally and simply supported with no end restraints, and loaded with a uniform load , we expect it to deflect downward.
It does so because there is compressive stress in the top and therefore the top shortens, and there is tensile stress in the bottom, and therefore the bottom lengthens. Simultaneously a horizontal shear is induced in the web - Vh=1.5V/A.
Does it not seem logical that anything that would create that same curvature in the beam would cause the same internal stress?
What is less obvious is how a vertical load manages to create a horizontal stress in the rectangular beam. Would not a horizontal load like PT forces seem more direct and more obviously create horizontal stresses?
If we think of a pack of paper for the printer, and we want to prevent the sheets from sticking together we flex the sheets to cause them to slip on each other. That slip would represent the horizontal shear from the curvature. If we prestress the sheets on the tight curve side just enough to shorten them and maintain alignment with the sheets above, that shear is negated. But don't we have to transfer some of that PT force to the sheet just below the top sheet and to all the sheets from there to the PT force?
If the object were a truss and not a beam, if it were 18 feet deep and 174 feet long, and if prestress forces in the deck were sufficient to cause maybe an inch of camber in the truss, would it not have lifted off its interior falsework and be spanning 174 feet, and therefore the axial load in diagonal 11 have been the same as when setting on the pylon? Would the load in member 11 been negated if loads were placed on top of the truss to force it to stay straight? Or would the load in 11 be increased?
This brings to question the issue of concrete strength at the time of tensioning and the E value at that time.
I think the drawings allowed tensioning at 6000 psi.

RE: Miami Pedestrian Bridge, Part XIV

I am slowly working away on the PT model when I can.

The forces in the diagonals due to the initial PT of the deck are 14Kips and 7Kips respectively for #2 and #11. That will go down with creep and shrinkage. Keep in mind that this is only looking at the initial PT of the bottom deck in isolation. I am getting a DL compression in #2 of 1800Kips and 1280 Kips in #11. IN the north end, I get a vertical shear of 110 Kips in the bottom deck, 58 Kips in the canopy and 12 Kips for the weight of #12. So the net vertical component to the diagonal is (938Kips-100-58-12)=770 Kips

The camber up is 0.92" (this excludes the DL which is obviously in the opposite direction.

The initial dead load deflection is 1.3".

The south end reaction is slightly higher than the north end reaction.

The initial deck PT contributed 0.5% to the interface shear at the north end.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Earth314159 (Structural)5 Nov 19 02:34
I am slowly working away on the PT model when I can.)

Awesome work. Thank you!

RE: Miami Pedestrian Bridge, Part XIV

Earth,
Thanks. Does your model give an estimation of longitudinal shortening due to drying shrinkage and applied PT?

RE: Miami Pedestrian Bridge, Part XIV

Quote (hokie66 (Structural))

In my experience, drying shrinkage has been in the order of 5/8 inch in 100 feet in normal weight concrete. I have no experience with Florida aggregates and 8500 psi concrete.

Quote (Earth314159 (Structural)The camber up is 0.92" )

That value is for only PT D1L and D1R? So there are five more sets of strands to be tensioned?
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

Vance,

The upward camber is short term with all the PT in the deck (no other PT applied yet). I am trying to set the initial temperatures for the PT strands. The temperature will be kept the same through out the analysis. Shrinkage is models as a negative temperature on the concrete. How much high strength concrete shrinks in a Florida environment is a good question. I usually use 0.0003 strain but 0.0002 may be more appropriate. 5/8" 100 feet is about 0.0005 which is realistic but maybe on the high side (once you include thermal changes it may be more realistic).

The model is 2D and stick elements for simplicity. Even at that is takes a while to put in all the PT, determine temperatures etc.

Hokie,

Right now, I have only included the initial PT (full bottom deck) and the initial concrete modulus. I will include for shrinkage and creep later. I will rerun the model with a lower E for concrete to account for creep. The temperature on the concrete will be negative to account for shrinkage. I am getting there but it is a lot of work and I still have to my regular work.

The concrete shortens with the application of the PT.

RE: Miami Pedestrian Bridge, Part XIV

Hokie,

I am back on the computer. The total shortening of the bottom deck from the initial bottom deck PT is 0.56" but that is taken up mostly by camber of the truss.

I made a slight error. The DL deflection is closer to 1.5" rather than 1.3".

I work more on this later. I have to go back to work.

RE: Miami Pedestrian Bridge, Part XIV

Thank you for doing that. Certainly no hurry. That PT shortening is about what I would expect, and as Vance reported, the drying shrinkage shortening is usually about twice the PT. So overall, in the range of 1.5" or a bit more, similar to what I said before.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Earth314159)

You are not going to bill us for this, are you? If you are considering that, we need to talk about billing rates.surprise

RE: Miami Pedestrian Bridge, Part XIV

Vance,

No worries, it is just Canadian money.

I am a Stuct. Eng., so I have to get CPDs hours done anyways.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Earth314159)

In a true truss, there are zero shear forces due to PT.

Hi Earth314159. I am not sure what you are suggesting here and am curious to understand the function. Are you suggesting that in a true truss there are no shear forces to be found if post tensioning (PT) is provided? I'm not sure if PT will remove shear forces from a truss.

Thanks for your thoughts on this.

RE: Miami Pedestrian Bridge, Part XIV

Hi 40 years,

No, that is not what I am suggesting. There is still shear in the structure due to gravity loads. The PT in a "true" determinant truss does not add to any of the member forces (including shear). It only adds minimally to the horizontal shear at #11 and #2 in this truss which is not determinant.

The compression in the concrete created by the tension in the PT is equal and opposite with a net zero member force.

PT in a true truss can camber or otherwise deflect the truss but the net members forces will be zero if you look at that PT case in isolation. You use the laws of superposition to add back in the other cases.

RE: Miami Pedestrian Bridge, Part XIV

Hi all, I've been distracted by the wildfire and power cutoffs here in the SF north bay (everything is fine at home, except for the frayed nerves), but I just had some time to look at:
FIGG Bridge Engineers, Inc Party Submission Findings Conclusions Recommendations, and Attachments 628567
and I noted some differences between the way FIGG presents the WJE analysis and the WJE report itself.
I have two questions that are bothering me.
One: If the truss was not supposed to crack when the shoring was removed, why didn't this raise questions about how it was constructed?
Two: Pate headed down to FIU to calm people down. Would it have turned out different if he had made an effort to understand what they were upset about?

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

Quote (SFCharlie (Computer))

Stay safe there - hopefully this will lead to some reasonable forest management procedures.
One - Some concrete cracking is normal and not of structural significance. I do not have the dimensions of "acceptable cracking" at my fingertips - I recall FDOT (I think - from NTSB meeting) sets 1/2" deep and either 0.002 or 0.006 inch width. That is for normal structures and there was nothing special about this one, right? (Hint - it was a truss).
I also recall an NTSB comment that on the morning of March 15 the cracking was 40 times an acceptable limit.
Two - I don't think the EOR made an engineering call on March 15.

RE: Miami Pedestrian Bridge, Part XIV

While contemplating cracking, please do not overlook the significance of the sound reported when shoring was removed.

RE: Miami Pedestrian Bridge, Part XIV

Per jrs_87 links:
https://www.enr.com/articles/48016-what-florida-br...
Unfortunately, the ENR article ends seemingly incomplete and it incorrectly states the bridge design would use cables attached to the pylon instead of faux stay pipes.

https://www.miamiherald.com/opinion/op-ed/article2...
This opinion from a member of the NTSB is pretty straight-forward in stating the design error was clear yet the group think behavior overrode any dissenting response/concern.

What I still have not seen addressed officially is the issue that has been broached by many posts in this forum, regardless of all the talk/calculations on the applicability of shear capacity at the critical nodes - based on joint preparation and such, shouldn't the main span have been designed to be a fully safe and functional self-supportinhg span, not reliant on the future addition of the small back span to 'capture the 11/12 node'? Is this a common design/construction scheme - build one section of a multi-span structure with knowledge that it is reliant on another yet to be constructed section? Shouldn't individual sections stand alone unless they share beams? My naivete shows . . .

RE: Miami Pedestrian Bridge, Part XIV

Quote (jrs_87 (Mechanical)P.S. What is going on with the widespread misuse of the word 'sheer'?)

As in "the sheer incompetency of not recognizing a shear failure when you see one"?
Or maybe " the sheer magnitude of the consequences of the shear failure before your eyes"?
That sheer looks like a problem to me. :)

RE: Miami Pedestrian Bridge, Part XIV

Quote (Brian Malone (Industrial shouldn't the main span have been designed to be a fully safe and functional self-supportinhg span)


It certainly should have had the capacity to support itself and any construction loads, as well as withstood the rigors of transport, without failure and with some room to spare. And I believe this was the intent.
The concept of every piece having its final capacity at time of erection is not efficient and not required in many stages. An example is a precast girder which can support formwork for a yet to be placed concrete slab. The slab can be considered to be composite when completed and provide far more support in the final structure but the girder acting alone only needs to have the capacity to support (safely) the construction activities and the wet concrete and forming. Once cured, the intended strength is developed. If the designer does not screw up.

Quote (My naivete shows . . .)

Not at all. You are right on point. There are calculations by the design firm that address shear in node 1/2. but I have found no calcs for node 11/12. It appears node 11/12 was not properly addressed in the design.
But I have to wonder about the long term service of the improperly treated joints. If FDOT wants them chipped and cleaned for good sealing and to prevent deterioration, these joints would not be protected as intended by FDOT. They will be wet when it rains - they are effectively in the gutter. And deterioration of reinforcing will not be visible. Perhaps this is what Berger saw when they requested guidance for treatment of the joints.





RE: Miami Pedestrian Bridge, Part XIV

Quote (John Coil, SE)

Unconventional Reinforcing Bar Layout in Diagonal Members
John Coil, SENovember 1, 2019
A review of the plans show that the longitudinal reinforcement in the diagonal member were encased in #4 ties at 12 inches on center. The longitudinal bars were not located at the tie corners, however. The plans show the bars to be located 6 inches from the corners on the member. The ties were detailed to be open ties and there were no cross ties specified. Thus there was not adequate resistance to buckling of the compression steel in the diagonal members under compression i. e. diagonal member No. 11. In addition the post-tension bars in member No. 11 were located significantly outside of the kern points. If the upper bar was re-tensioned first which I suspect is the case, it would have resulted in significant tensile stress to the lower portion of the diagonal member which it appears from the photos was already cracked. By my rather simple quick analysis of the truss as a simple pin ended member truss i. e. simple static analysis for dead load was over-stressed by about 30% per ACI 318 standards. This is not enough to cause failure but it should have raised concern for the peer reviewer.

Source: Comment to this article> https://www.enr.com/articles/48005-ntsbs-fiu-bridg...

Can someone please elaborate on this comment? I suspect point of comment is diminished if the bar tension changes were staggered at 50 kips. Are two balanced bars outside kern area effectively inside area? Was 50 kips small enough a limit for concentric loading? What complication is caused by positioning of dead end anchors? Did the bars in fact cause excessive tension in 11 cross-section?

RE: Miami Pedestrian Bridge, Part XIV

I've gone through a fair amount of the interviews and reports and I have a few 'main' questions:

I understand that Figg had multiple models and that some predicted greater shear forces across the 11-12 joint, but they only used the "simple-span" model and the "fixed-pylon" model which predicted lower forces. It is unexplained why they did this, but I could see them thinking that these 'should' be the worst case and thus not really checking/looking into it (or just missed it). Either way the model at the time of the collapse would have been the simple span model. I also understand that they had a solids model (Lusus) and a beam element model (Larsa). They said that these correlated fairly well. My question is - why was shear demand in these models so different (lower) than the FHWA's check and even the Figg 'hand-checks' that were part of the 3/15/18 slide presentation.

Regarding the 3/15/18 presentation. It appears that Figg checks the shear strength at this node and finds that it has adequate strength. What was wrong with this hand check?

Also I didn't realize that two of the designers were on vacation immediately following the bridge move. Including the lead project engineer. Not sure that things would have been different, but interesting to note.

EIT
www.HowToEngineer.com

RE: Miami Pedestrian Bridge, Part XIV

Quote (jrs_87 (Mechanical)Did the bars in fact cause excessive tension in 11 cross-section?)

That is a possibility because of the stressing sequence at Stage 2.
There we learn that about 11% of the deck PT was first applied ( D1) and about 24% of the canopy PT was stressed ( C2) when the PT rods in member 2 and member 11 were to be stressed. There is no specification as to sequencing the tensioning between the rods.
From calcs now underway by Earth 314159 we find that full PT in the deck will cause about 1 inch camber and dead load about 1.5 inches deflection so member 11 may have been under some load but likely much less than when spanning 174 feet or when on the falsework with full PT in the deck. Stressing one rod to max before stressing the other will induce a moment but whether that causes tension depends on the total load in the member at the time. PT acting at the kern could cause sero tension in an otherwise unstressed column, and PT outside the kern can cause tension. Axial loads at that time could overcome the tension stress and the section remain in compression across its full area. Only the numbers know.
Perhaps the analysis by Earth314159 can provide some numbers to work with here. We need the compressive load in member 11 while on its falsework and with PT tendons D1 and C2 only stressed to answer this question.
Without a computer analysis I would consider that the PT force in the deck was about 960 kips and in the canopy was about 880 kips. While those forces are somewhat close to the same value the compressive force STRESS in the canopy is much greater because of its relative size being much smaller. So the effect of the early PT would be a downward camber or a lifting of the ends. That could induce tension in members 2 and 11, making it more likely that cracking did occur from stressing one PT rod located outside the kern.
Or that PT sequence may have caused some initial cracking at nodes 1/2 and 11/12.

RE: Miami Pedestrian Bridge, Part XIV

After reading the ENR link posted by jrs_87: "What Florida Bridge Collapse Report Leaves Unexplained", I clicked on the author's name to see what else he may have written for ENR and then read this item by ENR co-authors Richard Korman and Scott Judy, "Did Concrete Error Doom Florida Bridge?" Link. What eventually jumped out at me was the paragraph: Under its design-build contract agreement, FIGG’s role explicitly states that it would not serve as a resident engineer but would “at appropriate intervals visit the site to determine if the construction is proceeding in accordance with the construction documents.”

When I was going through MCM's Contracts & Purchase Orders it became obvious that MCM was really only performing as the Construction Project Management firm and the actual Contractors, were all Subs. MCM was ranked 276, down from 245 in the 2015 ENR 400. So they were a big dog in construction but I was starting to get the feeling they didn't really have the chops to be doing this level of Heavy Civil Construction. It was one of the first things I looked at, when FIU made the Bid/Submittal packages available for public viewing. Link I knew MCM had people on staff that were Florida Licensed Professional Engineers, but where were they on this project? Was MCM a "General Engineering Contractor" on paper only? FDOT also doesn't give the CEI firm any engineering authority over design.

The FIU Request for Qualifications - Request for Proposals, pg 51 lists the requirements for the Design-Build Firm’s Project Manager. Link. Specifically, The Project Manager assigned by the Design-Build Firm must be proficient with the English language, and shall possess a Registered Professional Engineer License in the State of Florida and three (3) years of specific experience in construction management on limited access facilities or have a minimum of five (5) years of specific work experience providing construction management in limited access highway facilities.

Here is the organizational chart MCM submitted with the proposal on September 30, 2015.


The Design-Build Manager for MCM was one, Joe Martin, P.E. (LinkedIn: Link) The chart wasn't worth the paper it was printed on. Joe Martin, P.E. left MCM in October 2015 for Odebrecht. His role seems to have remained unfilled and the FIU Bridge Project put under the management of Rodrigo Isaza (LinkedIn: Link). You can see MCM's descriptions of Mr. Martin & Mr. Isaza starting on pg. 25, of their submittal. Link

It is entirely possible that Rodrigo Isaza met the minimum of five 5)years of specific work experience providing construction management in limited access highway facilities but there was a clear lack of critical understanding & thinking related to the execution of the work.

Using a 19th century nautical comparison. MCM was a big dog and as such, when they sailed into port, they anchored and waited their turn, to unload and load but when they weighed anchor and headed out to sea; they were just dog legging, direct reckoning sailors, who made the error of thinking they could blue water sail without a Celestial Navigator.

RE: Miami Pedestrian Bridge, Part XIV

Quote (epoxybot (Structural)starting to get the feeling they didn't really have the chops to be doing this level of Heavy Civil Construction.)

And yet they competed for the skyway project at an estimated $800 million and sued when they were not chosen. That project was more than 50 times larger.
This project was a reality check for all.

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley - Before that project was mentioned in this forum, I was wondering just how MCM & FIGG had come together. They seem like Oil & Water as a team.

RE: Miami Pedestrian Bridge, Part XIV

Oldfieldguy…

Unfortunately, this article is poorly written. For just two examples (there are others):
-- "The report went on to say that Berger knew he should have checked the math...." Louis Berger, in this case, is a company, not an individual.
-- "In other words, the contractors did crappy math." The article hits this theme quite a few times. But, it was the engineer working for the contractor who did crappy math, not the contractor.

Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: Miami Pedestrian Bridge, Part XIV

Fred - As I read the article I thought it was written by FIGG. But deeper into it the criticism included FIGG.
The comments by the 'public' if you will provide an insight to what a jury might think/say/decide.

RE: Miami Pedestrian Bridge, Part XIV

The conclusions and board member statement at the end of the report are very damning stuff. I'm a little surprised by how strong the verbiage is, but as others have mentioned since the NTSB meeting, it's clear they've been flabbergasted this failure was allowed to happen.

As an aside, who ultimately determines if the EOR loses their PE due to the miscalculations and apparent disregard for public safety? I assume it'd fall to the professional departments of the various states he's licensed in?

RE: Miami Pedestrian Bridge, Part XIV

2

Quote (RFreund (Structural)10 Nov 19 17:46
My question is - why was shear demand in these models so different (lower) than the FHWA's check and even the Figg 'hand-checks' that were part of the 3/15/18 slide presentation.)

Like you, I have yet to identify where FIGG developed the lower design number for the connection of 11/12 members to the deck - the shear friction failure location. NTSB suggests they did not correctly interpret the results of the computer models.
There is a hand calculation in the FIGG design calcs that addresses node 1/2 using a "punch thru" set of shear planes thru the deck and out the end. I do not see a similar calc for node 11/12.

Quote (Regarding the 3/15/18 presentation. It appears that Figg checks the shear strength at this node and finds that it has adequate strength. What was wrong with this hand check?)

The slide presentation calc of the shear in node 11/12 was a similar calculation to that at node 1/2 in the design calc. That calc and a subsequent calc presented to NTSB included contributions from any reinforcing which could be identified in the region, not being limited to reinforcing intended to resist shear friction.
The FIGG calcs were made using a coeff of friction of 1.0, while the actual joints had received no roughening and a correct coeff of friction would be 0.6, reducing the contribution of all components by 40%. The NTSB concluded the joint prep was not a factor - it would have failed anyway.
As I recall now, that slide presentation did not address the presence of two 4" dia pvc sleeves vertical thru the deck and together at both sides of member 12. Nor did it consider the extensive cracking of diaphragm 2, which it assessed as having cracked from the vertical component of members 11 and 12. The horizontal thrust was greater than the vertical component and the joint had slipped maybe 1/2 inch when the slide presentation was made. Tests from WJE show that maximum resistance of a shear friction joint is developed at a slip of 0.020 to 0.025 inches - long before the EOR saw the actual conditions on the morning of March 15. It was downhill from there.

Quote (and thus not really checking/looking into it (or just missed it).)

You may have answered your own question. It appears that almost no attention was given to tne need for node 11/12 and member 11 to "stand alone" during Stage 2 erection and until the entire double span structure was completed.
When complete, member 12 was to be "wrapped" and integrated into a concrete pylon 5 feet by 6 feet in dimension cast with the north span. And the north span was a shortened mirror image of the main span, so member 14 of the north span counters part of the horizontal force in member 11 in the final structure. The mental image of the huge pylon seems to have created a confidence in this area that overshadowed the importance of capacity at Stage 2.

And - can anyone confirm the actual reinforcing in member 11? The drawings leave some question of whether member 11 should have been considered a member "without PT rods" and therefore should have had 10 - #7 bars, or whether member 11 was a member with "with PT rods" and therefore was to have only 8 - #7 bars - in which case the amount of reinforcing does not meet the minimum requirement for a structural member. In either case, I see a visibly evident lack of appropriate reinforcing in the web members.
Thank you.


RE: Miami Pedestrian Bridge, Part XIV

Does the phrase "feet of clay" spring to mind when considering the talents of FIGG?

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley)

...It appears that almost no attention was given to tne need for node 11/12 and member 11 to "stand alone" during Stage 2 erection and until the entire double span structure was completed...

That's what makes this more a failure of imagination than of calculation.

Quote (Vance Wiley)

...When complete, member 12 was to be "wrapped" and integrated into a concrete pylon 5 feet by 6 feet in dimension cast with the north span. And the north span was a shortened mirror image of the main span, so member 14 of the north span counters part of the horizontal force in member 11 in the final structure. The mental image of the huge pylon seems to have created a confidence in this area that overshadowed the importance of capacity at Stage 2...

I agree that Figg seems to have concentrated too much on the final structural configuration, but I'm still on the fence about how much it mattered. There is no evidence that there was to be any substantial connection between the decks of the main span and back span of the finished bridge. So I think that while the completed span could not have failed in the same way as the Stage 2 span, there would still have been plenty of room for failure. Given time, the horizontal component of the force in 11 could still have separated the 11/12 node from the main span deck. But instead of kicking that node off the deck to the north, it would have shoved the entire main span to the south until it used up all the available travel in the expansion joints. Depending on the detail design of how the main span was to be anchored to the pier and at the southern abutment, it might or might not have collapsed fully, but it certainly would have made a scary mess.

RE: Miami Pedestrian Bridge, Part XIV

Quote (hpaircraft (Aeronautics)Given time, the horizontal component of the force in 11 could still have separated the 11/12 node from the main span deck. But instead of kicking that node off the deck to the north, it would have shoved the entire main span to the south until it used up all the available travel in the expansion joints)

The final canopy PT force to be added was the continuity force of 1786 kips in the full 270 foot length of the canopy. I think they intended that to "clamp" node 11/12 - at least on the morning of March 15. I question the efficiency of that force to clamp the decks together - earlier discussions here propose the deck PT forces cannot influence the canopy so why could the canopy PT influence the deck? Some 270 foot PT in the deck would have been far more effective.
The south expansion joint is 1-1/2 inch wide so that is the point where the south stairs would begin resisting forces. Whether that would have kept it on its supports is a guess at this time. NTSB Member Homedy discussed the stairs as providing forces to keep it on its supports. The staff responded that there was an abutment at the south end. Hmmmm....
Member 11 was so badly damaged and under reinforced it could then have been the weak link.

Quote (That's what makes this more a failure of imagination than of calculation.)

Imagine this - the force in member 11 is 1.61 times the truss reaction at the pylon. For the reaction to equal the force value in member 11 would require a span length of about 240 feet (deducting any increase in the end span reactions of the canopy and deck ). This is where the imagination comes in -
Can you imagine an engineer supporting one end of a 240 foot long structure just like this on a 24 foot long column of the dimensions 21" X 24" and reinforced with 8 - #7 bars?
Can anyone?
Would cracking and splitting in that column like that seen in member 11 have caused any concern among the people at the meeting of March 15?
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

I watched the NTSB Hearing last weekend. It is unfortunate that they did not make a more deliberate effort to distinguish between FDOT & the FDOT LAP in their comments. The FDOT LAP is an out sourced Contracted Service, which in many respects is established under FHWA guidelines. While it can be stated that FIGG failed at all levels of their staffing, I think it can also be said that the LAP Contractor, BPA & MCM all failed to exploit the depth of their staffing, as presented, when they prequalified for the project.
Just as an example, when during the meeting on March 10, 2018, the day of the collapse, FIU ask BPAs opinion of FIGG's analysis, they deferred to their in-house superiors (Jake Perez and Luis M. Vargas) and requested time to give a response. One wonders why Jake Perez and Luis M. Vargas, were not, already involved and at the meeting. Luis M. Vargas' CV on his BPA Profile certainly establishes him as an engineer familiar in dealing with concrete failure. If BPA had brought the full measure of talent that they had claimed would be supplied to the project, when conditions in the field degraded, perhaps tragedy could have been averted.
I also think the NTSB spared FIGG the coup de grâce. The NTSB puts little emphasis on the cracking that was photographed between 3:16pm & 3:18pm on March 10, 2018, 2-1/2 hrs after the transports were removed and 1 hour before the PT rods were detensioned. There was already a visible crack in member 12, longitudinal cracking in #11 and the spalling on the deck at the edge of the diaphragm was great enough to have been accompanied by cracking on the north face of the diaphragm. The bridge was most likely already damaged beyond repair. There was only one opportunity, to attempt to repair the concrete and that was while it was still on the SuperShores in the casting yard.

Emails between Figg & MCM regarding when the detensioning took place and if the photos taken between 3:16pm & 3:18pm, were before detensioning, establish that FIGG knew the new cracking had begun before detensioning.
When Louis Berger's, Dr. Shama, first modeled the complete bridge, he did so with the PT bars in #2 & #11 fully tensioned. He found the compressive forces in #2 & #11 far too high. FIGG assured Dr. Shama that #2 & #11 would be detensioned immediately after the bridge was set. It may be that the FIGG employee that Dr. Shama worked with, was one of the FIGG staff on vacation at the time of the collapse and Dr. Shama's concerns not known to others at FIGG. It seems, detensioning possibly slowed the the failure of #11 by reducing the compressive force in #11 and the decision to retension precipitated the collapse.

RE: Miami Pedestrian Bridge, Part XIV

Quote (epoxybot)

There was only one opportunity, to attempt to repair the concrete and that was while it was still on the SuperShores in the casting yard.

I would disagree that there was ever any opportunity at all to "repair" the concrete. The design itself lacked any shear steel within the concrete for connecting strut #11 to the deck. The only "repair" that could have been done would have been to introduce a massive steel tie sufficient to "capture the node" at #11/#12 and tie it back to the previous node on the deck. There was not one possible modification to the concrete that could have had any preventive effect capable of avoiding collapse.

RE: Miami Pedestrian Bridge, Part XIV

I ran across this news item a bit ago, about a major bridge being built in Corpus Christi, Texas:
https://www.ccbiznews.com/news/corpus-christi-harb...
What I find is rather confusing. The "design" has been suspended but I found other links that seem to indicate the bridge is half-built. One article mentioned the contractor was the same one as the FSU bridge, but it's not. This article indicates Figg is designer of the new Harbor Bridge, but that's not mentioned anywhere else that I find.

RE: Miami Pedestrian Bridge, Part XIV

Ooooh, look, concrete trusses! Good thing they contracted an engineering firm with a proven track record for making that work, right?

Quote (Flatiron)

...Twin precast concrete box girders with precast delta frames provide maximum durability for the harsh coastal climate, with shapes for maximum strength and stability in extreme wind conditions...

RE: Miami Pedestrian Bridge, Part XIV

epoxybot (Structural)15 Nov 19 20:04
Please check post for typo. Third paragraph, should be March 10, not 15.

RE: Miami Pedestrian Bridge, Part XIV

Re: Repairability

When a girder fails, usually the entire girder is replaced as a repair. Since this bridge was designed as a single girder, the only practical form of repair is total replacement. Since the whole idea of the single girder was to lower cost, it should therefore not have been a huge burden to scrap it in the casting yard. This bridge was treated like a red-headed stepchild.

RE: Miami Pedestrian Bridge, Part XIV

Quote (jrs_87 (Mechanical)16 Nov 19 02:39
Re: Repairability
This bridge was treated like a red-headed stepchild.)

How funny - that exact phrase has been running thru my mind.
That brings us back to the beginning - and the question of "Who decided to abandon the whole project?"
The collapse would have at least aroused the attention of the design team. With 30 days of study, the problems could have been identified and corrections made in the detailing while 90% of the forming would have been reused or at least reconstructed to known dimensions. Start building the back span immediately and concurrently with the casting of the replacement to the main span. Add 6 months to finish date - that would seem better than what we have now - nothing.
Or better yet design it using steel - much lighter so foundations are adequate - the size has already been approved, the location determined - just need a concentrated effort to redesign and save a project.
Had there not been loss of life this project might have been resurrected. Those taking action and closing the street would have been heroes.
If only the street had been closed immediately at the end of the March 15 meeting.
As we know now, there would not have been time to shore it and save it, and that fact would have pointed out the wisdom of closing the streets, thereby certifying the heroes.
Was there a knee jerk reaction that led to the decision to abandon this 'red headed step child'?
Professional reputations would have been stained, and the costs would have been significant, but there would have been a completed project.
I have always wondered why the back span was not constructed concurrently with the casting of the main span. I do not see anything significant that would have prohibited that.

RE: Miami Pedestrian Bridge, Part XIV

4
Vance, consider what might have happened if they had closed the road after the March 15th meeting.

They would also have suspended the retensioning of the PT bars in 11, since putting up a crew would have been unsafe. If the PT bars had NOT been retensioned it is possible that the bridge could have held until the shoring was in place. It really depends on how quickly they could support the node in question.

What I’ve long found ironic is exactly what jrs_87 said: this was the red headed stepchild project! That exact phrase has been running through my head since the week after the collapse.

After going through everything in the docket, watching the October 22nd meeting twice and reading the final report, I can only conclude that for all that this was a special, novel project for FIU, to FIGG and MCM it was just a little pedestrian bridge. It did not get the attention it needed from the design or construction firms.

While MCM was ultimately in charge of the build site, they abdicated their oversight to the mighty FIGG EOR rather than listening to what the bridge itself was saying. Frankly, with the rush to retension the PT rods they proceeded even though the Coranado group PT inspector was not present- which should never have been allowed to happen. There was no one in the March 15th meeting who had the courage to stand up to the FIGG EOR and call BS on the plan.

This project was too small to garner FIGGs real attention. Clearly the EOR was very busy with more important projects. I am still rather stunned that the reason the peer review was abbreviated was because FIGG dropped the ball by planning to use their internal offices for the review rather than a seperate firm. Then they had to rush to get the design rubber stamped in time. I find it telling that even FIGG can not explain how they used their models to extract the original calculations for capacity and demand on the nodes. That’s how little this project meant to them, until it blew up in their faces.

RE: Miami Pedestrian Bridge, Part XIV

jrs_87 (Mechanical) My bad, now I've started a chain of March 15, 2018 references. It is indeed March 10, 2018.

FortyYearsExperience (Structural) While I agree that there was never a point at which a repair could be 'effective', I can't help but mark the point in time, when an examination of the cracking should have taken place and was summarily dismissed by FIGG. I did contemplate the futility of the suggestion of a repair. Figg was given opinions/concerns regarding the node/s by FDOT-Tom Andres, BPA-(Cold Joint) & Louis Berger (Compressive Forces in PT tensioned #11), so when a problem developed in the casting yard, it should have been the wake up call.

Kestrel42 (Bioengineer) How does one shore the bridge? You cannot send workers under the bridge. Do you bring the transporters back? There were cracks in the #1/#2 node as well. The cantilever resisting PT action cannot be restored.

RE: Miami Pedestrian Bridge, Part XIV

Quote (SFCharlie (Computer))

Is there a way to explore a thin crack to determine it's depth?

Impact-echo, ultrasonic impulse-echo.
Impact-Echo is a nondestructive test method for evaluating concrete and masonry structures. The test utilizes stress waves (sound) that is normally generated through striking concrete by an impactor (Impact), and recording the reflections and refraction from internal flaws and other boundaries (Echo).


RE: Miami Pedestrian Bridge, Part XIV

I think ultrasonic methods are unwieldy for most in-situ applications. Maybe someday we will be able to use nanobots for mapping cracks.

RE: Miami Pedestrian Bridge, Part XIV

CAB -- surely if there was a proactive attempt to determine the extent of cracking (rather than waving a hand over it in blessing as happened, or scrapping and recasting as some are suggesting), this would be a prime case to have employed some more advanced methods.

I'm speaking out of ignorance, as I'm not familiar with the process for concrete. But it's hard to imagine the procedure being that much more unwieldy than ultrasonic weld testing, which is completed regularly for major bridge structures under construction.

----
just call me Lo.

RE: Miami Pedestrian Bridge, Part XIV

Those cracks were so bad that NDT, advanced or basic, was unnecessary.

IC

RE: Miami Pedestrian Bridge, Part XIV

You're right. It seemed they forgoed non destructive testing and went straight to destructive testing. cry

RE: Miami Pedestrian Bridge, Part XIV

2

Quote (Kestrel42)

After going through everything in the docket, watching the October 22nd meeting twice and reading the final report, I can only conclude that for all that this was a special, novel project for FIU, to FIGG and MCM it was just a little pedestrian bridge. It did not get the attention it needed from the design or construction firms.

Sometimes it takes a moment to consider this from a higher level and in two sentences I think you've captured the essence here about what went wrong.

The cult of the FIGG EOR also looms large in this disaster and hopefully in the future it will embolden some of the others in the next meeting to step up to the plate and demand action (or inaction) despite what "the mighty EOR" says.

I think (hope) state highways departments will already be preaching the safety first philosophy more than they already did.

To go through that amount of data must have taken you days so congratulations for that assessment.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

Dummy post to boost this current one (Part XIV) back up above its predecessor (Part XII) in the presentation order.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Denial (StructuralDummy post)

Hi - help me here. I scanned XII and did not find a post by you.
Perhaps a link to where we should go? Then maybe a return link?
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

from Miami Pedestrian Bridge, Part XIII

Quote (JAE (Structural)(OP)3 Nov 19 00:15)

Earth314159 - please stop posting here - go to Part IV. Thanks.

Quote (Denial (Structural)24 Nov 19 19:51)

JAE's instruction should read:
G O T O P A R T X I V
(...just the facts mam)

RE: Miami Pedestrian Bridge, Part XIV

Vance,

All that has happened is that someone posted in Part XIII (or 13 in most peoples language) and shouldn't have been. Hence the post above by Denial just put part XIV back as the top listing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

(OP)
Ya I screwed up. Thanks Denial for the clarification.

RE: Miami Pedestrian Bridge, Part XIV

Quote (LittleInch (Petroleum)

Thank you. Happy Thanksgiving to all!

RE: Miami Pedestrian Bridge, Part XIV

Quote (SF Charlie)

Is there a way to explore a thin crack to determine it's depth?

I haven't checked in a long time but I read a few of the latest posts.

Cracks usually go straight through a member. In the NTSB board meeting, they mentioned a standard that considers cracks to be an issue if they are more than 0.5" deep. 99.9% of cracks are more than 0.5" deep. So I don't know where that standard originated from but it makes no sense. The width of the crack is more critical. Also the consistency (does the width vary and how), shape, location of the crack are all more important than the depth. You can usually determine what is happening by an external review of the crack. Even in research, you usually don't look at the internal shape (probably too difficult). You can in theory use ground penetrating radar (GPR) to determine the internal shape. We used to use x-rays for locating rebar and PT (We use GPR now) but I don't ever recall seeing an x-ray and being able to identify the shape of a crack (I am not saying it can't be done) but likely at least as difficult to identifying bone fracture shapes on x-ray (not that easy).

RE: Miami Pedestrian Bridge, Part XIV

When they talk about cracks less than 1/2" deep (12 mm), they are talking about normal flexural and shrinkage surface cracks in the cover concrete, which don't reach down to the embedded rebar (or only just), so the "core" of the section is intact, and all rebar is encased in sound concrete. Such surface cracks also typically have a very small surface width - less than 10/1000" or so (0.3 mm) at the surface, tapering to effectively zero width at or near the outer face of the embedded rebar, so do not generally pose a risk for moisture ingress and rebar corrosion etc (although the permissible crack width will be smaller in aggressive environments). Surface cracks of this type are expected in normal reinforced concrete design. One of the common aims of post-tensioned concrete design is to close up even these fine surface cracks, through the application of compression on the whole section, overcoming the flexural and shrinkage tensile stresses.

When we see full-section cracking, with crack width such that you can insert your fingers, then we are looking at a whole different situation.

http://julianh72.blogspot.com

RE: Miami Pedestrian Bridge, Part XIV

Quote (jhardy1 (Structural)When we see full-section cracking, with crack width such that you can insert your fingers, then we are looking at a whole different situation.)

Good description.
And to comment on your last sentence, particularly cracking of that magnitude in an area having designed (??) and purposeful reinforcing. Put simply, if the designer put reinforcing there for a purpose and it cracks that badly, it is time to consider the total consequences of a failure.
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

(OP)
A very short ENR article (Link):
Seems that Figg now brings with them a skeptical reputation.

FIU Collapse Report Spurs Texas Bridge Design Review
The Texas Dept. of Transportation has suspended design work on Corpus Christi’s Harbor Bridge pending a safety review of plans prepared by FIGG Bridge Group, one of the firms faulted by the National Transportation Safety Board for last year’s deadly pedestrian bridge collapse at Florida International University. Although construction of the $803-million cable-stayed structure’s initial phase remains unaffected, the review, expected to take at least 30 days, could further delay a project already behind on its original 2021 completion schedule. Tests of elements already in place have revealed no issues.


RE: Miami Pedestrian Bridge, Part XIV

Re: Corpus Christi Harbor Bridge. Does anyone here know what kind of waiver or exemption allows children to be in construction yard? See photo gallery here: https://harborbridgeproject.com/gallery/

RE: Miami Pedestrian Bridge, Part XIV

I'm not aware of any prohibition of children in construction yards that would require a waiver or exemption. But it looks like a public event, similar to groundbreaking, ribbon cutting, etc.- not just kids but other non-construction workers there.

RE: Miami Pedestrian Bridge, Part XIV

Yes, looks like a good day out to me. There should be more events like this. Otherwise, how does the public, especially the next generations, learn a bit about construction?

RE: Miami Pedestrian Bridge, Part XIV

I suspect they have to rebuild something otherwise they have to do something else with the money that they received back.

Plus also I suspect that the student accommodation that was going in on the other side of the road in anticipation of the bridge is now all complete and full.

So they have a huge issue with them crossing the road anyway possibly killing more kids than the bridge collapse did inside 5 years if there isn't a bridge.

RE: Miami Pedestrian Bridge, Part XIV

There is another construction issue that I don't understand.
The use of the back span to "capture the node".
I think that this was just some after the fact bafflegab to cover their assets.
If it had been a design intent that the back span would provide needed support to the main span, then why was the back span not completed and in place before the main span was placed.
If the back span was seriously intended to provide needed support to the main span, then in addition to the other shortcomings we can add an inappropriate construction schedule and another failure of oversight.
The point is probably moot as the span was badly damaged even before it was placed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

If that was to be considered they would have cast it as part of the pier and butted the span against it and grouted it. So I'll put that in the burn bin along with the (paraphrased) 'we used multiple longitudinal post-tension cables to make the bridge a redundant structure.'

The pylon was going to use the two spans as part of its formwork so it could not be in place until the two spans were in place. Perhaps the audience for that comment was meant not to notice.

RE: Miami Pedestrian Bridge, Part XIV

Yep the same thing occurred to me. Everything I've seen suggests this was designed as two simply supported trusses with an architectural pylon and faux pipe stays. The presented calculations show that their previous calculations were all based on independent trusses.

RE: Miami Pedestrian Bridge, Part XIV

Quote (3DDave)

'we used multiple longitudinal post-tension cables to make the bridge a redundant structure.'

This is close to an admission of liability for the failure. They provided multiple longitudinal post-tension cables, but they did not connect those cables to the 1.5 million pound load heading north in diagonal strut #11. The 1.5 million pound load moved northward because it was not restrained by any other force. (Newton, First Law of Motion, discovered 332 years ago; see, "Principia Mathematica Philosophiae Naturalis" pub. Cambridge University, 1686)

RE: Miami Pedestrian Bridge, Part XIV

Aaaaghhhh! There's that old Newton stuff again. That guy has been dead 300 years and who remembers what he said?
Heck, it probably doesn't even work anymore, it has been so long. We have computers today and they take care of all that stuff so we don't have to worry about it.
Wiley's First Law of Computers: Garbage Out is directly proportional to Garbage In.

RE: Miami Pedestrian Bridge, Part XIV

40YE,

"because it was not restrained by any other force"

It was restrained, but just not enough.

I do agree though with sentiment - all they really needed to do was connect those cables via strong enough end beam to resist the end force.

I just hope that the results of this investigation prove to be a key learning point in bridge / concrete truss design for decades to come.

However I don't think there will be too many designs similar for a long time due to this collapse. There was no redundancy and the design and execution was flawed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley (Structural))

Garbage Out is directly proportional to Garbage In.
Definitely, but remember, computers can multiply (garbage)

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

2

Quote (LittleInch)

There was no redundancy . . .

I would respectfully submit that the oft repeated assertion of "no redundancy" misses the mark.

The subject bridge lacked any functional connection between strut #11 and the multiple longitudinal post-tension cables in the deck. If the absence of such connection were to be repeated in two trusses running parallel to each other, such a bridge would have "redundancy" but would certainly also collapse. The problem was not a lack of redundancy. It was a lack of a critical connecting element. No matter how many times you repeat that error, you are not going to fix the bridge. And yet, if that connection had been provided only once, the bridge would have stood for 100 years without possessing any "redundancy".

If the term "no redundancy" is defined to mean "lacks a critical element" -- well then we have an argument about the meaning of language, and not about civil engineering.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley (Structural) 20 Dec 19 07:04)

Aaaaghhhh! There's that old Newton stuff again. That guy has been dead 300 years and who remembers what he said?
Heck, it probably doesn't even work anymore, it has been so long.

Newton's Laws of Motion were actually replaced in 1915 by Einstein's relativity theories (1905 & 1915) but they're still "good enough" for everyday engineering.

Was doing some reading about Scientic Laws and came across these nuggets:

Quote:

(Physical) Laws differ from scientific theories in that they do not posit a mechanism or explanation of phenomena: they are merely distillations of the results of repeated observation. As such, a law is limited in applicability to circumstances resembling those already observed, and may be found false when extrapolated.
:
Some laws are only approximations of other more general laws, and are good approximations with a restricted domain of applicability. For example, Newtonian dynamics ... is the low-speed limit of special relativity ... Similarly, the Newtonian gravitation law is a low-mass approximation of general relativity.

So the definition of a scientific law kinda sounds a lot like a traffic law: just because a 35MPH speed limit means "up to 40MPH is OK" in your city doesn't mean it will be the same in all cities.

RE: Miami Pedestrian Bridge, Part XIV

Review of FIGG calculations on another bridge, and the consultant has "identified an issue" with design of the pylon legs. Link

RE: Miami Pedestrian Bridge, Part XIV

Quote (wetlander)

Review of FIGG calculations on another bridge, and the consultant has "identified an issue" with design of the pylon legs

You'd pretty much have to wouldn't you?

RE: Miami Pedestrian Bridge, Part XIV

From the Constructiondive article:

Quote:

"...WJE’s detailed research, in-depth analysis, and physical testing shows that faulty construction of the Florida bridge — which FIGG had no hand in — was to blame for the collapse, not its design.”

That is such completely unmitigated horseshit, one wonders whether anybody would trust WJE if they said the sky was blue.

RE: Miami Pedestrian Bridge, Part XIV

It is hard to see Figg surviving. Who would hire them? They have to keep 100 employees busy.

RE: Miami Pedestrian Bridge, Part XIV

Quote (one wonders whether anybody would trust WJE if they said the sky was blue.)

That piece was from FIGG - naturally that is their interpretation of the WJE test - but the test and how it was performed - what it addressed - missed the point that FDOT and AASHTO joint requirements are much different. In other words it is double talk or conflagration or - horse+++t - how much WJE is to blame is a question. Never cease being critically objective.
I recall the Sunday show that exposed all the bad things - and a lot of them really got me steamed up.
When they got to the point of criticizing JEEP because they could be turned over by an idiot driver I quit watching. They lost all credibility with me. Then I learned they rigged the Chevy PU gas tank explosion.
In court they say "one lie, all lies" or something to that effect.
Thanks, and Happy New Year to all.
What do we hear from the EOR these days? Looking for a job or hiring an agent to sell his book?

RE: Miami Pedestrian Bridge, Part XIV

From the Florida board a while back: "FBPE will monitor developments as the other agencies complete their investigations. If the cause is determined to be an engineering issue, FBPE will take any necessary steps, including disciplinary actions, to ensure public health and safety." So I expect that's an upcoming little obstacle to deal with at some point.

RE: Miami Pedestrian Bridge, Part XIV

So can anyone with a knowledge of proceedings let us all know what happens next / is happening?

NTSB report was pretty damming of FIGG amongst others, but they hide behind their defence of "if the joint was made properly" which seems not able to stand p to serous scrutiny.

So any more hearings?

Court cases?

Anything new?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Business News)

...that the move could result in further delay of the $930 billion project, already two years behind schedule

I hope this project isn't almost a trillion dollars...

RE: Miami Pedestrian Bridge, Part XIV

Wait until the extras to contract come in... the US is already at 23... what's another one?

Dik

RE: Miami Pedestrian Bridge, Part XIV

Wetlander... they did't pull any punches or make it 'sugar coated'.

Dik

RE: Miami Pedestrian Bridge, Part XIV

I think $930 billion is a typo. This link says $930 million, which is more reasonable.

Link

RE: Miami Pedestrian Bridge, Part XIV

For the curious or skeptical, WJE webinar tomorrow:

https://event.on24.com/eventRegistration/EventLobb...

If my bridge relied on an apprentice mason scouring a few square feet of concrete in an extremely tight area to remain aloft, I think I would be standing over his shoulder with a trowel in hand whilst doing so..........

IC

RE: Miami Pedestrian Bridge, Part XIV

Wasn't that the team that was hired by FIGG to come to that exact conclusion; that it wasn't FIGG's fault for having insufficient reinforcement but that the concrete people didn't properly read the minds of FIGG engineers to see a requirement that was never analyzed, written down, or otherwise seemingly considered?

Why yes, they are:

"FIGG also detailed that it had hired forensic structural engineering experts Wiss, Janney, Elstner Associates to conduct an investigation, which revealed that the construction joints at the failure point were not roughened in alignment with Florida standards. (Roughening would have also contributed to the strength of the connection.)"

https://www.paintsquare.com/news/?fuseaction=view&...

RE: Miami Pedestrian Bridge, Part XIV

It's the bit at the end I don't like " that demonstrated that, had the construction joint been roughened as required by the project specifications, the collapse would not have occurred."

This bridge was fatally flawed in design and construction.

To try and blame a non roughened joint doesn't work for me. I haven't read all the back words here but I'm pretty sure the NTSB refuted that suggestion. Sure it didn't help, but this bridge would have fallen down even with a roughened construction joint.

I also can't see how anyone can say something like this "would not have occurred" when there were a number of failure modes, lack of strength and re-inforcement and probable damage whilst it was moved and placed on its supports

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

Like LI said, I don't understand how WJE can so strongly state the bridge wouldn't have failed when it was cracking excessively the moment it was moved into place. Is this a PR piece for court cases or something?

RE: Miami Pedestrian Bridge, Part XIV

Anyone else think it's just a coincidence that WJE is presenting this just 4 days before the NTSB meeting? Didn't think so.

Wasn't there significant design errors made by FIGG? Where did admitting your wrong and accepting at least partial responsibility go?

RE: Miami Pedestrian Bridge, Part XIV

What does Figg and WJE hope to gain? The failure was a fundamental engineering design flaw and it is an understatement to say that the response from the design engineers was unacceptable even if the surface was roughened and the "the collapse would not have occurred". It is just not that the joint had to be roughened, it had to be designed correctly as well and that design had to be communicated to the contractor. The engineers were either incompetent or unethical. How can a structural engineer see those cracks and not know it was a shear friction failure?

Notice how the event summary says "had the construction joint been roughened as required" and does not say the specifications or drawings required roughening. Codes require it to be roughened and the engineer is suppose to communicate those requirements in the specifications and drawings. Someone was suppose to tell the contractor that the joint had to be roughened and even in that case, it was still under designed. Regardless, it did not meet the minimum level of strength to protect the life and safety of the public. The response to the cracks was unacceptable and for any competent experienced structural engineer that works with concrete, it boarders on criminal negligence.

The roughening issue is a moot point and is irrelevant to the question of responsibility.

RE: Miami Pedestrian Bridge, Part XIV

It seems that in addition to a flawed design and ignoring clear warning signs of impending failure, they also failed in their duty to diligently inspect the work as it progressed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

...are we beating a dead bridge? (to paraphrase Monty Python...)
The bridge is not pinin'! It's passed on! This bridge is no more! It has ceased to be! It's expired and gone to meet it's maker! It's a stiff! Bereft of life, It rests in peace! If you hadn't set it on the pier It'd be pushing up the daisies! It's metabolic processes are now 'istory! It's off the twig! It's kicked the bucket, It's shuffled off it's mortal coil, run down the curtain and joined the bleedin' choir invisible!! THIS IS AN EX-bridge!!

Note: FDOT is going to build a pedestrian bridge in it's place, hopefully made of steel, with no faux supports...

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

I'd be interesting to see their results on the difference roughening makes.

However I agree the above that that statement is a bit rich. That is like saying had we not placed that last piece of straw on the camel then its back would not have broken.

RE: Miami Pedestrian Bridge, Part XIV

Human909 - Without stretching the metaphor too far it's more like saying

"hey, that last piece of straw must have weighed so much more than the previous pieces of straw..."

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

At this point, I don't care to go back and research everything, but what I remember reading was that Figg basically said "If the surface had been roughened, the bridge wouldn't have fallen down." But that is a completely different statement from saying "the bridge was designed correctly and the only reason it failed was that the surface wasn't roughened". I got the impression they worked backwards through the design and were able to calculate a factor of safety of 1.001 if that surface was roughened and hung their hat on that.

I pity the designer and contractor on the next bridge. There will be more inspectors than workers, with inspectors inspecting the other inspectors.

RE: Miami Pedestrian Bridge, Part XIV

Just listened to the WJE webinar. Found it interesting. They really seem to be defending FIGG and focusing solely on the fact that the surface was never roughened. The defense of FIGG hinges on an email that references a FDOT spec for roughening the hardened surface. The spec doesn't give any guidance to the amount of roughening nor does FIGG's email. WJE says pretty much any amount of roughening would have been enough to prevent collapse.

What I don't understand is how FIGG ignored that the bridge was telling everyone there was major problems. How did FIGG come to the conclusion that adding more compression to the joint was going to help matters?

I can't come to the conclusion that FIGG is not at least partially liable here.

RE: Miami Pedestrian Bridge, Part XIV

My hope is that the concept of shear-friction will be reexamined. This relatively new, and in my opinion illogical, design method should not have been relied on for such critical joints in a novel structure. Many will say it is not new, but it was not part of ACI 318 when I worked in the US, and is still not part of the Australian code.

RE: Miami Pedestrian Bridge, Part XIV

Is there a copy of the WJE presentation? i only got to see part of it.

RE: Miami Pedestrian Bridge, Part XIV

What does WJE have to say about the fact that the firm hired to perform construction inspection, had advanced the concern, before the diagonals were built, that congestion arising from the deck made roughening of the joint during casting ineffectual?

RE: Miami Pedestrian Bridge, Part XIV

As I recall, and as a recap, the only reference on the drawings regarding construction joint preparation was at a horizontal joint in a pier, with dimensions of about 6 feet by 10 feet.
Finding no specific requirements for the truss joints, the engineering firm overseeing construction requested specific direction in this matter.
FIGG responded by instructing the joint to be prepared to meet FDOT requirements. But FDOT as I read it a year ago gives NO specific direction beyond removal of loose materials and cleaning the joint and FDOT does not specify an amplitude of roughness, normally specified at ¼ inch.
Then, after failure, WJE prepared and tested joints which were prepared to intentional roughness of ¼ inch amplitude and used that as a “woulda – shoulda” joint, implying that their preparations and testing proved the likely results had the FIGG requirement been followed.
But there is a large gap in this logic – somehow they made the jump from FDOT requirements of "clean" to an intentionally roughened amplitude of ¼ inch.
A bit of a “bait and switch” maneuver, it would seem.

RE: Miami Pedestrian Bridge, Part XIV

The end of the deck (the diaphram across the end??) was cracking and moving outward under the diagonal. How can anyone say that roughening the deck to diagonal joint would have stopped that from happening while keeping a straight face? It seemed clear to me that all the pictures of cracks before as well as the after collapse pictures showed that the end of the deck under the diagonal failed, not that the diagonal slid off the deck.

RE: Miami Pedestrian Bridge, Part XIV

Quote:

How can anyone say...?
Legal training.
His lips were moving.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

Quote (LionelHutz (Electrical))

The performance of the construction joint at the deck surface is only one step along the way to a disaster. Member 11 was in failure, pipe sleeves beside and below the failed block defined weakened failure planes, reinforcing was inadequate, and NTSB found they used the wrong load for the joint - all these resulted in progressive cracking and failure in progress for days before the actual collapse.
So I am in agreement with you - this joint preparation could not have saved it. The handling of the joint preparation and the double talk attempting to obscure a lack of specificity in the instructions to the job and the subsequent laboratory testing show an attempt to deflect the focus.
Sadly, the EOR did not make an engineering call on March 15.

Quote (ImminentCollapse (Structural)27 May 20 15:05
If my bridge relied on an apprentice mason scouring a few square feet of concrete in an extremely tight area to remain aloft, I think I would be standing over his shoulder with a trowel in hand whilst doing so..........)

That would be most prudent at that jucture.

RE: Miami Pedestrian Bridge, Part XIV

2
WJE has heretofore had a good reputation for forensic work. They have blotted their copybook now.

RE: Miami Pedestrian Bridge, Part XIV

hokie66 (Structural),

I think the usefullness of this forum is there are other experienced professionals who can voice our opinions.

Like Vance Wiley (Structural) pointed out the Member 11/12 has already been separated from the deck so the adhesion at the construction joint is minor if not trivial comparing with the loss of effective concrete at the critical section displaced by the 4 No. of vertical 4" I.D. pipe sleeves, various cable ducts and one large horizontal 8" I.D. embedded drain pipe. These embedded items are the real killer because not only they (1) robbed the section of precious bonding concrete they also (2) obstructed placement of reinforcing bars so desparately needed, (3) created weakness points and paths (stress concentrations) due to their presence and (4) provided sources of defective construction due to the difficulties of compacting concrete in small, tight and confined spaces.

In the update to the preliminary BTSB report on May 23, 2018 NTSB's Fig 1 and 2 show, below the the construction joint, the deck had already developed diagonal shear cracks, typically about 45 degree, in a pattern exactly predicted by FDOT Engineer Tom Andres on Mar 2016 when he first review the preliminary design. The crack on one side was 4" deep shown from the insertion of a tape and appeaed at least 20mm wide. Above the construction joint NTSB showed by Fig 3 Member 11 had a crack 7" deep against by the insertion of a tape. The extensive cracking of Fig 1, 2 and 3 were taken prior to the collapse and also based on them Figg declared the FIU bridge safe! NTSB and most of us were critical of such claim as it shows Figg did not even know the bridge had gone and continued to fiddle with the postensioning rod until the bridge broke up.

The key evidence prior to the collpase shows bridge had suffered fatal and structural damage below and above the construction joint and any suggestion that the adhesion of old deck concret could have hold the bottom of Member 11/12 without separation/failure cannot come from an experienced or qualified reinforced concrete designer. Any engineer experienced in reinforced concrete can confirm it is normal for such a constrcution joint to have minor cracks just by shrinkage in many existing serviceable structures.

RE: Miami Pedestrian Bridge, Part XIV

Thank you Saikee119
I would also note that the WJE modeling did not include the conduits for the post tensioning rods, etc. It also modeled the deck as effectively infinite depth, when in fact the deck failed. They seem to have set out to prove that the un-roughened cold joint was to blame, and saw what they were looking for.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

SFCharlie (Computer),

I haven't followed WJE modelling recently but I know the accuracy of every mathematical model depends on the assumptions used. Rubbish in rubbish out.

I post the follow sketch to higlight some of the important considerations

I have shown three section of the deck in A, B and C. C and A were highly stressed laterally by the transverse tendons whereas Member 11/12 was unstressed in the same direction. Thus the interface is prone to crack when there is a major change in stresses in the components. These members were initially held tight together by the PT rods in 11. According to OSHA report 11 has no structural crack at the CJ prior to the bridge removal from the casting yard.

The CJ was seen open up (or cracked) after the bridge had been installed at its final position and the PT rod stress was removed. At this point 11 would have its highest compression load (less only the live load) and it could only hold its position if its joint with the deck did not break.

It can be said with certainty in any court of law that the joint in question had already failed structurally prior to the collpase and re-tightening of the PT rods. The following sketch shows the effects from the embedded items.



Prior to the collapse we know the CJ had cracked or the interface shifted horizontally. The failed components show the failure plane could include the horizontal CJ indicated by the orange dotted line, a vertical plane between the deck and the bottom of Member 12 (show in red dotted line) and two horizontal plane , in green dotted line on either face of 12) along the embedded 8" drain pipe's centreline. In transverse direction possibly two more vertical planes, on either face of 12, shown as B1 were contributing shear resistance against failure.

OSHA has called it a blow-out failure when the bridge collpased.

The shear resistance of concrete and the shear resistance of rebar across the above dotted line interfaces or plane are the main forces to hold the bridge together. Some enhanced shear resistance from the clamping action may be at play but not much in this failing mode.

The design has a modest but not generous amount of shear steel across the CJ interface. At the deck interface with the front of 12 reinforcement is lacking because of the two PT rod anchors there plus the semi-circular cutout for the drainage pipe had left little room for passing the reinforcement through. The two sides bonding of 12 with the deck, Area maked B1, have been curtailed severely vertically by the 4x4" vertical sleeves and horizontally cut off by the presence of 8" drian pipe.

I have not gone through every steel bar to compute the capacity of this joint but it is a lot weaker than any drawing could reveal if the full set of embedded itmes were not included in the examonation.

Only looking at how little room availabe for inserting rebar so that it can have adequate development lengths, on both sides, then one realises how hopeless is the situation. If WJR could not find how this joint fails then its modeller has not enough practical knowledge of reinforced concrete in the field. If any rebar cannot be anchored adequately in both directions across an interface then that rebar should be written off structurally (for not able to realise its full material stress). In the OSHA report one can find a rich set of examples showing exposed bars with intact hooks and clean lengths which indicate the failures were by bonding and not shear.

RE: Miami Pedestrian Bridge, Part XIV

The horizontal component force in Strut #11 was known to be 1,500 kips (1.5 million pounds) at the time of the design

Back of envelope, to hold 1.5 million pounds in place requires 25 square inches of steel. (before we get onto factors of safety etc.)

NOWHERE in these drawings can we find 25 square inches of steel, let alone that amount being configured to restrain the horizontal force in strut # 11.

And, as saikee119 points out, the small amounts of steel which are visible are too short to provide any development of tension force. Effectively, there is ZERO square inches for meeting the requirement of 25 square inches of tension steel.

So, the FIGG argument is that 1.5 million pounds could have been held in place by cleaning the cold joint better.

HAHAHAHAHAH

RE: Miami Pedestrian Bridge, Part XIV

FortyYearsExperience (Structural),

For Member 11/12 To initiate a failure by pushing out from the deck the concrete has to break along the line of least resistance. Most of the resistance is the shear capacity of the concrete and the rebar from shearing and tensile strengths. Tension is concrete is normally a write-off in RC design unless postensioning is involved.

There are many combinations but the failed bridge has helped us to establish the mode of the blow-out. I have stopped to count the steel bars as the drawings are quite poor in revealing the information. May be the latest drawings with a complete set of bar bending schedule would help.

What is certain many bars if adequately anchored would have to fail by shearing off the circular section or snapped off by tension but not many of those are visible. If the rebar could not hold the node together as a rigid joint then the theoretical models would have little value. The primary failure wasn't in the structural calculations but in the detailing of the rebar because the node was not made rigid enough to perform its structural duty.

NTSB criticised the design for the load and capacity calculation errors. This is equivalent to say the capacity of the node should have been downgraded due to the imperfection of rebar installation (caused by PT rod anchorages, limited concrete dimensions, large amount of embedded pipe sleeves and drain pipe at critical locations creating stress concentrations and load path interruptions).


The 4 vertical sleeves were in good condition after the bridge broke away. Could the plastic stronger than the reinforced concrete? These sleeves formed a weak spot for Member 11/12 to leave the deck "cleanly".

All the reinforcement across the CJ had sheared off properly although only a few cross sections could be seen or made out from Fig 63.




Looking at the above OSHA drawings, showing a massive amount of rebar unable to be gripped soundly by the concrete, it is a waste of time to go the extreme length to prove the concrete adhersion at the CJ if constrcuted perfectly could have made a difference.

Any engineer investigating the 11/12 connection with the deck needs to appreciate the installation of 4 No. of 4" vertical plastic pipe sleeves and one 8" PVC drain pipe have severely compromised the integrity of the rebar in the vicinity by depriving the full development of the concrete bond with the steel bars. In another word there is no use in having sufficient rebar development lengths when there isn't sufficient concrete surrounding the steel bar!

RE: Miami Pedestrian Bridge, Part XIV

Quote (Saikee119)

It can be said with certainty in any court of law that the joint in question had already failed structurally prior to the collpase and re-tightening of the PT rods.
Yeah, but If I were in any way culpable in all of this, I'd sure hope that all the esoteric talk about an un-roughened CJ might be just enough to convince (or confuse) a juror enough to believe that I didn't belong in prison.
I'm not sure the thought of bringing criminal charges has been dropped yet.

Brad Waybright

It's all okay as long as it's okay.

RE: Miami Pedestrian Bridge, Part XIV

thebard3 (Computer),

A construction joint with concrete poured at different times, resulting unavoidably slightly different shrinkages and creeps, is always a plane of weakness no matter how perfect the construction.

The Fig 63 by OSHA I posted last time in fact shows the performance of the CJ rather well. To any experienced RC professional the construction result looks normal, acceptable, average, no particular bad or good. The shear surafce is quite rough, irregular and deep at places because a significant amount first stage concret had been forcibly removed substantiating a significant amount of successful integration, bond or adhesion with the second stage concrete.

There were altogether 10 vertical bars (2x4x7S01, 2x6S07, see drawing B61) plus 50% of the axial rebar from Member 11 (2x4x7S11+2x7S03, see drawing B40) and one PT rod sheared off cleanly at this CJ.



The failure of the connection is actually due to what happens the beyond the CJ to the rear end of Member 12 where the majority of the rebar from Member 12 was exposed and had no bonding concrete. With the exception of a few small diameter bars severed, like the 4S01 in the deck from both directions, nearly all the other bars, 1x11S03, 2x3x7S01, 2x9S01,2x9S02, 2x2x8S07 from drawing B47, had the concrete stripped off revealing the bare steel. This is not the result of a good design when half the structural components didn't get stressed, let alone failed.

Pinning the hope in the soundness of the CJ won't hold any water because NTSB has already shown 11/12 has been massively underdesigned. NTSB would have by default assumed the Member 11/12 node constructed soundly without defect in ordedr to carry out the analysis.

RE: Miami Pedestrian Bridge, Part XIV

No PT bars were sheared at the construction joint, at least not until the portion of the deck was cut loose for examination.

RE: Miami Pedestrian Bridge, Part XIV

Of all the errors, and there seem to be many, the worst was probably the decision to keep the road open during the "repair" efforts.

RE: Miami Pedestrian Bridge, Part XIV

3DDave (Aerospace)3 Jun 20 01:05 SAYS:
"No PT bars were sheared at the construction joint, at least not until the portion of the deck was cut loose for examination."

That is correct 3DDave. That's because the PT bars were not connected to any other bars. They simply ended in 'space' embedded in concrete. Thus, they moved intact, together with the concrete surrounding them.

RE: Miami Pedestrian Bridge, Part XIV

The lower bar lower attachment moved with the deck; the lower bar upper attachment moved with the canopy; between the two it tore the lower 1/3 of member 11 off and sliced through the reinforcing cage within 11. The concrete around the lower PT bar in 11 did not remain intact.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Wetlander)

Of all the errors, and there seem to be many, the worst was probably the decision to keep the road open during the "repair" efforts.

Politics. If you shut the road then ABC is a failure. So they gambled they could fix it.

Quote (Rabbit12)

Wasn't there significant design errors made by FIGG? Where did admitting your wrong and accepting at least partial responsibility go?

No-one does that voluntarily. Everyone tries to offload as much responsibility as they can. Despite all our "codes of ethics" it's how engineering works when things don't work.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Tomfh)

Everyone tries to offload as much responsibility as they can.

So true.

Figg's hired-gun (WJE) report dated Sept 18.2019, page 128 states:



Even though Figg's own EoR (Pate) was on site at the meeting hours before the collapse, whereby instructing the contractor and sub to continue with the stated re-stressing operation, before he traveled back to Tallahassee.

Engineers are becoming too much like f'n lawyers!

RE: Miami Pedestrian Bridge, Part XIV

Anyone notice the post by

Quote (saikee119 (Structural)3 Jun 20 00:35)

00:35
FIGURE 31 D/C RATIOS FOR MAIN SPAN NODAL REGIONS ?
Node 1/2 has a D/C Ratio of 2.15 while Node 11/12 has a D/C Ratio of 1.62.
Would we not expect the failure to more likely have been at Node 1/2 if the cause was inadequate design capacity?
Did FHWA not consider the capacity loss due to the pipe sleeves when arriving at the D/C of 1.62??
The differences between Node 1/2 and Node 11/12 have been discussed - size of Member 2 vs Member 11, thickness of diaphragm, amount of reinforcing across the CJ, angle of Member 2 vs Member 11. In short, Node 1/2 appears to be much more robust and obviously was not expecting assistance from future construction.
But the finding was that the D/C Ratio of the failed heel joint ( Node 11/12 ) was less than that of the heel joint at the other end of the truss which did not fail.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Ingenuity )

Even though Figg's own EoR (Pate) was on site at the meeting hours before the collapse, whereby instructing the contractor and sub to continue with the stated re-stressing operation, before he traveled back to Tallahassee.

Wasn’t my fault! I’d already left!

RE: Miami Pedestrian Bridge, Part XIV

Quote (Rabbit12)

Wasn't there significant design errors made by FIGG? Where did admitting your wrong and accepting at least partial responsibility go?

Don't know about USA, but I've been told by three employers here that the insurer might walk away if you admit any fault without the insurer's prior approval.

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural),

Although 1/2 was underdesigned by a bigger margin than 11/12 its members have more generous dimensions. All of then are 1'9" thick but 1 and 2 are both 3' wide whereas 11 &and 12 are 2' and 2'-10.5" wide respectively. 1/2 is type 1 and the drawings show no 4" pipe sleeves. This makes structural sense as in a bridge we always fix one end and allow the other end free to expand to avoid any thermal stress from the evironmental changes. Therefore if 11/12 were to be held down there is no need to install holding down bolts and sleeves at 1/2 end.

Having demand/capacity ratio exceeding 1.62 or higher doesn't necessarily cause an immediate failure. The bridge wasn't taking the full load at the time of collapse. Also using say a common load factor for dead load 1.4 and live load 1.6 the bridge at collapse didn't have an extra 40% dead load and the live load was zero so the structure was far from being stretched. NTSB's declaration of demand exceeding capacity is a judgement that the bridge design did not comply with the bridge design code.

As far as I know capacity reduction analysis including embedded items is never done for a footbridge in this catagory as no one would pay for knowing this information. Like many have said it is far easier to pass the problem down the line of the food chain. The designer, if he is experienced and has a conscience would probably add a note in the formwork drawing asking additional trimmer steel bars around the embedded pipe sleeves and leave the technician preparing the rebar drawing to sort out the rest. I use the word conscience because a good experienced engineer knows his bad arrangement, oversight or exclusion could kill and would try to discharge his duty with intregrity, so alerting others of a problem is the minimum one should do. A good rebar detailer, say in UK, will know the problem already and would attempt to distribute the stress around the sleeves with extra bars. After all a few steel bars cost very little but the attention to details is the quality of the engineering. I am not saying a few extra bars is what needs to save this bridge but it would surely force the rebar detailer talk to the engineer to hammer out a solution for an obviously poor arrangement.

That reminds me of FDOT Tomas Andres who marked up countless warnings on FIGG's initial design drawings about the obvious shortcomings. It was FDOT's responsibility to do peer review so he just offered his experience to a safer design.

There cannot be a better way to understand what is a good and bad rebar detailing just by comparing the condition of 1/2 with 11/12 after the collapse.

1/2 didnot suffer any visible structural damage when viewed from the ground. It was just moved.

11/12 on the other hand had 11, 12 and deck totally separated but the separation could be consequential to the joint connection failure.

RE: Miami Pedestrian Bridge, Part XIV

That quote from WJE brings shame to the engineering world. They could no more eliminate the cracks by re-tensioning than Humpty Dumpty could have been put back together by lifting him back onto the wall.

RE: Miami Pedestrian Bridge, Part XIV

Ingenuity (Structural),

I am one of those who stopped reading the thread after a while when no new information came in and picked it up again so I wasn't aware of WJE's work. I wonder if it is me only having a problem to understand WJE's logic.

The cut and paste WJE's defense suggests :-
(1) WJE had accused no one monitored the cracks. This lets FIGG down.
(2) Structural/VSL own drawings mandated restressing operation to stop if existing cracks widen or new cracks observed.
(3) Evidence showed CJ was not roughened. This could cause existing cracks to widened when restressed and Structural/VSL would have several opportunities to observe.
(4) Structural/VSL could stop the operation if existing cracks widened and collapse could have been avoided.

By (1) I assume no one made any record/note of the cracks, no photo and no video. This also means nobody knew if existing cracks had narrowed or widened or any new cracks developed during the restressing. By (3) WJE now tells everybody the CJ was not roughened and the existing cracks could only widen and not close if restressed.

Since there was no crack monitoring, any restress would widen the cracks and the agreement was to halt restress if cracks widen so what was the purpose instructing VSL to restress? Was FIGG intentionally getting VSL into a trap knowing VSL was unlikely to carry out monitoring so it would not know the cracks widening and continue to destroy the bridge?

SInce there was no crack monitoring during restressing so no new crack information was available. WJE's evidence that the CJ wasn't roughened must be the same information that every contractural party had before VSL did the restressing. What was the evidence convinced FIGG/WJE that the CJ was not roughened and a restress would only widen the existing cracks? Why such information wasn't communicated to the contratual parties to stop the restressing?

LionelHutz (Electrical)

Spot on!

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)3 Jun 20 14:48)

The cut and paste WJE's defense suggests :-
(1) WJE had accused no one monitored the cracks. This lets FIGG down.
(2) Structural/VSL own drawings mandated restressing operation to stop if existing cracks widen or new cracks observed.
(3) Evidence showed CJ was not roughened. This could cause existing cracks to widened when restressed and Structural/VSL would have several opportunities to observe.
(4) Structural/VSL could stop the operation if existing cracks widened and collapse could have been avoided.


1. A supervisor was on the deck during the retensioning. Sadly he sustained brain damage during the collapse.

Quote (TheGreenLama (Structural)15 Oct 19 15:45)

The WJE report that you noted (p. 151, https://dms.ntsb.gov/public/62500-62999/62821/6285...) seems to be coming at the collapse from several different angles all at once. ...

This report shows they did not test the structure of the end of the deck/member-12/diaphragm

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

3DDave (Aerospace)3 Jun 20 01:05 SAYS:
"No PT bars were sheared at the construction joint, at least not until the portion of the deck was cut loose for examination."

Hi 3DDave. What is the inference that we can draw from this fact?

Many thanks for your helpful input.

RE: Miami Pedestrian Bridge, Part XIV

40 - if you mean that the embedded end of the bar was still in place - no kidding.

It's because you left out the qualifier - PT NUT remained in place. Sorry that you mistyped that, and depended on what you believed you had typed instead of what was actually put into words.

The bar did not remain embedded in concrete.

Here's what I think you heard in your head when typing:

"They simply ended in 'space' embedded in concrete. Thus, they the embedded ends of the PT bars moved intact, together with the concrete surrounding them."

In fact the entire upper bar remained in place and only the one end of the lower bar was embedded in concrete. The tension nut was not ever embedded.

RE: Miami Pedestrian Bridge, Part XIV

Sorry if I have misled others on the lower PT rod sheared off at the CJ as I quoted off directly from OSHA Fig 63. 3DDave is correct. The lower PT rod was firmly attached to the deck during and after the collapse. The embedded anchor plate is clearly shown in OSHA Fig 64 to 70 inclusively although I only depicted Fig 63 in my earlier 2 Jun 20 01:24 post.

Here was the scene at the CJ showing the exposed PT rod and how it rip out from the Member 11, severed the links on it way out while the links cut the protecting plastic conduit of the PT rod. The breaking up of the lower section of Member 11, multiple cuts of the conduit and severance of the shear links were only possible if the lower PT rod was intact and firmly anchored during the collapse. Thus OSHA Fig 63 without the presence of the lower PT rod at the CJ must be the result of the post-colapse cutting by the workmen.



Initially I was intrigued by multiple cuts of the plastic conduit. Only after looking at how the PT rod left Member 11 could I put the two together and realised the cuts were done by the shear links.

RE: Miami Pedestrian Bridge, Part XIV

3DDave (Aerospace)3 Jun 20 01:05 SAYS:
"No PT bars were sheared at the construction joint, at least not until the portion of the deck was cut loose for examination."

Hi 3DDave. What is the inference that we can draw from this fact?

Many thanks for your helpful input.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Steve)

Don't know about USA, but I've been told by three employers here that the insurer might walk away if you admit any fault without the insurer's prior approval.

Yes. Same as car accidents. Your insurer carries your liability, so your mistakes are now theirs to admit to, not yours. That’s the deal.

Quote (Saikee)

Since there was no crack monitoring

I think this is part of the general defense -Look at this shonky contractor! Not even monitoring the cracks! No wonder the bridge fell down!

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural) 1 Jun 20 19:18)

According to OSHA report 11 has no structural crack at the CJ prior to the bridge removal from the casting yard.

Don't believe everything you read.

RE: Miami Pedestrian Bridge, Part XIV

Sym P. le (Mechanical)

A structural guy working with reinforced concrete may see cracks differently from a mechanical guy.

When an engineer declares structural cracks the remedial work will have to be structural repair able to restore the original structural integrity of the structure, Otherwise the repair is just cosmetic.

Your Fig 42 is the same as OSHA Fig 18 which came from a group of photo Fig. 17 to 20 inclusively from the OSHA report depicting the general condition around the CJ.

When the bridge bottpm was fully supported in the casting yard the application of PT rod stress in Member 11 would shortened the disgonal member slightly as both concrete and rebar have elastic modulus. The cracking in your Fig 42 is a consequence of it. At that condition the bridge structural integrity had not been compromised. That would be my view from a retired engineer who designed, accepted , rejected, modified, strengthened and repaired reinforced concrete structures. Legally say at a court of law the reciver of this bridge at that condition would not be able to reject the work but may be able to demand sealing of the cracks say using epoxy resin injection.

The "structural" cracks I was talking about can be seen in OSHA Fig 32 to 38 inclusively. OSHA Fig 35 is a direct comparison of your Fig 42 but after the bridge had been placed in its final position and the PT rod stresses in 11 removed. As the CJ at that time shown a complete separation and a visible 11/12 bodily displacement of about 1/2" (OSHA Fig 40) relative to the deck. At this point the bridge could be considered damaged structurally and would no longer be servieable. Additionally the 4" crack depth in the deck showed by OSHA Fig. 30 and the 7" crack depth in diagonal 11 in OSHA Fig. 39 confirmed the bridge was beyond repair and must be partially demonlished, re-constructed, modified and strengthened if it were to be serviceable again.

Any engineer who has worked with reinforced concrete for say 20 years would know the bridge was gone just looking at the OSHA photos mentioned above.

RE: Miami Pedestrian Bridge, Part XIV

saikee119, thank-you for your considered response to my short post and for clarifying the perspective of the language. It is true that I have a different approach to this issue and cannot match your (and others) experience nor your responsibilities. As such I have tried to understand the dynamics of the failure free of the encumbrance of codes and calculations. Given the unusual design application and the eventual outcome, I struggle to view the initial CJ and associated cracks as the usual affair but rather an initiation of the larger failure. I can understand that the initial CJ and filet crack presentation, pre-relocation, may not have raised extraordinary concern, however the longitudinal cracks along 11, extending from the CJ, give pause.

With the benefit of hindsight and with the mindset of critical analysis to understand the failure, I do not agree that the initial CJ crack represents a shortening of member 11 or for that matter, the slab. The photo was taken after concerns were raised when loud cracking noises were heard as the shoring was removed. Again, with hindsight, the structure was already straining, unexpectedly, under its own weight. Cracks due to shortening would and did occur prior to shoring removal.

RE: Miami Pedestrian Bridge, Part XIV

I disagree with saikee19 on that crack. In my opinion, the structure had failed at that point. And rather than shortening of member 11 because of applied force, the crack was due to shortening of the deck due to drying shrinkage and PT.

RE: Miami Pedestrian Bridge, Part XIV

I had a look back at some of the earlier posts and this slip point was noted pretty early on.

The key issue which I can't find is any pictures of what was sticking out of the deck slab before the pour of the no 11 column. But from the picture above at 00.35 3rd June the only thing seemingly holding this joint together is that weedy looking green cage. Mention before has been made that they have no where near the amount of steel you need for the forces on member 11 and in fact it was only the PT rods which were really holding it together. Loosen them as they did and it literally fell apart and couldn't be recovered.

As soon as you got significant movement on that joint surely these bars would have sheared off?

The really criminal part for me was the action of tightening them up without really understanding what was happening / could happen.

This was the real straw which broke the bridge, but it may well have collapsed a bit later anyway.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

hokie66 (Structural)

All concrete structures crack. It is a fact of life. What matters is how severe and where.

I have been interested in looking out for concrete that has received the best quality control and attention to details. Top of my list are the Charle De Gaulle airport in France and Oslo airport in Norway where bare concrete is part of the architectural features. Yet you can find cracks there. Some have been carefully patched up and some were left unattended but the amount is among the minimum you could find.

I shall explain the crack not structural in Sym P. le (Mechanical) 4 Jun 20 22:36 Fig. 42 which is depicted again below for quick reference. The bridge was still in the casting yard and after PT rod stresses applied.



The same crack changed in nature and severity after the bridge had been installed at its final position and on completion of the release of the PT rod stresses in Member 11.



That crack width was measured suggesting the one side had separated from the other side by approximately 1/2". I ignored the >1" width at the surface as it is the crack's interior that should be of interest to us.



If one looks at the design drawing B47, especially Section AA, one may be able to pick up information why the same crack changed by so much.



If PT rod tensions were applied while the bridge was in the casting yard the Member 11 would be axially compressed. The compression would also squeeze the deck axially because the PT rods anchors wanted to move from left to right. The deck was not taking load at that time.

When the bridge was placed at its final position the deck would be forced to take its design dead load and the member 11 would have one of the highest compressions in the structure. Member 11 could only develop the expected high compression if the deck provided the necessary reaction from which the deck must be stretched to develop tension to partly neutralise the previous compression from the PT rod stresses. By releasing PT rod stresses Member 11 anchores were no longer compressing the deck but wanted to deflected in the direction exactly opposite to what was before.

Thus in the MCM Fig. 42 the Member 11 was compressing the deck inward while in OSHA Fig 35 the Member 11 was stretching the deck outward. It should be obvious that the deck has a much greater scope in resisting Member 11 in compression, by buckling, than by tension of letting Member 11 to depart from the end connection. Thus the same crack in MCM Fig 42 and OSHA Fig 35 are different in nature. I for one would not lose sleep for minor concrete cracks permanently compression zone.

What stopping Member 11 moving outward and way from the end of the structure was its strcutual connection with the deck. Looking at Section AA many structural engineers would drop their jaws because there were no serious restraint availabe to prevent the whole thing from sliding out along top of the 8" drain pipe!

If we look at the rebar provision in drawing B47 there were some hefty 8S01, 8S02, 8S03 and 8S04 (1" diameter) at the rear of the deck but due to the physical obstruction of the 8" drain 8S03 and 8S04 were discontinued at the middle and turned 90 degree to avoid the drain pipe. OSHA Fig 62 shows all 8S01 to 8S04 inclusively intact after the failure but one side of 8S04 seems to have cast in the wrong position. The 8S01 is the lowest horizontal bar in OSHA Fig.62 below, then upward it is 8S02, 8S03 and 8S04. 8S03 was seen embedded between 8S01 and 8S02 on the right side. 8S01 to 8S04 were exposed after the surface layer of the concrete came off with 11/12. My point is had similar size horizonatal rebar were able to place above the 8" drain pipe the chance of a failure could have been significantly reduced.



Design drawing B47 Section AA alone can tell us why and how the bridge failed.

In horizontal direction the only reinforcement to stop 11/12 from sliding out of the deck were just two 4S01 (1/2" diameter). One snapped at the middle and evident in OSHA Fig 63 I post previously. In the vertical direction the designer did provide a modest amout of large diameter rebar but their effectiveness was severely compromised by the presence of the 4 No. of 4" vertical sleeves and the 8" drain which destroyed bondable sections of the reinforcement. In other word even if adequate development lengths had been provided in the vertival steel some sections of the concrete were weakened, too slender and were unable to distribute the full material stress. OSHA Fig 61 to 71 inclusively confirmed none of the vertical reinforcement had failed. They were just stripped off the concrete after failure.

RE: Miami Pedestrian Bridge, Part XIV

saikee,

I don't need a lecture about cracking. Have seen all too many.

But my opinion stands. The crack while still in the casting yard should have been investigated further, as it showed separation between the flange and web.

The design was fatally flawed, and hopefully will result in concrete trusses never seeing the light of day again.

RE: Miami Pedestrian Bridge, Part XIV

There are other concrete trusses (although they are rare) and they sometimes make sense. It is just that they have to be designed properly. Any material used can fail.

RE: Miami Pedestrian Bridge, Part XIV

Hokie66 concluded that - "The design was fatally flawed, and hopefully will result in concrete trusses never seeing the light of day again."

Hokie, I believe the correct conclusion would be that, "concrete trusses with fatal flaws should never see the light of day again." Concrete trusses have been used endlessly in civil structures, and will continue to be so used.

RE: Miami Pedestrian Bridge, Part XIV

I agree with Hokie66 that it’s the same crack. It simply opened further when it experienced serious loads. I don’t understand the distinction between 11 pushing and the deck pulling either. They’re one and the same.

I agree with saikee too that most of us dismiss cracks of that magnitude as “non structural”. I wouldn’t be prepared to condemn an engineer for dismissing initial cracking of that magnitude. We have the benefit of hindsight. They didn’t.



RE: Miami Pedestrian Bridge, Part XIV

FortyYearsExperience,

There have been a few, but "endlessly" is stretching it. I have never seen one in person, and have more than forty years experience.

RE: Miami Pedestrian Bridge, Part XIV

hokie66, I walk past this building every day. Ever been to Chicago? These concrete trusses can be found in just about every modern city. Sometimes they are concealed within the outer cladding of the building. Concrete trusses are well understood, and endlessly used.

RE: Miami Pedestrian Bridge, Part XIV

Poor example. Those are structural steel.

RE: Miami Pedestrian Bridge, Part XIV

hokie66 -- been in the game for more than 40 years, but never seen these ?


RE: Miami Pedestrian Bridge, Part XIV

3DDave -- and the other three concrete trusses ?

RE: Miami Pedestrian Bridge, Part XIV

And no, no matter how many examples you find on the internet, I haven't seen any of them.

With the exception of the picture at the bottom left, which may or may not be a component in an intended bridge, I think the others are steel. But if you can provide some documentation otherwise, I am interested. Early on in these discussion of the Miami footbridge, I did some googling as you have done, and didn't find much. There was one example of a concrete truss bridge in Europe, maybe Germany, and it was very bulky in appearance.

RE: Miami Pedestrian Bridge, Part XIV

3
The miami bridge was a concrete truss in the middle of two decks complete with a mismash and total mess of tubes, PT rods and a construction system which had cold joints all over the place. I'm sure those trusses, if they are actually concrete trusses, were made in a formwork laid on its side to avoid cold joints.

A pure concrete truss with decks resting on it may have been much better. It is likely the end re bar would have been tied back into the lower flange with something more than the equivalent of chicken wire or with PT rods running the full length of the truss properly capturing the forces from the lateral member.

It's not the truss per se which was at fault here, but the way the whole bottom deck, top canopy and intermediate asymmetrical struts were put together complete with all sorts of weak points. Member 11 just didn't have enough reinforcement to attach it to the bottom deck and blew out. Poor design.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

2
LittleInch nailed it, especially his remarks about building concrete trusses on their sides.

A truss needs three things: compression, tension, and joints. Concrete is great in compression, useless in tension so requiring reinforcement, and tedious at best in joint design.

This was not just a truss, it was a truss/frame where the frame action was not addressed, and it had dissimilar chords without a rational way of getting the forces transferred between chords and webs. Differential volume change due to the construction sequence was the initiator of cracking. Altogether, an abject failure in design concept as well as execution. In addition, as a novel concept, it should have been tested. They had ample opportunity while it was still on the ground to load test it, probably using water bladders or sand bags as the test load.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural) 5 Jun 20 12:23)

What stopping Member 11 moving outward and way from the end of the structure was its strcutual connection with the deck.

My argument has long been that the connection of Member 11 with the deck was rendered irrelevant by the faulty design. Hence, Member 12 became the constraint preventing 11 from moving north. The meager rebar passing from the deck through 12 only serves to tear at the deck adding to the illusion of punch out. The lower PT rod becomes the mechanism of failure but is also the final snag holding the structure together until it tears out of 11. As I've stated previously, the lower PT bar initially held the structure together but once the tension was released, the structure relaxed and snagged. Adding load to the structure only exaggerated the deflection to its demise.

Importantly, the sleeve around the PT rod would limit the relative movement of 11 to the deck to approx. 1/2 inch. Further movement is allowed by the tearing of 11.

We have not seen images of the initial crack at 12 while the structure was still in the yard.

Consider the following diagrams:




Also, the deck sag is greater along the center longitudinal axis than along the deck perimeter as is evidenced by the longitudinal crack along the deck underside in the drain cut-out.


Bridge Factors Photo 71 – View of crack in cut-out for drain pipe under deck on 3/12/18 at 10:20
a.m. (Source: Munilla Construction Management, MCM)

RE: Miami Pedestrian Bridge, Part XIV

Also, the Shear Plane Fallacy

RE: Miami Pedestrian Bridge, Part XIV

Sym P. le (Mechanical),

To me Member 12 wasn't an impenetrable fortress against the translation shear against Member 11. In fact it was almost like a lump of concrete attached to Member 11 that can be moved bodily with 11. That was the heart of the whole problem. If we temporarily ignore the contribution from the joint then structurally the only resistance of 12 against 11's horizontal shear is by bending its connection at its top with the canopy. Since 12 is about 18' high and the canopy is only 1' thick the rotational resistance at the canopy connection would be small even when there is a large displacement at the bottom of 12.

Your indication of the CJ sheared off along a horizontal plane, part red and part green, in your last sketch is not the path of the least resistance. The OSHA Fig. 58 to 70 inclusively show the shearing path was stepped. There was no concrete left on top of the 8" drain pipe in every one of the OSHA Figures. This substantiated after the CJ the failure plane had changed level to the top of the drain. The NTSB's version of the failure path is depicted by its Fig. 32 which is enclosed below.



In a way it was the combination of the 4 No. of 4" pipe sleeves and one 8" drain pipe that broke the back of the camel. These embedded items apart from disallowing the necessary rebar to be placed, especially in the horizontal direction where it was needed most, they created a weak pocket for 11/12 to slide out once the reinforcement in the A-B-C part of the CJ had sheared off.

In NTSB Fig. 32 the part C-D would be in tension and in design offered zero resistance from concrete. If you scrape the barrel the concrete tensile resistance is around 0.5 to 2 N/mm2 which is insignificant. I believe there were only 4 No. #4 bars, availoable to resist tension, from the deck longitudinal reinforcement which again not much a fight against the huge shearing force from the Member 11.

The last defence against the shearing from diagonal 11 is the shearing capacity along part D-E-F plus the two vertical shearing faces indicated by hatched areas C-D-E-H-C, on each side of Member 12. Unfortunately the end of the deck has 12 No. of end anachors from the longitutinal strand tendons. They prevented any substantial rebar to be inserted into the hatched areas but the drawing B47 did show a couple of #4 bar. Part D-E-F has a decent amount of Vertical reinfrecment available to withstand the horozontal shear but unfortnately none of them failed in shear but by bonding accaording to the OSHA figures.

RE: Miami Pedestrian Bridge, Part XIV

Thanks saikee, I'll post a response in a few days when I get home.

RE: Miami Pedestrian Bridge, Part XIV

saikee, Thank-you again for your comments. I believe you have given an excellent summary of the issues encumbering the 11/12/"deck" node though you have understated the issue of the blue electrical conduit on either side of 12. These conduit are equivalent to an additional pipe sleeve on each side of 12. As such, I have considered these surfaces as contributing "zero" to the strength of the structure. As most agree, this node is the fundamental flaw in the structure.

In my stick diagram above, I attempted to show relative movements in the nodal region and also proposed an offset displacement of 11. My reasoning for this is that as 12 bows, the base of 11 tips upward, and as the bridge sags, the top of 11 tips downward. My depiction of the movement of the top of 12 and both the canopy and deck deflection is greatly exaggerated. A one inch relative motion between the deck and 11 would not result in the same movement elsewhere in the structure, rather, significantly less.

It seems to me that 11 is ill equipped to support the loads imposed on it both in its intended role and far less in its effective role. There is no question that the base of 11 is being torn apart. I suggest the following image, the significance of which has been overlooked, reveals that the failure of 11 was the "next step" in the collapse. The upper PT Rod has yielded at the failure point in 11 which indicates that its base was still firmly positioned in 12 as 11 sheared. The failure cascaded from that point as 11 pancaked while being guided along the lower PT Rod. Note that the lower PT Rod was free to extend out of the blister cap while the upper PT Rod was not.


Evidence Testing and Results - Adrienne Lamm


(epoxybot (Structural) 18 Mar 18 17:25)

As I reviewed the dash cam video, I noted that prior to 12 descending, the 10/11/canopy node dropped significantly while the deck had also rotated significantly. This suggests that the slab rotated off of the base of 12 and I've previously noted evidence of torsional failure in the diaphragm. The collapse of 11 would have hammered its own base, the 11/12 node and also the lower portion of 12, increasing the likelihood of the rebar being stripped clean.

The Shear Plane Fallacy as I posted only refutes the suggestion that surface treatment or lack thereof played any role in the collapse. Although weaker shear surfaces are identified, I do not find it plausible that the bulk of concrete and rebar in Member 12 was weaker than the distressed Member 11.

You rightfully raised the issue of language. It has me thinking that when the cracks first appeared as the shoring was removed the question that needed to be asked was "Is the structure fit for moving?" and this should have guided the inspection and review at that time. Figg was well aware that the node was critical and that the structure needed to survive a certain amount of jostling. As such, I think that the cracks were structural issues prior to the move.

(edit: removed blank space)

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

The upper PT Rod has yielded at the failure point in 11

i do not see an indication of failure of the upper PT rod in Member 11.
Is there a better image?
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

I'll post another one tomorrow, your image happens to be directly across the plane of failure. I had to remind myself that any "kink" in the PT Rods should not be there. The lower PT Rod, of course, was "kinked" at the slab from the structure snagging on it.

RE: Miami Pedestrian Bridge, Part XIV

We now all know the 11/12 blew out from the deck. In this failure mode there is no need to stretch either PT rods in 11. There was also no force available to stretch them.

After failure the rigid PT rod, especially the bottom one, was severely bent so one can say the PT rods might have yielded but the yielding is consequential to and not contributory to the bridge failure.

It may difficult for non-structural engineers to appreciate but if the connection failed the first thing happened to a PT rod would be a total stress relaxation, or zero stress, due to a total loss in the anchorage. If a PT rod later yielded it could only be the bending stress from distorting an initially straight 45mm diameter rod. This is totally different from the unform post-tensions in the brdige design.

RE: Miami Pedestrian Bridge, Part XIV

Five different vantage points of the upper PT rod in Member 11:









RE: Miami Pedestrian Bridge, Part XIV

saikee, in response to your post, I submit the following:

What we should all appreciate is that the 11/12 node detached from the deck. The structure accommodated with various deformations and held (though momentarily) until shortly after load was reapplied to the two PT rods in Member 11. Whereas the design intended this node to be inclusive of the deck, it was not.

The detachment of the 11/12 node resulted in the lower anchorages of the PT rods having competing loyalties to the detriment of Member 11.

If Member 11 failed due to excessive load and offset ends (by deformation), the PT rods would unload their energy into the collapsed member (I believe the rods are stretched about 1.4") and then be vulnerable to deformation as the member crushed further. The different deformations of the upper and lower PT rods are consistent with this failure scenario. In fact, had Member 12(edit) the upper PT rod not deformed in this manner, it would disprove my theory.

If Member 11 pushed through Member 12, the lower PT rod would tear out of the bottom of Member 11 but Member 12(edit) the upper PT rod would be protected, at least initially.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley (Structural) 11 Jun 20 02:42)

i do not see an indication of failure of the upper PT rod in Member 11.

Interestingly, your posted image goes more to support my theory than a multitude of alternative perspectives. Your image indicates that the two deformations are in the same plane which is more indicative of a single assault than multiple assaults which might have occurred with the collapsing structure.

RE: Miami Pedestrian Bridge, Part XIV

2
A picture is worth a lot of words.
Is this the area which as supposed to be roughened?


The KISS principle applied to the failure.
Member 12; Negligible contribution to the structural strength.
Member 11; In compression.
The connection between member 11 and the deck; all of the calculations and speculation may be reduced to:
The connection will fail when the force on the connection exceeds X.
Member 11 is in compression and exerts a force of Y on the connection.
The upper PT rod is anchored such that it acts to add to the compressive force on member 11 but does not add to the force on the connection.
The lower PT rod is anchored in the deck.
Call the force on the lower PT rod Z.
Tension in the lower PT rod adds to the force on the connection.
When Y + Z exceeded X, the connection kicked out.

I can see the use of the PT rods to support the cantilevered portion of the span during transit, but once in place the rods were mostly superfluous.
Member 11 was under compression and the PT rods served to increase the compressive force, and in the case of the lower rod to increase the force on the connection.
The tension on the lower PT rod acted to open the crack rather than to close it.

All of the arguments and calculations come down to: When Y plus Z exceeded X, the bridge fell.
And it was done intentionally.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

waross,

The technical concept and ideas you are expressing are fundamentally correct. I would just say that it is better to say that the PT increased the concrete compressive stresses in member #11 or increased the shear stresses at the pour joint. I am being picky but the compressive force in the member was not increased due to the PT since the compressive member force is actually the sum of the PT (PT force is negative since it is tension) and the concrete (this is equal to the member force from the frame analysis). It is an important concept to think about the members this way when doing PT design.

Without any external forces, I can tighten a rod in a chunk of concrete and the concrete compressive stresses increase and the PT increases (in tension) but the chunk of concrete takes no more compressive load/force. It's member load or force is still zero. PT is equal and opposite to magnitude of the concrete.

When you think about post tensioned concrete or other residual stresses (tempered glass, wood shrinkage, cooling rolled steel elements etc.) in this way, it makes it less confusing/complicated and that is the only reason that I point it out.

RE: Miami Pedestrian Bridge, Part XIV

Link to Brady Heywood's latest post on this topic: Link

RE: Miami Pedestrian Bridge, Part XIV

Warros, you assert as a proposition:

"Tension in the lower PT rod adds to the force on the connection."

But this is wrong. By definition, the force "on the connection" is a force that is applied BETWEEN #11 and the deck. That means we are looking for a force OUTSIDE #11, that is BETWEEN #11 and the deck. This is the shear force between #11 and the deck. It is commonly agreed that this force was about 1,500 kips (1.5 million pounds.)

Note: tension in the lower PT rod is INSIDE #11. Therefore, it is not outside #11.

The bridge would have failed no matter what the tension in the lower PT rod. i.e. The failure condition of the bridge was independent of the tension force in the lower PT rod. Failure was caused by the fact that there was no tensile steel provided in the deck that could have prevented 1.5 million pounds in #11 from travelling north.

RE: Miami Pedestrian Bridge, Part XIV

Is there some distinction between pure frames where joints are fully moment carrying and pure trusses where the joints cannot carry moment loads; perhaps semi-truss, where the joints will develop moment loads?

It is certain that this is neither designed as a frame, nor is it designed to avoid moment development which clearly caused a variety of cracks that a pure truss could not have.

I mention this as the linked Brady Heywood article calls this a truss; it's form is more of an openwork beam, not much different than if it had been first cast with a solid web that had triangles cut into it, rather than an assemblage of individual parts that normally comprise a frame or a truss. The main difference there is the localized web reinforcement to account for the loss of shear continuity from chopping so much out of the web.

RE: Miami Pedestrian Bridge, Part XIV

Also - Brady Heywood is entirely wrong that parallel truss construction offers redundancy. The I-35W bridge was a parallel truss that lost a connection on one side and completely failed. There is no way for a structure that shares loads across two members to survive the loss of one of those members due to the imposition of torsion loads that won't be supported and the secondary loss of that joining structure (often roadway) that destabilizes the remaining truss.

If there are examples where a truss member failed under load and the structure didn't collapse it would be interesting to see them, but just two doesn't seem to be enough.

RE: Miami Pedestrian Bridge, Part XIV

40YE
The lower PT rod was anchored to the deck below the crack.
After the collapse and the destruction of a lot of concrete, the end of the lower PT rod appears to be still anchored to the deck, despite being ripped almost entirely out of member 11.

The crack is opening in a horizontal direction.
This strongly implies that the concrete had already separated and that the joint was held together by the reinforcement.
Here is the first indication of failure.




Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

3DDave, This is more of a truss. In reality there is no such thing as a pure truss but this is close enough that you can make a reasonable assumption for calculating loads. It is analyzed on a computer so it is almost irrelevant that we call it a truss or a frame because the computer accounts for the moment continuity at the joints. In the days before a computer analysis was so easy, this could be assumed to be a truss. Making a beam analogy is not so good. For beam analogies to work, plan section need to remain plane which is not the case here although the net bending and shear forces will be the same since this is very close to a statically determinant single span structure.

I agree, having trusses on both sides does not make the structure redundant.

RE: Miami Pedestrian Bridge, Part XIV

OK, OK, ?
I'm confused. I thought that because the lower PT rod was anchored to the deck at the bottom, that tension in the rod created compression in the joint, but because the joint was not perpendicular to the PT rod, that a shear force was also applied to the joint? (Since the top of the upper PT rod was not exposed, I assume that it was not Post tensioned.) ((yes I know about assumptions.)

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

Continuation of my post:
Here the crack is opening in a horizontal direction.
Yes, it's being measured diagonally but the movement is horizontal.
If the crack was opening diagonally, the horizontal crack to the right would also be opening.
The actual displacement is greater than the measured amount.


Looking at this sketch, the lower PT rod passes through the plane of failure.
The tension in the PT rod may be resolved into horizontal and a vertical components.
The vertical component adds to the friction of the failure plane but does nothing to close a horizontal crack.
The horizontal component of the PT tension acts to open rather than close the crack.

[quote Earth Pi]Without any external forces, I can tighten a rod in a chunk of concrete and the concrete compressive stresses increase and the PT increases (in tension) but the chunk of concrete takes no more compressive load/force. It's member load or force is still zero. PT is equal and opposite to magnitude of the concrete.(/quote)
The point is that one end of the lower PT rod is anchored outside of member 11 and did exert a force such as to cause the collapse.
Your comment is valid for the upper PT rod, but not for the lower PT rod.

The attempt to capture the node and to close the crack with a force in the opposite direction was particularly ill advised.
Talk about dumb and dumber.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

3DDDave,

In contrast with the previous answer you received to your question, I have always been of the opinion that the failure to recognize frame action was a primary reason for this failure. In steel trusses, you can sometimes get away with ignoring joint moments, due to the inherent ductility of steel. Not so with concrete, and that is why concrete trusses are exceedingly rare and not at all practical. This was a novel structure, and should have been tested before erection. A load test while it was still in the casting yard would have exposed its flaws.

RE: Miami Pedestrian Bridge, Part XIV

3DDDave == "This was a novel structure, and should have . . . etc." Not true.

Trusses and frames have been around since before the middle ages. It is well known that both trusses and frames must have shear reinforcement at each node, where the forces in each element change direction. This particular truss/frame had no shear reinforcement in the deck connecting with the northward force in strut #11. Thus, the northward force in #11 kept moving northward. The rest is all hot air.

RE: Miami Pedestrian Bridge, Part XIV

I suggest that you are all hot air, FYE. Or maybe you have a year experience, repeated 40 times.

Perhaps you can share with us an example of another structure of this type.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Wiki)

A Howe truss is a truss bridge consisting of chords, verticals, and diagonals whose vertical members are in tension and whose diagonal members are in compression. The Howe truss was invented by William Howe in 1840, and was widely used as a bridge in the mid to late 1800s.
Is this part pertinent?
"whose vertical members are in tension and whose diagonal members are in compression"
So they took a design that was patented by a millwright over 175 years ago, pimped it up, failed to understand the basic forces and broke it.
I wonder if the drain, sleeves, and conduit were steel instead of plastic if it may have held together long enough for someone to condemn it without anyone dying.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

Nice point Waross. I quite agree.

Hokie 66 says "Perhaps you can share with us an example of another structure of this type."

Yes, every single truss/frame on the planet is a structure of a similar type. Every single truss/frame on the planet has got shear reinforcing at EACH NODE - because EACH NODE in a truss/frame necessarily connects a force in compression with a force in tension or an opposing force. This Miami truss/frame has got, effectively, zero shear steel at the node connecting the compression force in #11 with a tension force in the deck (north force in #11 with south force in deck). If you assert that it does have such shear reinforcing, please point out where it is.

RE: Miami Pedestrian Bridge, Part XIV

We are talking about a concrete truss/frame here, not steel, cast iron, wrought iron, wood, or aluminum.

Howe, Pratt, and Warren are the most common configurations of trusses, and they are all commonly used in steel trusses.

There have been few, very few, examples of concrete trusses, mostly in Europe. I know of one concrete truss footbridge in California, now abandoned.

RE: Miami Pedestrian Bridge, Part XIV

I think the point here might be that there were no gussets as there would have been in a steel truss.
Also FIGG proposes that it would be like an I beam.They seem to have forgotten that in an I beam the flanges are welded to (or rolled with) the web?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

Hokie66, you say that, "We are talking about a concrete truss/frame here, not steel, cast iron, wrought iron, wood, or aluminum."

Hokie66, are you suggesting that because this truss is formed in concrete it is exempt from the requirement that EVERY truss/frame requires shear reinforcement at the nodes ?

Strut #11 has a load of 1,500,000 pounds pressing north. Can you identify any structure within the truss/frame that balances that force to prevent strut #11 from travelling north ?

RE: Miami Pedestrian Bridge, Part XIV

Quote (SFCharlie (Computer)15 Jun 20 22:19
OK, OK, ?
I'm confused. I thought that because the lower PT rod was anchored to the deck at the bottom, that tension in the rod created compression in the joint, but because the joint was not perpendicular to the PT rod, that a shear force was also applied to the joint? (Since the top of the upper PT rod was not exposed, I assume that it was not Post tensioned.) ((yes I know about assumptions.))

You got it! The lower PT rod added shear force to the joint. Bad call by the EOR.
To your second point - as I recall, both PT rods were re-tensioned, alternating in 50 kip increments.
Kinda scarySo tragic to think of working on a joint having 1500 kips load which was less than 50 kips away from failing and causing the structure to collapse and then intentionally adding that last 50 kip load. And the damn thing killing so many.
(edit - I apologize for being so glib. That was grossly insensitive.)

RE: Miami Pedestrian Bridge, Part XIV

Quote (FortyYearsExperience (Structural)16 Jun 20 03:55

Trusses and frames have been around since before the middle ages. It is well known that both trusses and frames must have shear reinforcement at each node, where the forces in each element change direction.)

While I am not old enough to remember the middle ages, I have been concerned about the interior nodes of this truss/frame structure where PT rods in tension terminate and must transfer connection forces to/from normally reinforced concrete having compressive forces using only the concrete in shear and with nominal if any dedicated containment reinforcing.
Were I to have designed the joints, there would have been generous weldments to firmly anchor PT forces and distribute bearing and rotational eccentricity forces to significant areas of concrete, with the steel providing shear transfers. And I would offer those weldments at nodes 1/2 and 11/12 for connection to your 25 square inches of tension reinforcing.
Together we could bring over 80 years to a project.



RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural) -- I'm glad you agree that this node has no shear reinforcing at all.

RE: Miami Pedestrian Bridge, Part XIV

Quote (waross (Electrical))

I wonder if the drain, sleeves, and conduit were steel instead of plastic if it may have held together long enough for someone to condemn it without anyone dying.

I have thought about this too. I believe if the embedded items were strong enough to distribute the internal stress like a rebar the bridge could have lasted a lot longer. For that to happen the sleeves and pipes would have to be pretty thick and physically structural elements.

When the embedded sleeves, conduits and pipes are plastic with radically different in elasticity to the concrete and rebar the concrete surrounding them would be forced into higher stress due a reduction of area. The areas around these embedded services had formed hinges once the concrete had been crushed internally allowing the rebar a much higher degree of movements not permitted in a serviceable structure. The crushed concrete around the hinged areas would be loosened resulting the structural bonding of the concrete with rebar confined further high up in the elevation until the remaining available bonded length could no longer sustain the structure. In other word because of the embedded items wereable to nullify a section of the bonded length of the rebar the provided original development length had suddenly become inadequate.

In any design in reinforced concrete the fundamental assumption or requirement is there should be no slip between the concrete and the steel reinforcement and the two must have identical strain. Once this breaks down the design fails.

Like I mentioned previously all the rebar in the CJ were sheared off but all the vertical rebar in 12 were intact. None of them failed structurally. The concrete just couldn't grip the bars.

RE: Miami Pedestrian Bridge, Part XIV

Quote (FortyYearsExperience (Structural)16 Jun 20 18:33
Vance Wiley (Structural))

I'm glad you agree that this node has no shear reinforcing at all.

Respectfully I beg to differ. No shear reinforcement is not the same as inadequate shear reinforcement provisions.

With reference to the NTSB Fig 32 posted most recently by Waross 15 Jun 20 23:06 I have established the rebar across the failed interface in the enclose table.



The area CD was in tension so strictly speaking not shear reinforcement but were available to stop the failure.

There were substantial vertical rebar of 2x#11 and 8x#7 across DF. They could have been able to resist the horizontal shear had the bars' development length not compromise by the crushed concrete around the embedded item. In the end these bars failed by bonding.

Apart from the sheared off reinforcement in the CJ (showed in construction drawing B61, B40) nearly all the rebar I quoted in above are visible from the OSHA report. An isometric cut out view prepared by NTSB final report as Fig 17, which I posted on 3 Jun 20 00:35, also shows the extent of the shear reinforcment available.

RE: Miami Pedestrian Bridge, Part XIV

Quote (FortyYearsExperience (Structural)16 Jun 20 18:33
Vance Wiley (Structural) -- I'm glad you agree that this node has no shear reinforcing at all.)

Hold on a minute - " at all" applies to some joints and some parts of joints but there was reinforcing in place that was intended to resist shear at the top of the deck in Node 1/2 and 11/12. Woefully inadequate and poorly detailed, but intended to resist shear. And some incidental reinforcing crossed shear planes in the sides of the blown out block below Node 11/12 but was also inadequate.
I can agree that, in the case of Node 11/12, the results suggest it may as well not have had any shear reinforcing "at all".

RE: Miami Pedestrian Bridge, Part XIV

Rebar anomaly: on the east face of Member 12, 9s01, 9s02 and 9s04 are inside the 7s01 verticals. How's that for a twist??







RE: Miami Pedestrian Bridge, Part XIV

I wouldn't stake my forensic analysis business on regurgitating the NTSB report.

RE: Miami Pedestrian Bridge, Part XIV

I can't imagine how nauseating it was to watch this happen. You've carefully placed the shims and then the structure heaves some more. Pushing, twisting, rotating, and leaning on one corner ... and you don't know why. Can you hear the record needle scratching as someone glibly says "Let's retension the PT bars, that'll work!" Do you think they were too dumb to sweat?

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

Rebar anomaly: on the east face of Member 12, 9s01, 9s02 and 9s04 are inside the 7s01 verticals. How's that for a twist??

The north face of rebar 9s01, 9s02 and 9s04 8s07 can be seen from every OSHA figure depicting the rear of the deck. Their east face can be seen on both south and west sides after 11/12 was blownout as evident in OSHA Fig 60 and 63, most recently posted on 2 Jun 20 01:24 here.

I believe these bars, forming a group of 7 on each side of 11/12, were shear reinforcement for the the longitudinal tendon forces applied to the 2'-3.785" thick deck against the rest of the 4'-3.875" deep edge beam. They were never intended to restrain 11/12 from blowing out.

During the course of bridge failure the two sets of 9s01, 9s02 and 8s07 were suddenly called upon to stop 11/12 from moving out. They could have failed by shearing off each steel cross sectional area if sufficient length of the rebar had been embedded above and below the shearing plane. As it happened the shearing plane was at the level of 8" PVC drain pipe and evidently above this level the embedded length of 9s01, 9s02 and 8s07 were insufficient (because a failure plane was never envisaged there) so the concrete broke off instead of steel bars sheared off. Structiurally speaking the force needed to break the concrete bond off from the rebar here was less than that required to shear off the 4 number of #9 and 4 number of #8 steel bars. The bond had also been severely compromised by the close proximity to the 4 No. of the 4" vertical plastic sleeves.

To me there was no anomaly of 9s01, 9s02 and 8s07. They were just bars designed for other purpose inside the connection that the blown-out surface interfering with. Approximately a length of 45 times the diameter of a rebar is needed to be fully embedded in sound concrete to allow the steel stress to be fully developed. By inspection 9s01 and 9s02 do not have 45*1.125 = 50" above the failure surface.

RE: Miami Pedestrian Bridge, Part XIV

The rebar in Member 12 is indeed asymmetrical, and were it not for the catastrophic failure it wouldn't be much of a big deal. Others noted some time ago that the distress revealed in the structure was also asymmetrical. The 11s03 in the SW corner of Member 12 is also misplaced and asymmetrical to the placement of its equivalent in the SE corner.

This speaks not only to the level of care taken during construction, but also to the attention to detail taken by those who are appointed to review this sorry affair and did not point this out. This is not a figment of my imagination. I am not interested in comparing reality to the theoretical performance expectations of mythical structures.



RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical)17 Jun 20 18:10)

(bottom photo)
Thanks for calling our attention to this photo. It looks like Grand Central Station in the deck below 11's position.
I see curvy black things that weave around the vertical 2by. They look like rebar but are blacker. Also all the blue conduit running side by side seem to leave no room for concrete between them. And whats the smooth black pipe hung with ziptie under the deck floor? I'm mystified.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

What would be the ultimate strength of member 11 under compression.
Would the 1,500,000 lbs plus the added tension of the PT bars be close to the failure point of # 11 under compression?
Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

SFCharlie, I'm not sure what you're looking at.

waross, I believe this has been covered with the conclusion being that it is close. But that would be a theoretical limit below which significant confidence could be placed that failure would not occur. In this situation, the design intentions were not met so the additional question is what parameters should guide the calculations.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

The rebar in Member 12 is indeed asymmetrical, and were it not for the catastrophic failure it wouldn't be much of a big deal. Others noted some time ago that the distress revealed in the structure was also asymmetrical. The 11s03 in the SW corner of Member 12 is also misplaced and asymmetrical to the placement of its equivalent in the SE corner.

This speaks not only to the level of care taken during construction, but also to the attention to detail taken by those who are appointed to review this sorry affair and did not point this out. This is not a figment of my imagination. I am not interested in comparing reality to the theoretical performance expectations of mythical structures.

I wouldn't be too harsh on the quality of workmanship here as it wasn't a contributory factor to the collapse.

From the available photo the rebar of 12 were seen in reasonable format and position before the pour. By comparing them with the heavily distorted final positions after the failure can be misleading and even dangerous because we do not have a 3D image showing the exact embedded position of every bar. The important consideration should be had any bar been omitted, changed size or cast wrongly?

Fixing reinforcement is not an exact science. No matter how clever is the designer's concept we still have to rely on mostly emi-skilled labourers to execute the works of fixing the bars and erecting the formwork. The accuracy they use is about an quarter of an inch if we get lucky.

The main failing of this bridge is the designer has not thought of the congestion from the embedded sleeves, ducts and pipes resulting omission of vital reinforcement to link the 11/12 solidly with the deck. The drawings show a symmetrical design, for drawing office economy, but the site work show a few odd bars lapped at slightly different locations asymmetrically for avoiding services is a fact of life in every reinforced concrete structure.


RE: Miami Pedestrian Bridge, Part XIV

Thanks Sym P. le

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

Quote (SFCharlie (Computer) 17 Jun 20 18:45)

I see curvy black things that weave around the vertical 2by. They look like rebar but are blacker. Also all the blue conduit running side by side seem to leave no room for concrete between them. And whats the smooth black pipe hung with ziptie under the deck floor? I'm mystified.

I think the curvy black things are grout bleed tubes for the PT rods. The smooth black pipe looks like a screed rail to aid pouring the deck.

Is there any redeeming structural value in the Member 11 PT rods by grouting them had the structure been successfully positioned?

Several photos of the work in progress. The top mat of deck rebar has not been installed in the first photo (Photo 8). The final installation of the grout bleed tubes extend upwards in the filet (Photo 15). They probably required a hole to be drilled in the form work (Fig. 30)

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

I would grout the smooth PT rods to protect them and the concrete sections from the elements. And the PT code requires that tendons be grouted, of course. So grouting would express a semblance of conformity and care.
The relaxed rods would have no structural contribution in a compression member. Actually, as the compressiomn members like M11 "creep" and shorten under long term compressive loads, the rod will push on the closure at the top, and probably should have a void provided to maintain a cap or plug at the top.

RE: Miami Pedestrian Bridge, Part XIV

Quote (waross (Electrical))

What would be the ultimate strength of member 11 under compression.
Would the 1,500,000 lbs plus the added tension of the PT bars be close to the failure point of # 11 under compression?

I try to answer your question as I think you could be barking on the wrong tree by thinking the compressive stress played a significant role in this bridge. Speaking from a civil/structural background I would say a reinforced concrete structure rarely fails in compression.


Above is the Figg's calc submitted to NTSB. Most of it was computer print out but some hand calculations were added as sanity check. The design force in 11 was 1664k.


The design compressive stress in concrete can vary slightly with design codes but the main stream ACI318 prescribes 0.85f'c for 4000psi concrete and reduce 0.05 per 1000psi above 4000psi. Thus the charcateristic compressive stress is 0.75f'c fora specified strength 6000psi (see construction drawing B38). 11 has a dimension of 2'-0" by 1'-9" so the compression capcity from the concrete alone should be at least 0.75x6000x21x24/1000 = 2268k or 1012 ton without even consider the contribution from the compression reinfrcement. Since the bridge weighs 950 ton (construction drawing B37) so compression isn't a problem.

After the collapse NTSB cut 8 cores out of the bridge structure. The three cores from the deck have recorded failure stresses between 8580 to 10770 psi which are comfortably above the specified strength of 6000 psi. It is normal for the site concrete to be higher than the specifed strength. If a contractor supplies a poorer mix design that fails below the specified strength he could be asked to demolish the bridge before it leave the casting yard. Due to the standard deviation in the concrete quality control there is usually a modest safety factor available but never included in the design.

Therefore the concrete compression in 11 will not be a significant contributor to the failed bridge both in theory and practise.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley (Structural))

I would grout the smooth PT rods to protect them and the concrete sections from the elements. And the PT code requires that tendons be grouted, of course. So grouting would express a semblance of conformity and care.

Must admit I am not up to date with various design code changes but conduits for post-tension tendons and rods were used to be designed either grouted and ungrouted.

It is possible for long term durability the tendons and rod should be protected against corrosion and it makes sense to have them surrounded and protected by grout.

However there can be occasions where design conditions mandate a change in the tendon or rod tension so leaving the system not grouted is the only way. Over 40 years ago I designed a 2m thick roof experimental hall to house a nuclea fusion JET device. I had to cast the roof in 1m wide beams. Each had to be cast with shear key abutting the other. Each beam was a standard reinforced concrete design in one direction. On completion the other direction was stressed together by PT rods to act as a monolithic slab. This arrangement is for the eventual decomissioning to allow the roof to be demolished in small pieces after it has been soaked with years of radiation. The building is still operational today and I do not think the design codes would or could dictate the grouting arrangement of every post tension tendon or rod.

RE: Miami Pedestrian Bridge, Part XIV

Sorry for switching here.

Consider the following as the explanation for the collapse sequence. It's about the split at the bottom of Member 11.

If the lower tab loses its integrity, the upper tab takes the load and kicks out. Member 11 pancakes guided by the lower PT rod. I still contend that the slab rotates the diaphragm off of the bottom of Member 12.

There is a plethora of cracking all throughout the 11/12/diaphragm from a variety of causes but none of those other causes advanced to the extent of collapsing the structure.









RE: Miami Pedestrian Bridge, Part XIV

Two cracks of significance on the North portion of Member 12, one heavier than the other. The lighter one is at the level of the drain with a horizontal trajectory in keeping with saikee's failure plane.

The heavier one is above the drain and shows some displacement. It seems to follow the trajectory of the upper PT rod and would seem to align with cracking in Member 11.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural))

I must learn to be a bit more specific.
Not sure what the last 15 years have brought, but 20 years ago it was required that PT be grouted in exposed structures and grouting was not required in protected structures, as in a structure with a roof membrane. That seems to make sense. As one architect loved to say "It seemed like a good idea at the time", I guess.
As a bit of history, one practice early in the use of PT in slabs, and from the period of paper wrapped strands to prevent bond and allow post tensioning after the concrete has cured, the system was touted as being waterproof if the compressive stresses were 300 psi or greater. The benefit was parking structures did not need a waterproof membrane, thereby avoiding the associated wear and repair.
Not the best idea - but it did provide a lot of opportunities for repair work. And the paper wrapping served well to maintain moisture in contact with the strands.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

It's about the split at the bottom of Member 11.

My interpretation of the spliting at bottom of Member 11 is a lot simpler than yours.


Before the collapse the inner vertical face of the 2'-0" thick diaphram beam was flush with the inner face of the North pier. When the bridge started to fall it kinked at 9/10 with the deck and 10/11 with the canopy. Thus 9/10/deck and 10/11/canopy were two hinges and the bridge was no longer a structure but a mechanism.

The deck was well designed and didn't break into pieces but was virtual intact after the fall, right? So during the drop of 9/10/deck to the ground the deck's underside or more precisely the underside of 12 would have uplifted from the bearings and momentarily leaned against the southern edge of the North pier.

If the deck deformed to a "V" shape then the physical distance between the two extremities of the deck had to be shorten. This is just the property of a triangle that the sum of two sides is bigger than the third side. We now know at the Southern end the 1/2 of the bridge was resting on the Southern pier after the failure so if there were longitudinal shortening a relatively small amount would have taken place there. At the North end due to the "V" geometry and the underside of 12 was leaning against the edge of the pier there had to be a sliding action at the pier edge as a result from any reduction in the bridge's overall length. In fact the shortening was at least 2'-0" in order to enable the deck plus the diaphram beam to clear off the Northen pier.

Rather unfortunately Member 12 is 10.5" wider than the width of the diaphram beam so at the precise moment when the diaphram was able to clear the pier and commenced the drop Member 12 would be still caught up at top of the Northern pier because it still had 10.5" for sliding out. To me it was the dropping of the heavy deck that rip out the lower PT rod in Member 11 and assisted in further breaking out the 11/12 connection.

RE: Miami Pedestrian Bridge, Part XIV

For some reason I can't edit my last post to add the following diagram so I have to post it in a separate thread.



At the beginning of the collapse the 11/12 support would move inwards due to 9/10 formed a hinge resulting the bridge deflected downards and shortened the span. This is indicated by the first sketch showing the diaphram started to lift off from the bearings. At some point the underside of diaphram would be able to lean against the southern edge of the northern pier. The motion from that point onward would be sliding.

When the bridge kept on deflecting there came a time when the diaphram could clear off the northern pier but the Member 12 couldn't because it has an extra 10.5" to go yet. At this moment the half span weight of the bridge would suddenly supported by the 21" wide Member 12 instead previously by the entire length of the diaphram which is 18'-2" long. The support length was abruptly reduced by a factor of 10. Thus Member 12 would split. The deck was able to drop freely down and rip out the lower PT rod in Member 11. I estimate the lower PT rod could have about 3" concrete from the external surface so it is easier to rip it out sideway than breaking in tension the 1.75" diameter PT rod.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural) 18 Jun 20 17:52)

Rather unfortunately Member 12 is 10.5" wider than the width of the diaphram beam so at the precise moment when the diaphram was able to clear the pier and commenced the drop Member 12 would be still caught up at top of the Northern pier because it still had 10.5" for sliding out. To me it was the dropping of the heavy deck that rip out the lower PT rod in Member 11 and assisted in further breaking out the 11/12 connection.

Refer to my "cut-and-paste mashup" in Part 12 - MikeW7 (Electrical) 22 Jul 19 03:24. It's crude, but the sequence of events seems to follow what was observed in the truck dash-cam video. Image 2 assumes the 11-12-canopy triangle remained relatively intact after the hinging began, in which case the canopy hinge combined with the rigid 12-canopy "L" section would have forced the 11-12 joint off the deck almost immediately and crushed the lower end of 11 in the pinch point where the lower end of 12 overhangs the deck end.

RE: Miami Pedestrian Bridge, Part XIV

[quote Refer to my "cut-and-paste mashup" in Part 12 - MikeW7 (Electrical) 22 Jul 19 03:24.][/quote

Your "cut and paste" is pretty descriptive and definitely helpful. I think it needs a bit of scale, and removing M11 in pic 4 may be deceptive in the case of M12. Specifically, pic 4 shows M12 falling behind the pylon. That begs the question how did it climb back atop the pylon? I suggest M11 was still attached to the bottom of M12 and prevented M12 from dropping behind the pylon. There was 4 feet of the top of the pylon available to catch M12 before the deck started sliding south. This could explain some of the damage to M11 and the missing concrete from M11 and M12. It may also have contributed to the bend seen in the upper PT rod of M11. If the concrete near the base of M11 were damaged, either before initiation of collapse or during, and M12 were falling over the end of the deck or pylon with the lower anchor plate of the upper PT rod still embedded, the 35 kips or so DL in M12, supplemented by the force necessary to fail the canopy at Node 10/11, would bend the upper PT rod of M11, resulting in the bend seen in the upper PT rod in photos of M11 leaning on the pylon. There has been discussion of the upper PT rod being bent due to axial compression, but that seems unlikely to me because the upper end of the rod was free to move (it had just been tensioned for a second time) and the rod was not grouted so was not bonded.
The scale of pic 4 shows the structure having dropped maybe 20 feet (top of truss about level with top of pylon) at Node 9/10 while Diaphragm 2 is still on the pylon. The spreadsheet posted here
https://res.cloudinary.com/engineering-com/image/u...
shows the diaphragm likely slipped off the pylon when Node 9/10 dropped about 11 feet.
The comments of 22 Jul 19 20:46 seem to hold today.
The numbers show that as Node 10/11 passes what was the original elevation of the top of the deck , N10/11 has moved north about 1 foot and the geometry shows diagonal M11 to be about 5 feet longer than the length of the canopy from the top of M12 to Node 10/11. Thus the base of M12 can be pushed about 6 feet northward from its original position. Then as Node 10/11 continues to fall below the top of the pylon it is M11 that restrains M12 and/or drags the bottom of M12 to its final position. M11 experiences a bit of damage in doing so.
Thank you,

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley (Structural) 19 Jun 20 18:26)

Specifically, pic 4 shows M12 falling behind the pylon. That begs the question how did it climb back atop the pylon?
NOTE: I'm using my YouTube video below because it doesn't add any interpolated frames - it's "slowed down" by adding multiple copies of each frame.
  • The truck dash-cam video clearly shows 12 starting to dropping behind the pier at about the 11 second mark when the deck hinge has fallen about 1/3 the way to the street.
  • I can't surmise an exact description of what happened to the lower half of 11, but whatever remained of it was most likely crushed, in or before the deck-12 pinch point, as soon as member 12 slid off the deck.
  • When the "falling man" is visible a couple of frames later the deck hinge has almost hit the street (maybe one foot of daylight is still visible) and the 12-canopy "L" has fallen so far the canopy hinge appears to be resting on the remains of 11. Also note at this time that the deck end appears to still be on the pier.
  • As the north section collapses on impact with the street you can see the deck slide off the pier, the "elbow" of the "L" appears to rotate on top of the pier until 12 is level, then the canopy hinge drags the "L" to it's final resting position with the base of 12 on top of the pier.
  • With the exception of the actual collapse of the north bridge section, these events are what I tried to show in my cut and paste diagrams. The diagrams aren't accurate to the millimeter, but they're "close enough" to understand the torturous journey of member 12 as shown in the dash-cam video.
NOTE: numerous small edits since I first posted this.

RE: Miami Pedestrian Bridge, Part XIV

MikeW7 (Electrical),

Your postulation assumed the hinge at 9/10 formed first, secondly the 11/12/canopy remained relatively intact and thirdly the rigid canopy “L” would have forced 11/12 joint off the deck.

The weakness in the model are :-

(1) The collapse was initiated by the hinge at 9/100.
(2) There was no joint blowout or a need of it.
(3) The deck finally rest against the northern pier and not flatly on the ground. If this wasn’t the intention the model has no explanation how the deck ended as it did.
(4) The canopy “L” was rigid and able to forced 11/12 out of the deck.


Answer to (1) The design of the bridge resembles to an “I” beam with the canopy as the top flange (in compression) and the deck as the bottom flange (in tension). The diagonal members are the equivalent web. In performing this structural duty the bridge’s neutral axis, where the stress is zero, would be somewhere in the middle of it 18’ height. If for whatever reason the web fails, say by the deck could not hold Member 11/12 in position, the support of the whole bridge would passes to the 2’ thick deck as it is the only bit left resting on the two piers. The second moment of area, which is proportional to the cube of structure height, would then be drastically reduced. The height reduction from 18’ to 2’ would be accompanied by a shockingly high stress increase in the order of 729 times. Clearly the deck had no such resisting ability but to buckle or hinge. Thus between the hinge of the deck and a functional loss of the web the former is less likely. Additionally the whole deck has been uniformly reinforced and post-tensioned by tendons without one part made stronger than the other. If the deck structure fails it should be at the point of maximum bending moment near the mid span and not at the end span.

Answer to (2) It was OSHA who used blow-out 10 times in its report. The importance of a blow-out is that it is sudden and most probably the starting point of a major structural collapse. This blow-out occurred when 11/12 was detaching from the diaphragm.

Answer to (3) I have already explained with sketches how the deck dropped flat onto the ground with my post on 19 Jun 20 12:14. This ties in well with geometry computation mentioned in Vance Wiley (Structural)’s post on 19 Jun 20 18:26.

Answer to (4) Member 12 and the canopy are structurally superfluous to this bridge as it will stand if you chop them off and install the canopy post-tension anchors at the next bay. In fact a standard Warren truss does not have them. The easiest way to appreciate their insignificance is at each node the member forces must hold each other in equilibrium. At the top of 12 there was no opposing member. Neither there was any member on the other side of the canopy to balance whatever axial force inside the canopy. Therefore these two members have no significant internal forces. Structurally top of 12 has to support a portion of the canopy’s dead weight while the canopy provides anchorage for the prestressing tendons, in additional to its function as a weather shield.

The canopy “L” was NOT rigid enough to be able to dislodge 11/12 out of the deck.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural))

The canopy “L” was NOT rigid enough to be able to dislodge 11/12 out of the deck.
Member 12 had an interesting ride. From the spreadsheet posted just after your posting of the original "cut and paste" model, the geometry shows that , as the structure fell and rotated about the bearing pad on the south pier, the following happened:
The top of M12 was pushed north about 2 inches as Node 10/11 fell thru a straight line from the south bearing to the top of M12.
At the same time, as the deck failed in bending at Node 9/10 and dropped about the same amount, 3 feet, the top of the deck and diaphragm 2 rotated into the fall, translating the base of M12 south about (3' drop/40' bay*48" = ) 3.6 inches. So now M12 is leaning north 5.6 inches out of plumb while the deck is now sloping south and down 3 feet in 40 feet. That deck slope should indicate M12 would be moving south at the top 3/40 * 16 feet = 14.4 inches.
Rotations, strains, and moments in M12 at the top of the deck are now equivalent to the top of M12 being forced north 20 inches from its original relationship to the deck. The remains of M12 suggest a bending failure with tension on the south face at the bottom of M12. Until it failed, M12 was applying torque on the top corner of the "blow out" block. While M12 possessed stiffness due to its 34.5 inch dimension, it only lapped ontothe deck by 24".
What was the magnitude of that moment at the base of M12? I hope someone with access to some design aids or computer programs can tell us that. I suspect M12 failed early in the collapse, maybe before Node 10/11 dropped 3 feet. We can see some damage at the underside of the canopy and south face of M11 M12.
Also, the damage to M11, in viewing the dash cam video of the collapse in the excellent frame by frame video work ( the creator of which I cannot identify now after 15 minutes of searching so I might give proper credit) posted , I see what appears to be Member 11 folding toward the canopy as the structure drops maybe 10 feet or so. Did M11 fail at Node 10/11 early in the collapse?
Regarding the initial movements of M12, the numbers support the post by SFCharlie (Computer)8 Jul 19 02:15
and his comments on 3 frames from the dash cam "(continues as 12 appears to remain vertical)", "(continues as 12 appears to remain vertical)", and "(Finally 12 tips)".
He also noted an angle change between M10 and M11 early in the sequence.
Thank you,

RE: Miami Pedestrian Bridge, Part XIV

Quote (Brady Heywood "Look at the cracks in the photograph below, which was taken two days before the failure by MCM (at Member 2). Try convincing yourself it’s thinner than your nail and not of concern.")


Hard to watch him criticize FIGG for not checking their work while his work needs checking.
It is Member TWELVE.
Fortunately no one died from his work. And in this case he can make his own deadlines.
He does present it logically, as I see so far. Much more to read.
Edit: Thanks for the link.

RE: Miami Pedestrian Bridge, Part XIV

Sorry, Vance, but Member 11 was the diagonal in question, and where the telltale crack appeared at the lower end.

The Heywood presentation does succinctly tell part of the story of the fatal problems with this structure, and why the road should have been closed.

Edit: On further examination, I see that you were referring to the reference to "Member 2" in that one photo. I am yet to figure our the perspective of that photo. Taken 2 days prior to the collapse? Hard to believe.

RE: Miami Pedestrian Bridge, Part XIV

Quote (hokie66 (Structural)29 Jun 20 20:48)

I am yet to figure our the perspective of that photo.
The same three two blue flex tubes appear in

Quote (Sym P. le (Mechanical)17 Jun 20 18:10)

(bottom photo)
to the left (ultimately north) of the two white pipes that weakened the diaphragm twelve joint allowing the blowout, (edit) so I think it is a close in wide angle shot.

RE: Miami Pedestrian Bridge, Part XIV


Can anyone provide a link to PART 1: BRADY HEYWOOD BLOG please?
I cannot find it in Part 2 nor their web page.
I would like to read it.
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

Very interesting read from the Brady Heywood firm in Australia (thanks to the earlier posters for the links to Part 1 and Part 2). There doesn't seem to a Part 3 post to their blog. Hopefully, they add on to their analysis.

https://www.bradyheywood.com.au/blogs/

RE: Miami Pedestrian Bridge, Part XIV

Some thoughts about PART 1: BRADY HEYWOOD BLOG
Well written and clearly presented.

Quote (In this bridge, reinforcing steel was designed into the members to carry the tensile load – in essence, the steel is now preventing the concrete pulling apart.

In addition to reinforcing steel, the bridge also had post-tensioning tendons included in some of its members.)



I would have went directly to post tensioning here - I cannot think of one piece of reinforcing member in this structure that is primarily in direct tension as a working member and depends on normal reinforcing. As presented, this statement appears to describe the post tensioning as - "additional"?

Quote (The collapse had taken just 429 milliseconds.)

He starts counting time from the first indication of a problem - the blowout, and makes this statement after the picture of the bridge hitting the roadway.
Now if memory serves me from my elementary physics over 60 years ago "d=1/2at^2".
In 429 milliseconds a dropped bowling ball will drop less than 3 feet. Have I lost it? Has something changed that I did not notice?
The structure is about 18 feet above the street. The bowling ball will need about 1.05 seconds to hit the street. I would suspect a bit longer for the structure, because some progressive nature exists and at least minimal ductility. Rotating about the south bearing, the center of the bridge would drop 3 feet in 429 ms, ane the north end would drop only 6 feet.
Help me here.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sean Brady @BradyHeywood (on tweeter))

Jun 29
Yes, there will be two more parts to this article series, then I’ll do a podcast on the whole thing. That’s the plan...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

2
Vance W.,

The north end of the bridge falls faster than 9.81m/s^2. It is not a free falling object. There is a rotational force added by the south pier which makes the north end accelerate faster. That is why the falling person is "slower" than the north end of the south segment. This is not intuitive at all that the north end is actually accelerating faster to the ground due to the south support. The CG of the south bridge segment is falling at a lower acceleration than 9.81 m/s^2. And obviously the south end of the south segment has zero vertical acceleration.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Earth314159 (Structural))

Thank you for the response. Would you do the math for me, please?

RE: Miami Pedestrian Bridge, Part XIV

For a very straight forward example, consider a simply supported rigid horizontal beam, length L, mass per unit length m.
Kinetic energy = KE = 0
Potential energy = PE = 0
Instantaneously remove one of the supports.  Beam begins to rotate downwards.

When it has rotated by an angle R it has rotational velocity Q and
PE = -mgRL2/2
KE = Integral of [m(Qx)2/2] wrt x, from x=0 to x=L.
These two must sum to zero, leading to
Q = sqrt(3gR/L)

If the falling end was accelerating downwards at g, the beam would have
Q = sqrt(2gR/L)
So the free end is accelerating at greater than g.

(Not that this ~20% difference explains the much larger difference that tweaked Vance W's interest.)

RE: Miami Pedestrian Bridge, Part XIV

I reckon though that this blog misses a trick. It implies that the failure is due to concrete being in tension.

The real reason that we all know is that the member in compression, as it should have been, member 11, just wasn't attached to the lower part of the truss - the deck - with sufficient strength / re-reinforcement. The joint area at the deck, 11 & 12 node was not provided with enough steel to be able to connect member 11 to the bottom deck. The drain pipe, those 4 white plastic tubes and a host of other small black tubes plus the PT rods and their plates made it virtually impossible for member 11 to be connected to the deck with enough strength. THAT is the key issue with this concrete truss.

Now it's a bit of what iffery here, but IF those temporary tendons in member 11 hadn't been there would there have been more room for some steel to connect member 11 to the deck? Who knows?, but it won't have helped. Normally you wouldn't need to have 2 PT rods and their plates in such a truss. A bit like why those white plastic tubes were moved from somewhere where they caused no problem right into a highly congested location which again prevented sufficient steel from being included in that high stress area.

One layer of hole in the swiss cheese gone.

Add in that concrete trusses on their own may have worked as you could pour them in one go on their side to avoid all these horrible cold joints is a major downside to concrete trusses built into a slab / deck. Add in the cold joint was at a shallow angle to the compressive load and there's your hole in the swiss cheese appearing again.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Denial (Structural))

(Not that this ~20% difference explains the much larger difference that tweaked Vance W's interest.)
Thank you, DENIAL, for your time and your perception. At this point I do not remember ever doing the calculation you presented. After all, structures as such are not supposed to move. I did have to check a quite intricate mobile having at least 3 cross arms - one main and two suspended - with many hanging spires and about 20 feet in height from point of suspension. The point was to ensure that it could not impact any surfaces under anticipated seismic motions. As you know, even a simple 4 piece pendulum can require a very involved analysis, but the powers-that-be agreed that it could not displace farther than if it were rigid so I went to that limit first.
As a quick check, I normally go to limit conditions - even more so than your rigid and uniform beam assumptions. In my check for the time of fall I used the parameters of a point mass at center of span, weightless and rigid structure, and removed the north support. It seems to me under those conditions the north end would fall at the rate of 2g and the nuances of the problem could only slow the fall. Under these "ideal" conditions the fall distance at the north end becomes just less than 6 feet in 429 milliseconds.
In the actual case at hand, previous calcs found the north end was on the pylon until Node 9/10 dropped about 10 feet. As the deck folded somewhere near Node 9/10 the yielding of the PT in the deck maintained its moment capacity, however inadequate, and effectively slowed the collapse to some extent. The canopy also provided some resistance.
So my takeaway is that this collapse took much longer than 429 milliseconds and even the experts can benefit from a review of their work.
I agree with Charlie's suggestion that an incorrect frame rate was likely used.
Thanks again,

RE: Miami Pedestrian Bridge, Part XIV

I think we're splitting hairs here guys. OK less than half a second is a bit too fast, but one second - who really cares? The point is that is was so fast when it finally went no one had any time to do anything, either the poor buggers under it or the equally poor workers on top of it.

I still think this bridge got so much attention because of that dash cam footage being posted so soon after the collapse.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part XIV

One takeaway from this latest round of hair splitting is that now it's clear that the bridge literally fell out from under the people working on it. They didn't ride it down. It pulled away out from under them at greater than 1g, and they landed in its wreckage.

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural)1 Jul 20 16:51
The times and the state of the bridge are from the NTSB report (pages 11,12) (sheets 30,31 of 152 in the pdf)
Link"Highway Accident Report
Pedestrian Bridge Collapse Over SW 8th Street
Miami, Florida
March 15, 2018" figures 9 and 11

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

In furtherance to my previous posts, I submit the following as the final straw which led to the cascading collapse. I will not backtrack through shortcomings in my previous posts nor reiterate the strengths of my arguments. I do not submit to the fallacy of a concrete blowout.

Member 12 ultimately hinged along the failure plane tracking the upper PT rod. The following image labelled "PHOTO 8: Truss 12, North view cracks" from NTSB's Bridge Factors Attachment 24 – Email from Mr. Rodrigo Isaza of MCM to Mr. Dwight Dempsey of FIGG dated March 12, 2018 is indicative of such failure.

I suspect this explanation will resonate with most.



RE: Miami Pedestrian Bridge, Part XIV

This bridge got my attention because it was brand new and collapsed on eight cars, and we waited days to know the fate of the occupants.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

Quote ([quote LittleInch (Petroleum)1 Jul 20 17:16
I think we're splitting hairs here guys.)

[who really cares?]
You are completely correct - the time it takes to fall makes no difference in the outcome of the collapse or those damaged or lives lost.
But - if the expert makes this mistake in front of a jury and is called out on it he loses credibility and likely his influence in the case.
Now SFCharlie tells me the time is from the NTSB Report? This just keeps getting better. Maybe someone should carefully review the NTSB report. Was the time stamp made by the camera taking the video or assigned during some viewing/editing process? At any rate (oops - no pun intended) the interpretation is easily misleading.
The math presented in the BLOG is correct as to differences in time presented by the NTSB and apologies become due to the BLOG.
I will trust that the author prepares in more detail before testimony than before blogging.
Thanks for the link, SFCharlie.

RE: Miami Pedestrian Bridge, Part XIV

The full frame un-interpolated dashcam video is available on YouTube at Link. The start of the collapse is about 23 seconds in and the on screen time is displayed in seconds (not microseconds). Those of you with download and frame by frame software can have fun. I watched it at 1/4 speed and can get about half second resolution. After one second the bridge has not hit the road.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance W. Thank you for the response. Would you do the math for me, please?)


I just did a FBD with a rigid horizontal stick supported one end. Acceleration varies linearly from zero at one to max at the free end. I get an acceleration of 1.5g at the free end, 0.75g in the middle. The load at the support is 0.25wl. Hopefully I didn't make any mistakes.glasses

RE: Miami Pedestrian Bridge, Part XIV

Despite my better instincts, I downloaded the video, loaded it into VLC player, went to 23 seconds, counted frames per sec of on-screen time (5fps), single stepped forward to ejecta and movement (about 19.0-19.2 on-screen time) and then to deck hits the street (about 20.2 on-screen time)

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

EarthPi.  If you made a mistake, I made the same one.  Taking my workings one step further gives a starting acceleration of 1.5g at the free end.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

I do not submit to the fallacy of a concrete blowout.

In reinforced concrete a blow-out is a loose term. Generally it means when a part of the reinforced concrete under a very stress suddenly fails the local concrete is crushed into small pieces and disintegrated after the collapse.

OSHA is an officially appointed and professionally qualified investigator in the FIU bridge collapse and the word blow-out has been used in the report text five times and in the description figures also five times. NTSB used the word blowout also four times in the text and one time in the figure description. Thus the word blow-out or blowout would be the opinion of an expert witness in court and one I would concur based on having worked all my life with reinforced conceret. Thus I am sure it isn't a fallacy.

Comparing your figure posted on 1 Jul 20 17:57 with OSHA Fig 64 below (or any of Fig 58 to 70) your ultimate failure plane is nowhere near to what had happened on the bridge.



The bottom of 12 failed because the weakest plane, where the stress is the highest when the cross ection is the smallest, is at the 8" PVC drain and not somehwere above it where Member 12 still has a full cross section. The OSHA Fig 58 to 70 inclusively show no concrete above the 8" drain after the collapse.

The elastic mudular ratio between concrete and PVC is about 15:1 so the 8" PVC pipe could take hardy any laod. If both deflect by the same amount the PVC takes only 1/16 of the load when both have the same area. However the overall 8.6" OD PVC drain caused a 41% reduction in the 21" thickness of Member 12. As the drain pipe situated at the middle so the reduced section is split into two 6.2" thick pieces on either side of the PVC drain. Do you still think the 21" thick concrete is easier to break than two 6.2" sections when under the same load?

Lastly I like to re-post NTSB's Fig 32 below as this is the official failure plane concluded by NTSB and it matches all the post collapse photos. Any attempt to re-write the history or the offical verdict should be supported with similar rigorous analysis and investigation. Merely a personal opinion on something else not backed by proof is not a contribution to our understanding of what has really happened.

RE: Miami Pedestrian Bridge, Part XIV

Thanks again Saikee for your discussion. I have a response but it will me take some time to put together.

RE: Miami Pedestrian Bridge, Part XIV

Hi Saikee, much appreciated. But I have a question. Because failure is a three dimensional event, there must also be a vertical failure plane. Your excellent diagram shows only a failure plane in a horizontal plane - red line extending in an out of the page to form a horizontal plane. Can you clarify ? Many thanks.

RE: Miami Pedestrian Bridge, Part XIV

FortyYearsExperience (Structural)

With reference to NTSB Fig 32

Along the South to North direction

The failure section has A-B-C and D-E-F in horizontall direction. A-B-C failed by shear but D-E-F was likely by bond failure as the none of the reinforcement (2x#11+8x#7 on East and then on West face) failed by shear. The vertical plane C-D failed by tension. In reinforced concrete design concrete is assumed no tensile resistance in design and a small amount in analysis.

Along the East to West direction (East being Fig 32 while West is the back side of 11/12)

The deck is physically attached to 11/12 via area C-D-E-H-C but the two 4" dia vertical pipe sleeves destroyed part of the bonding or shearing area. What was left has been indicated by the hatched area and there are two sides both of which failed by shear.

RE: Miami Pedestrian Bridge, Part XIV

Hi Saikee
Sounds interesting, but confusing. Any chance you can provide a plan view of the deck showing what you consider to be the vertical failure planes ? many thanks for your input.

RE: Miami Pedestrian Bridge, Part XIV

FortyYearsExperience (Structural)

A plan view as requested

RE: Miami Pedestrian Bridge, Part XIV

FortyYearsExperience (Structural)

Also a 3D cutout view (originally from NTSB Final Report Fig 16)



Please note area bounded by C-D-E-H is the East face of Member 12 bonding with the deck. It has an identical West face at the back of member 12.

The flaw of the design is that along the East/West direction the longitudinal tendon anchors, shown in purpose, prevent placement of rebar linking Member 12 with the either side of the deck. The two size 4 bars, shown green in BTSB 3D sketch, were the only reinforcement.

In the South/North direction it is also impossible to place large rebar to link Member 12 with the deck because the obstruction by the two PT rod anchor plates.



The devastating effect of the two 4" vertical sleeves, extending to the full depth of 4'-3.875" of the diaphragm, on either side of Member 12 should not be overlooked. Their presences and locations served as a partial isolation joint between the deck and Member 12.

The positions of the two PT rod anchors can also be significant when examining the NTSB Fig. 32. It could be seen while the re-tightening of the lower PT rod could progressively compress the deck against the 11/12. The similar re-tightening of the upper PT rod, on the other hand, could encourage the concrete to debond along interface D-E-F which at the thinest point is only 6.2" wide on either side of the 8" drain pipe.

RE: Miami Pedestrian Bridge, Part XIV

Quote (FortyYearsExperience (Structural)3 Jul 20 23:35
Hi Saikee, much appreciated. But I have a question. Because failure is a three dimensional event, there must also be a vertical failure plane.)

I may be missing the question here, but does there have to be an actual vertical shear plane, in this case? It is my understanding that this structure is primarily a warren truss with some minor frame action due to the joint construction. As such, the vertical load of the truss is delivered to the support by an axial force in the diagonal web member ending over that support. The vertical reaction of the truss is supported by the vertical component of the axial load in that member. In a true truss, and with that diagonal member not being vertical or horizontal, there is a corresponding horizontal component of that axial force. When the joint fails horizontally and no longer resists the horizontal component of the axial force in the diagonal, the vertical component is lost also. Thus a horizontal failure results in a loss of vertical support.
I do not see this failure as being significantly different had there been an intentional slip joint, perhaps in the form of teflon pads, at the surface of the deck with Members 11 and 12 sitting on it and no reinforcing passing thru the joint. There would not have been a vertical shear plane. There would have been failure due to a lack of resistance to the horizontal component of any axial load in Member 11, preventing the development of any axial load in Member 11 EDIT ADD and therefore preventing any vertical component to support the structure.
It would have happened long before transporting and likely during removal of falsework. Actual joint construction, reinforcing, and post tensioning simply prolonged the event until 1:46 PM, March 15, 2018.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)the re-tightening of the lower PT rod could progressively compress the deck against the 11/12.)

Compressing the deck against 11/12 could be a good thing because it could generate additional clamping force for shear friction and thereby increase the joint capacity in shear. But that is not all the lower PT rod does when tightened. While it compresses the deck against 11/12 it does so because its tension force is delivered to member 11 as compression at the top of the canopy and is then delivered by Member 11 to the deck at the 31 degree angle, therefore it has a greater horizontal component in shear than a vertical component to compress. So retensioning the lower PT rod was not a good idea either. The lower PT rod was adding shear faster than it was increasing capacity.
The same thing could be said of the upper PT rod until Member 11 and the joint to Member 12 began to split. Until that point both PT rods were adding shear more than clamping compression, and instead were contributing to the cracks in the deck around Node 11/12 and the pvc sleeves. After member 11 split and the joint began to disintegrate, the failure was underway.
It could seem that the EOR found advantage in the low angle of Member 11 and sought to restrain the failure evident in the deck using the horizontal component of that added/restored PT force. It could maybe have worked if the upper end of Member 11 were fixed to some immovable object, but instead Node 10/11 just reversed the force and pushed back on Node 11/12.
I have yet to distinguish in my mind between whether the splitting of Member 11 happened before or after or during the shear failure of the deck block beneath Members 11 and 12 and between the 4" vertical sleeves and above the 8" drain sleeve.
Thank you for your discussions.

RE: Miami Pedestrian Bridge, Part XIV

Upon closer inspection of what I suggest is the ultimate failure plane through Member 12, I realized that it does not follow the trajectory of the upper PT rod in Member 11 but rather it is the 45 degree shear plane of Member 12, which is relatively fixed at each end but not fixed to the slab. Thus, the collapse occurred when Member 11 ultimately broke the back of Member 12 which was forced to act as a buttress to Member 11.

A trace of the crack in Member 12 from just above the lower slab rebar at the north face of 12 to just below the upper slab rebar at the north face of the deck reveals this possibility.





Edit: To clarify, Member 12 had to allow Member 11 room to move north. The shear plane in 12 was the weakness that was exploited but once enough northward movement had occurred, Member 11 failed just above the nodal block to start the cascade. The falling deck rotated the diaphragm off of the base of 12 while 11 hammered into the node. I believe that this sequence allows for the deformity in the upper PT rod in Member 11 to occur while the lower PT rod kinks at the slab.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

To clarify, Member 12 had to allow Member 11 room to move north.

The above postulation does not appear to be supported by the collaspe photo shown below. It is clear that after collapse the end of 12 was resting on the top of the North pier. Had 12 been allowed 11 to move north by shearing off as claimed then 11 had be further north of 12 but it ended up below and on the south side of the pier.

The second weakness of the postulation of 12 shearing off is that the shearing plane can only take place when 11 was moving but 12 did not. This is impossible because during the collapse a hinge was seen at 10/11 connection with the canopy. Thus the triangle formed by 11, 12 and the canopy was initially rotating as one entity and there was an absence of force available to shear off 12 within itself. Remember once in motion the bridge was a mechanism.

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley (Structural))

While it compresses the deck against 11/12 it does so because its tension force is delivered to member 11 as compression at the top of the canopy and is then delivered by Member 11 to the deck at the 31 degree angle, therefore it has a greater horizontal component in shear than a vertical component to compress.

This can be confusing as I was initially thought it too. But I now believe the stressing inside 11 does not increase the horizontal shear in the system.

The easiest way to explain it is to imagine the 12 is prestressed similarly by a PT rod. While the compressive stress inside 12 is indeed increased the reaction will not be increased and the north pier would not feel anything. Thus by similar argument there should have no increase in the horizontal shear in the 11/12 joint by the PT rod re-stressing simlpy because 11 was inclined at 31 degree.

RE: Miami Pedestrian Bridge, Part XIV

11 was separated from the deck at the time of re-stressing and the lower rod spanned the break. If I wanted to jack 12 out of the deck that's an arrangement that I'd say would work.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural))

I now believe the stressing inside 11 does not increase the horizontal shear in the system.
The easiest way to explain it is to imagine the 12 is prestressed similarly by a PT rod. While the compressive stress inside 12 is indeed increased the reaction will not be increased and the north pier would not feel anything. Thus by similar argument there should have no increase in the horizontal shear in the 11/12 joint by the PT rod re-stressing simlpy because 11 was inclined at 31 degree.

The force from internal PT in Member 12 which you describe would be perpendicular to the deck surface and therefore have no horizontal component. Member 11 is 58 degrees from perpendicular and has a serious horizontal component which must be resolved at the surface of the deck.
Of course the north pier would not feel anything - the reaction of the structure does not increase because the PT forces were internal to the truss and anchored in the deck which is a part of the truss. The PT forces were present in the truss while the truss was transported and erected in place. The pier is not a part of the truss.
l

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural)

How about imagining just 12 on its own but with an arm sticking out at 31 degree. You can stressed this arm with PT rods as much as you want and it would not affect 12.

Another way is to imagine if you have a flat equilateral triangle on the ground each side is like Member 11. By post-tensioning one side internally with PT rod you will not get the remaining two sides stressed to the same level. The post-tensioned side will shorten axially in responding to the PT rod stress but the other two arms will simply bend slightly inward to accommodate the exial shortening of the first side. So the internal post-tensioning is not additive to the summation of forces at a nodal point in the equilibrium calculation. That would be my interpretation.

RE: Miami Pedestrian Bridge, Part XIV

The lower end of the lower rod was not in member 12. It was in the separately cast deck. This is what the FIGG engineers missed.

RE: Miami Pedestrian Bridge, Part XIV

3DDave

FIGG couldn't Cast the lower PT rod any further into Member 12 as it would interfere with the 8" drain pipe. FIGG also can't change the drain pipe position as the drain connects to two spans and datum sensitive for gravity discharge.

On hind sight one should have fought the architect, who most probably would say the new arrangement looks ugly, and route the drain sideway, not penetrating into the diaphragm at all but attached to its external face and do the same for the short span. A few extra meters of PVC drians readily discharge to the river down below would have bought the FIU bridge a new lease of life by extending the low PT rod fully into Member 12.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)5 Jul 20 18:18
Vance Wiley (Structural)

How about imagining just 12 on its own but with an arm sticking out at 31 degree. You can stressed this arm with PT rods as much as you want and it would not affect 12.)


If the arm does not bear on the support below, the same problem exists at the face of the column. In this case the sliding force is vertical. On March 15 the sliding force from the PT was horizontal.
Here is a Member 12 with an arm and prestressing. If the acute angle between the arm and Member 12 is "A" it is my opinion the shear at the dotted "shear Plane" on the side of M12 is Clamp Force (PT) * cosine A.




RE: Miami Pedestrian Bridge, Part XIV

Vance, that's a great diagram and does not in any way represent the bridge in question.

RE: Miami Pedestrian Bridge, Part XIV

saikee, the lower rod was not in 12 at all. The casting of 12 also stopped at the surface of the deck with the same cold joint as 11.

RE: Miami Pedestrian Bridge, Part XIV

Quote (3DDave (Aerospace)6 Jul 20 02:17
Vance, that's a great diagram and does not in any way represent the bridge in question.)

Would neglecting the shear in the "shear plane" make it more relevant?

RE: Miami Pedestrian Bridge, Part XIV

Vance, trace over the diagram I copied from the post by SymP.le and you wlll see there shear plane is between 11 and the deck, not between 11 and 12 and that the lower end of the lower post tension rod is in the deck and not in 12.

RE: Miami Pedestrian Bridge, Part XIV

The force between the top end of the lower PT rod acts to force the 11/12 joint horizontally away from the deck.
The force of the lower PT rod was acting against the deck not against member 11.
The force exerted by the lower PT rod was trying to open the crack rather than pull it together.
The concrete had already sheered and shifted.
Only the re-bar was holding the joint together.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

Quote (3DDave (Aerospace))

Vance, trace over the diagram I copied from the post by SymP.le and you wlll see there shear plane is between 11 and the deck, not between 11 and 12 and that the lower end of the lower post tension rod is in the deck
Oh, I know - I am still chuckling about your response. Thank you for the compliment.
I was responding to " imagining just 12 on its own but with an arm sticking out at 31 degree. You can stressed this arm with PT rods as much as you want and it would not affect 12."
My concern is there seems to be a thought that the PT force does not have anything to do with shear at the top of the deck.
Hopefully I am just misunderstanding the points being made.
Thank you for your comment. I will hand it off to you for further comments.

RE: Miami Pedestrian Bridge, Part XIV

Has this been considered?
Once the crack had opened up, the concrete had failed and the re-bar had started to deform.
Why did the crack stop there and not progress further?
The lower PT rod was acting as a restraint.
It had some freedom of movement inside the sleeve.
When the PT bar was tight against the bottom of the sleeve, the movement ceased.
The PT bar was acting as a restraint.
As the PT rod was tightened further, the force against the bottom of the sleeve in member 11 increased.
The concrete had already fractured and the rebar had deformed and was supplying minimum restraint.
At this point the structure was held together by the lower PT rod by reason of the downward pressure against the sleeve and the lower part of member 11.
I suggest that the first part of the final failure was the PT rod ripping out of the bottom of member 11.
So, the construction joint 11 fails and the 11/12 joint shifts until it is restrained by the lower PT rod and the compromised re-bars.
Then the tension on the lower PT is increased.
The lower PT rips out of the bottom of member 11.
No longer restrained by the lower PT rod, the 11/12 joint blows out away from the deck.
The damage to the ends of members 11 and 12 is subsequent to the failure of the bottom of member 11 and was a result of the failure of the bottom of member 11, not a cause of the failure.
I have not seen any mention of the restraining action of the lower PT rod once the concrete joint had failed.
Forgive me if I have missed something.
Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

The necessary condition for the Lower PT rod to rip out of 11 is thet 11 has to move further north for that to occur. Also there would be some initial deformation of the shear ties in 11 that would delay the onset of the rip so I would wager that the rip occured later in the cascade.

Regarding the shear plane in 12, it is not an epic saber slice from the heavens. It is the expression of the tensile stresses built up from the bending deformation and was to be expected but was never identified. There are many vids of beam shear failure on the net and its easy to see the amount of abuse a beam will take (include deflection) without an epic complete failure. Hence, I wager that 11 lost its ability to support its compressive load first.

In essence, the failure is a complex arrangement of simple, well understood failure mechanisms with a wicked feedback loop.

3DDave, I will gave you a star because I like the diagram.

I can"t compete with Vances artistic abilities. I swear if I drew mouse ears on one of my sketches, some people would never get over it, even if they know the bridge design was mickey mouse.

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural)

I suppose you can create shear at the joint as your 6 Jul 20 01:42 post depicts.

I am think alone the line more like below.

RE: Miami Pedestrian Bridge, Part XIV

My interpretation of the event is quite simple.

The bridge was still standing but with worrying cracks described by the workmen "crack like hell". At this point the two PT rods in 11/12 have been de-stressed as per design.

11/12 was holding and could possibly for able to do so for weeks and not days. I support this statement by pointing out the shift of 11/12 was still relatively small according to the crack measurements at that time. The rebar across the shearing plane, as one shown by NTSB Fig 32 based on the failed bridge evidence, has possibly yielded at a few locations but not broken or snapped.

Personally I think the 11/12 was held in position by the vertical reinforcement inside Member 12 as it is quite substantial with 2x#11 and 8x#7 which if you like were acting as dowels. It is noteworthy to point out none of these vertical reabr has failed in the end. They were just stripped of the concrete an clear indication of bond failure.

In my own opinion the re-stressing of the PT rods was the the last straw that broke the camel's back. First by attempting to pull the cracked 11/12 joint together the restressing woul grind smooth the shearing surface, reverse the bearing concrete stress from back to the front of the rebar, asymmetrically pull the 11/12 shearing face with the deck as only one PT rod was stressed at a time and lastly the upper PT rod could literally pull open the north part of the shearing face due to the eccentricity.



I am highly critical of the proximity of the two 4" vertical flexible sleeves on either side of Member 12 as they are ideal instruments for crack inducements. The substantial verical rebar inside Memeber 12 have the thinest concrete cover next to the vertical sleeves which are almost certainly the initiation location for a blowout. The bond between Member 12 vertical rebar and concrete was forcibly reversed by destressing/restressing at the two points of thinnest cover is a bomb waiting to go off. The bond between concrete and steel must be totally rigid if two were to have same strain so destressing/restressing movements are simply cannot be tolerated. Once the steel and concrete each has its own deflection the bond is gone!

Therefore it is possible a bond failure local to the 2 No. of 4" flexible on either side of Memeber 12 increased the flexibility of the 11/12 hinge with the deck sufficiently to initiate a blowout. The record shows the bridge collapsed "during" the last operation of restressing the bottom PT rod after the upper PT rod had been fully re-stressed.

RE: Miami Pedestrian Bridge, Part XIV

Early on the concrete failed and a crack opened up.
At that point all that was holding the joint together was the rebar.
Given the relative movement of the joint, the rebar must have been compromised.
Why did the joint not fail completely?
After moving over 1/2 inch, why did the movement stop?
The lower PT bar was restraining further movement.
The lower PT bar would have been forced against the bottom of the sleeve.
The lower PT bar was not tight in the sleeve and some movement relative to the sleeve was possible.
The lower PT bar did not act as a restraint until it had taken up all possible movement within the sleeve.
I submit that the initiating blowout was the failure of the bottom of member 11.
When the bottom of member 11 blew out downwards due to the increased force against the restraint of the lower PT bar the 11/12 node was relatively free to move away from the deck.
The rebar had been unable to prevent the original failure or separation and was no compromised.
Without the restraint of the lower PT bar, the rebar provided little restraint as the failure progressed.
Consider:
1. The lower PT bar never failed.
2. The lower PT bar was firmly anchored in the deck below the plane of separation.
3. The PT bar remained anchored in the deck.
4. The 11/12 node could not move away from the deck as long as the lower PT rod was in place.
The first blow out must have been the failure of the bottom part of member 11 as the PT rod started to rip out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

waross (Electrical)

You may not be familiar with the design. The two PT rods are what we normally call "temporary work". It is there temporary to assist the constrcution and could be removed when the bridge is in service.

The bridge in service will have Member 11 permanently in high compression so it makes no sense to stress the PT rods there. However when the bridge was moved from the casting yard to the piers it could only do so by supporting the two "next" inner bays during the transportation so that the two end bays can be launched onto the piers. In this arrangement the end bay is a cantilever and Member 11 will be in tension. For this reason alone FIGG had to stressed the Member 11 for the purpose of transferring the structure from the cast yard to its final position. Naturally once in the final position the PT rods could be de-dtressed as happened on site.

My point is the two PT rods were never a permanent design and can be ignored in the strength assessment of the bridge in performing its structural duties in service.

Also the PT rods are straight bars of 1.75" diameter and they would not have been forced against the bottom of the sleeve before the collapse.

Quote:

Early on the concrete failed and a crack opened up.
At that point all that was holding the joint together was the rebar.
Given the relative movement of the joint, the rebar must have been compromised.
Why did the joint not fail completely?

The 11/12 joint at that time, in the permant position with PT rod stress removed, had probably shifted by 1/2". Such deflection would not necessarily cause a failure. The rebar could be bent or even yielded at some locations with some local concrete crushing confined only to the area around the shearing plane that has shifted.


In the above OSHA Fig 61 there are 2x1.375" plus 8x0.875" vertical reinforcementcast inside Member 12 now exposed between the two D1 tendon anchors. It is my belief that these 10 steel reinforcement were acting as dowels to hold 11/12 in position prior to the collapse.

In engineering we learn from mistakes. One of the most valuable lessons of the FIU bridge is that there was not a single failure from the above 10 vertical steel bars when Member 12 sheared across them completely. There is no better illustraion to show the FIGG's design deficiency in not able to make these bars to do what they were supposed to.

Quote:

1. The lower PT bar never failed.
2. The lower PT bar was firmly anchored in the deck below the plane of separation.
3. The PT bar remained anchored in the deck.
4. The 11/12 node could not move away from the deck as long as the lower PT rod was in place.

I do not have the dimension of the tube casing for the PT rod but my guess it would be around 4" so leaving at least 1" clearance all round the PT rod. In my experience it is highly probable when the concrete sheared by as much as 1" across the duct the Member 11 would have failed or broken already.

The 11/12 finally moved away from the deck. The lower PT rod anchor was still in its designed position with the deck. The upper PT rod was still inside Member 11. Member 11 was ripped open by pulling the lower PT rod against its bottom face where the concrete cover is at its thinnest. You can image between a 1.75" diameter high strength PT rod and a layer of say 3" concrete which one break first.

RE: Miami Pedestrian Bridge, Part XIV

saikee119
You have obviously misunderstood my post.
When the joint failed, why did the 11/12 node move just one inch and then stop?
The tension was relieved from the lower PT rod.
The only function now provided by the lower PT rod was that of a pin, preventing further movement, until the tension was re-applied, generating enough force on the bottom part of member 11 to destroy the bottom part of member 11 and probably cause collateral damage to the upper part of member 11.
Please explain how the 11/12 node could move more than one inch without either breaking, pulling loose from the deck or breaking out of member 11.
The upper PT rod was completely contained within member 11 and played no part in the failure.
The lower PT rod had issues.
It was anchored at one end at the top of member 11.
It was anchored at the other end in the deck.
It crossed the plane of failure.
Tension in the lower PT rod would cause opposing forces between member 11 and the deck in the direction of the failure.
When the joint moved, the lower PT rod was forced against the bottom of the sleeve.
How could it not be so?
The rebars failed to remain embedded in the concrete, while the PT anchor did remain embedded.
Sounds like a failure of the reinforcing design to me.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

waross (Electrical)

I suggest you have a read of WJE's sketches enclosed in FIGG report to see how the 11/12 developed the cracks and why it could remain in place until the day of collapse.

I suggest you take note of the reinfrcement, drawn to scale, along the failure surface.

The only thing you need to watch out in WJE's work, as it was used to fend FIGG, is the assumed cracked line marked in yellow. That we can agree or disagree. The rest is as per documented photos so should be credible.

RE: Miami Pedestrian Bridge, Part XIV

saikee119
Thank you for the considerable time that you have expended relying to me.
Allow me to retract an ill advised sentence in my previous post.

Consider the following possible sequence.
The crack originated in the fab yard, either during or after the tensioning of the lower PT rod.
The joint was broken but substantially held in place by the rebars.
When the bridge was placed the crack opened up approximately 1 inch.
With one inch displacement through the lap joint of the rebars the integrity of the rebars was substantially lost.
Further movement was restrained by the lower PT rod acting as a pin.
I take issue with your estimate of the sleeves as 4 inches in diameter.
I cannot find a spec, but one picture seems to show the sleeve at about 150% of the rod diameter.
The sleeve may have floated upwards against the bottom of the PT rod in the wet concrete.
If the rebar lap joint could not prevent 1 inch of movement it could not have halted further movement.
It would have broken its bond with the concrete during the first movement.
When the crack opened it relieved the tension on the lower PT rod.
When the lower PT rod was retensioned, it forced the joint apart.
As the PT rod was acting as a restraint at that time, that restraint had to be relieved by the bottom of member 11 breaking out.
As far as the fall ripping the lower PT rod out, during the first part of the fall the angle between member 11 and the deck is diminishing, rather than increasing.
This would not have ripped the PT rod out of member 11.
The bottom of member 11 had to relieve the PT rod for the movement of the 11/12 joint to increase beyond the previous 1 inch.

1. Original failure in the fab yard, with movement restrained by the rebars.
2. Second failure when the crack opened up about 1 inch and destroyed the bond and integrity of the rebar lap joints.
3. Final failure when reapplied tension on the lower PT rod simultaneously broke out the bottom of member 11 and pushed out the 11/12 node.
Full length rebars would provide considerable restraint even after a 1 inch relative movement.
Similar movement on the lap joint would act to break the bond of the bars to the concrete and destroy the greater part of their effectiveness.
I believe that you are putting too much faith on the rebar lap joint and are not at all considering the action of the PT bar as a pin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

waross (Electrical)

Quote:

Consider the following possible sequence.
The crack originated in the fab yard, either during or after the tensioning of the lower PT rod.
The joint was broken but substantially held in place by the rebars.
The joint wasn't broken. It had some cracks. Every structure of this size and complexity will have cracks.

Quote:

When the bridge was placed the crack opened up approximately 1 inch.
With one inch displacement through the lap joint of the rebars the integrity of the rebars was substantially lost.
Further movement was restrained by the lower PT rod acting as a pin.
The bridge was placed on piers on 10 Mar. PT rod tensions in 11 was removed within a few hours on the same day. Photos were taken on 12 and 14 Mar. Earlier photos show smaller cracks. Only on 14 Mar the crack was measured 1" on the outside but the interior was about 1/2". I urge you not to treat the PT rod as a structural element beccause within 11 it has no structural duty after destressing

Quote:

I take issue with your estimate of the sleeves as 4 inches in diameter.
I cannot find a spec, but one picture seems to show the sleeve at about 150% of the rod diameter.
The sleeve may have floated upwards against the bottom of the PT rod in the wet concrete.
This is pure speculation. Do you have proof of such bad workmanship? Such accusation is very dangerous if unfounded.

Quote:

If the rebar lap joint could not prevent 1 inch of movement it could not have halted further movement.
It would have broken its bond with the concrete during the first movement.
When the crack opened it relieved the tension on the lower PT rod.

You obviously lack the basic knowledge of reinforced concrete design. We are talking about a rebar perpendicular to a shear plane here. The bar has to be designed with a development length, approximatly 45 times its diameter, below and above the shearing face in order for the steel stress fully developed. A mear 0.5" lateral deflection of the bar inside a locally crushed concrete is not a big deal because rest of the bar can hold the structure if the situation deterioates no more. We are talking say about one diameter high of the steel and crushed concrete shifted by 0.5" but the rest of the length of 89 times the diameter is still soundly gripped by concrete. Here I urge you not to mention the PT rod because the 0.5" shift was the result of destressing. The 11/12 could have nearly negligible or not-measurable cracks prior to the destressing but nobody knows because the stress was removed from 11 within hours it was placed on pier.

Quote:

When the lower PT rod was retensioned, it forced the joint apart.
That is pure speculation again and suggests FIGG didn't know about the bridge more than you. It is obvious to everyone that FIGG was tightening the PT rod to close the cracks and not to force the joint apart. I believe nobody knows whether the cracks were closed up or pulled further apart as the result of the restressing. We only know from OSHA report "They had re-tensioned the upper bar to the desired tension of 280 kips and were at the lower bar at their last cycle to complete 280 kips when the incident occurred."

Quote:

As the PT rod was acting as a restraint at that time, that restraint had to be relieved by the bottom of member 11 breaking out.
As far as the fall ripping the lower PT rod out, during the first part of the fall the angle between member 11 and the deck is diminishing, rather than increasing.
This would not have ripped the PT rod out of member 11.
The bottom of member 11 had to relieve the PT rod for the movement of the 11/12 joint to increase beyond the previous 1 inch.
Can i ask you to read my post on 19 Jun 20 12:14 in which I drew two CAD sketches to explain how the lower PT rod was ripped out?


Quote:

1. Original failure in the fab yard, with movement restrained by the rebars.
2. Second failure when the crack opened up about 1 inch and destroyed the bond and integrity of the rebar lap joints.
3. Final failure when reapplied tension on the lower PT rod simultaneously broke out the bottom of member 11 and pushed out the 11/12 node.
Full length rebars would provide considerable restraint even after a 1 inch relative movement.
Similar movement on the lap joint would act to break the bond of the bars to the concrete and destroy the greater part of their effectiveness.
I believe that you are putting too much faith on the rebar lap joint and are not at all considering the action of the PT bar as a pin.
For (1) Please understand that you cannot use the word "failure" here. As an engineer you should be prepared being on on a stand in the court of law when giving out such opinion. I have said it before contracturally no one could reject MCM's work at the casting yard. You can complain and MCM would just ask a labourer to brush-paint the crack areas with a cement slurry. You can inspect it later and wouldn't find anything. I am not suggesting it is OK for MCM to cheat but the cracks were just one grade above trivial because such cracks are fact of life in the industry especially if the work is stressed in one part and not the other.
For (2) There is no second failure. The 0.5" crack was recorded one day before the collapse. No more photos on it was taken. Like I said previously the bar has 45xdiameter above and below the shearing plane. Your destroyed bond can only about 1 to 2 diameter at the crack interface.
For (3) If you look at any of the OSHA photo on the lower PT rod you should find none of them show broke out the bottom of 11 but attched firmly to the deck. Also whenever bars were provided they were in fully length. The problem here is FIGG cannot insert them at the right places due to congestion. Please be advised suggesting the PT rod useful as a pin is irrelevant. Even if both PT rods could act as pins they were useless against the collapse because none of them broke.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)6 Jul 20 11:44
)


If we remove the parts of M12 beyond the projection of the Arm thru M12, your point becomes clear. I have attempted to show this in Fig 1.


But what happens if there is a severely sloping construction joint somewhere between the two PT anchors?
Or an identifiable plane which could simulate a joint. Perhaps this is a problem with concrete as a material and not something that can be applied universally to all materials.
Were M12 and the Arm cast while laying on their side and and monolithically, then forms stripped and allowed to cure and dry, it would likely crack at the interface with M12 due to shrinkage and drying stresses – concrete always seems to crack at reentrant corners.
Were the Arm cast alone and in one piece and then PT added, I see no reason for a diagonal crack to form so there is not a defined sloping plane for failure. Joining another and different section of concrete may create an opportunity for cracking and may define a plane for consideration. Casting M12 at a different time than casting the Arm clearly creates a joint and a defined plane.
So IF, and it is an “if”, the Arm develops a defined sloping plane, the plane must transfer a component which creates shear at the plane. I tried to show this in Fig 2.


And I tried to illustrate my concern for not having significant axial load across a sloping joint in a compression member using Fig 3.


No one would think of allowing a joint as shown in Fig 3.
Plus – if anyone had seen cracking in a pier at a sloping joint and that in anyway looked like the cracking seen on the deck at Node 11/12 they would have known it was coming down and soon. To see a 1'9” X 2 ft column (same size as M11) supporting one end of a 174 foot 950 ton concrete Albatruss (spelling intended) should have alerted someone. And M11 had much more load – about 1500 kips axial.
As I recall, the ACI code allowed or addressed Shear-Friction design where (paraphrasing here and not complete) different materials interface, where construction joints are used, and where a shear plane can be defined. I doubt that they ever envisioned a condition like M12 with a PT'd Arm.
OK, DDDDave – (note – my decoder ring is working again) now there are three sketches to admire. Better watch your six, Industrial Light and Magic.
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

saikee, I struggle with what it is that you can't see. WJE's imagination is also limited. The whole of your vertical plane CDEH has virtually no connective value, the result being that Member 12 becomes a vertical beam which 11 is pushing against. 12 is connected to the canopy on top and the diaphragm beneath the deck. There is no slice through 12 that sees the top of 12 move north while the bottom stays put. That is not a requirement for the collapse.

12 merely has to bow sufficiently for 11 to fail.

At the same time that the weight of the bridge is pushing moving out with 11, the slab is weighing down and pulling the diaphragm in the opposite direction. Neither force vector passes through the connection at the bottom of the diaphragm.

As waross suggests, the lower PT rod is likely the snag that prevented the bridge from meeting an earlier demise, an otherwise indeterminent structure bent on collapse.

The latest diagram that I posted, (and reposted by 3DDave) shows a movement by 11 of 1 inch, resulting in a rotation of 1.3 degrees. For ease of effort, the slab is held constant, and other fine detail of relative movements and deformations were omitted.

The point being that the movement is subtle but the damage to 11 at this point is already severe while the damage to 12 is not. It begs credulity that 12 would suddenly become the critical member.

Beyond this, at some point we just have to agree to disagree.

P.S. The various tears in the deck and spalls on the diaphragm are collateral damage and as such are distractions since these components are not critical elements.

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural)

Thanks for your illustration. The sliding joint, as per 11/12 joint, will have shear across the CJ.

Can I have your thought on the action of 11 stressing and its structural duty? Both create comression inside the member and they are additive. I believe We all agree on that.

Would I be correct to say in prestressing Member 11 the 11/12 joint deflects to the South or towards the inner span?

When the bridge was droped on the piers by equilibrium the vertical compenent in Member 11 axial compression (plus self weight of Member 11) would be balanced by the vertical reaction from the pier while the horizontal comonent balanced by the tension in the deck. Member 11 had to act as almost like an arch but the 11/12 joint stoped it from kicking out so its deflection is to the North or away from the span.

Can Member 11 under two compressive cases deflect in opposite direction as I suggested above?

Also in the case of prestressing Member 11 do you expect the deck in compression, tension or no stress?


RE: Miami Pedestrian Bridge, Part XIV

Quote (Paraphrase)

While in the yard cracks were observed.
These cracks were superficial.
Coincidentally, these cracks later became the fracture plane.
Gibbs Rule #39: There is No Such Thing as Coincidence.
Gibbs Rule #51: Sometimes You're Wrong.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)6 Jul 20 20:07)

The only thing you need to watch out in WJE's work, as it was used to fend FIGG, is the assumed cracked line marked in yellow. That we can agree or disagree. The rest is as per documented photos so should be credible.
I'm sorry but "we" don't all see the photos showing any Testing of the deck as related to member 11. Testing member 11 as if it extended to infinity, does not address the complexity of the spaghetti blow of rebar, PVC pipe, hoses or cords, flex blue tube, etc. that visibly contribute to the failure. The diaphragm was cracked before the re-tensioning.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

SFCharlie's (Computer)

OSHA, NTSB and FIGG all offered reports on the FIU bridge collapse. The FIGG enclosed a report by WJE, a factual report by Turner Fiarbank Highway Resrach Centre and the NTSB Material Laboratory Study Report.

You can get a good idea how each describes the failure mechanism.

Like I said earlier WJE report was used to defend FIGG but the sketches, with the exception marked yellow lines stated as estimated cracks, were the crack mappings verifiable by the photos. WJE also show, via Exhibit 2.5.2 to 2.5.4 the broken out sections of 11/12 after the collpase. These are useful observed evidence, which are verifiable. to help our discussions here.

I would also point out the failure surfaces/planes between NTSB and WJE are substantially the same. In WJE case failure line between C-D-E is a striaght line instead of two sides of a right-angle triangle shown below.



I have intervened frequently because many discussions were not related to what had happen in the field.

RE: Miami Pedestrian Bridge, Part XIV

waross (Electrical) & Sym P. le (Mechanical)

After both of you insisted on that the PT rods could have acted as "pins" or "snags" I take another look at the PT rods and rebars in Member 12.

The two PT rods are 1.75" diameter enclosed inside plastic ducts which I estimated to be either 3" or 4". By comparing with the exposed 4" verical sleeves in the phoros the duct is more likely to be 4" overall but it has ribs making the inside diameter smaller. In any case the point is there is free movement inside the duct for which I previously discounted its pining effect.

The vertical reinforcement inside Member 12 that pass through the CJ are 2x#11 (11/8" diameter) and 8x#7 (7/8" diameter). The combined steel area of these reinforcment is 1.6 times more than the 2 PT rod. These reinforcing bars were cast tight inside the concrete so would have to be in play immediately when 11/12 commenced shearing horizontally relative to the deck.

I don't know how much the PT rod play in resisting shear but it has a much yield strength of 120ksi against the rebar's 60 ksi so probaly a corresponding higher shear strength. The only problem is to realize the pin effect the 11/12 must deflect to take up the free slack isnide the duct.

Since the two PT rods, just like all the vertical rebar inside 12, did not sever so there exists the possibility their shearing capacity could have helped to restrain 11/12 joint from failure at least initially.

RE: Miami Pedestrian Bridge, Part XIV

Sorry Vance - I specified they be traced over the existing design. ILM won't be returning your calls.

RE: Miami Pedestrian Bridge, Part XIV

saikee119 (Structural)7 Jul 20 16:24 First, you have my upmost utmost respect. Second, it is my observation that WJE didn't include all the voids (conduits, etc.) that were in member 11. I think they decided what to look for and then created a test to find that (and nothing else). ...just my opinion, more based on my experience with human nature rather than structural engineering.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XIV

I am making one more attempt to sell my interpretation of the collapse sequence as it may help to reduce confusion. It was recorded on video that during collapse hinges were formed at 9/10/deck and 10/11/canopy as indicated below.


No. 1 is the condition before the collpase. No. 2 is the commencement of the collapse.
[Apology: I posted the unannotated sketches by mistake. The Editor does not permit me to change or replace the sketches so please read them in the order they appear]


Since the bridge became a "V" shape it had to pull the two extreme ends inward. Thus allowing the diaphragm to move over the edge of the pier as shown by No. 3 & 4.


The diagragm is 18'-2" long for the first 2'-0" width. Thereafter the length drops to 1'-9", which is the thickness of Member 12, for the remaining 10.5" width. Based on the post collapse evidence it is likely that the deck fell to the ground once it had clear the pier edge. The caught last 10.5" width was split and broken up as indicated by No 5 & 6. sketches.


OSHA Fig 62 shows the sepration of the 1'-9" by 10.5" section managed to remove part of the surface layer of the diaphragm exposing some of the horizontal rebar at the north face.

(by the way if you wonder what is the steel bar at the bottom of the diaphragm under the right D1 my guess is it could be a holding down bolt trapped inside the 4" vertical sleeve)

My interpretation may not the actual event but it is based on photographic evidence. If the part of Member 12 did split as suggested above the concrete would pulverize at the bottom anchor of the upper PT rod due a sudden to release of the high energy stored in the rod. The remain of Member 12 can be seen from the photo I posted in 5 Jul 20 11:52 post. Detail description of its interface plus off-site photos are also available in Exhibit B TFHRC Factual Report "Concrete Interface Under Member 11 and 12" of the FIGG Report.

RE: Miami Pedestrian Bridge, Part XIV

There are hinge failures in the decks but those are secondary. You can't get a hinge failure in the decks until the much stiffer "truss" starts fail in some manner (unless there a localized hinge due to the spanning of the deck between joints which is not the case here). You also need a high force to pulverize the concrete. As soon as you have another failure mechanism (other than the shear friction/punch and pulverize mechanism), most of the load on #11 is relieved and you don't have the force to pulverize the concrete at the lower #11 joint.

The secondary hinge locations as seen on the video are as expected for a shear friction failure of #11 (or any other failure that would relieve load on #11).

RE: Miami Pedestrian Bridge, Part XIV

Quote (Sym P. le (Mechanical))

The whole of your vertical plane CDEH has virtually no connective value, the result being that Member 12 becomes a vertical beam which 11 is pushing against.

There are several pre-existing design arrangements which weaken the connectivity of area CDEH. Amount them are :-

If you look at the area CDEH of NTSB Fig 32, available in 7 Jul 20 16:24 post, you will find the 4" vertical sleeves have taken up more than 50% of the available surface. The imposition of the sleeves displaces the concrete making locally insufficient amount to grip the reinforcement. The thin concrete layers between the sleeves and the rebar could break easily and trigger the bond failure of the whole bar.

The area around the vertical sleeves is congested with reinforcement too making even less concrete to bond the reinforcement. You can see from OSHA Fig, 70 for yourself.


The stress direction inside 11/12 is significantly different from the deck. The deck is uniformly post-tensioned by tendons both longitudinally and trasversely except the plan area of 11/12. Once the deck was post-tensioned the south portion of 11/12 could be influenced by the D1 tendons on both side of Member 12 but the rear of 11/12 was not stressed. Then when the PT rod stresses were applied the strain direction was about 31 degree to the horizontal. Thus area CDEH is the interface of two highly stressed zones each pulling it own direction. This explains why the concrete could pulverize and able to leave the flexible vertical sleeves almost undamaged. The deck section after collapse is also solid around the prestressing tendons zones because the stress is homogeneous there.

Due to the presence of deck's longitudinal tendon anchers, label D1 to D6, on either side of 11/12 it was impossible to place decent size reinforcing bars in the south to north direction through area CDEH to connect Member 12 with the deck.

Finally from the failure suface/plane established by NTSB and WJE it should be obvious that the lower PT rod was able to anchor into the deck and could be difficult to overtighten. The upper PT rod on the other hand seems to pull the 21" thick 11/12 joint mainly and with little participation from the deck. This could be a recipe of disaster because the rod stressing can also inadvertantly exert stress and help to break the CDEH connectivity with the deck.


RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)7 Jul 20 11:02
Vance Wiley (Structural)
Can I have your thought on the action of 11 stressing and its structural duty?)


Your questions evoke issues requiring some thought. Thank you for asking my opinion.

Can I have your thought on the action of 11 stressing and its structural duty? Both create comression inside the member and they are additive. I believe We all agree on that.

Member 11 is highly loaded and in my opinion seriously under reinforced. Adding PT to support the cantilever condition under transport was a good idea. It provided the tension force for support of the cantilever plus kept the Member 11 in compression to prevent cracking. Under full span support conditions, more compression was not a good idea. The south end had sufficient capacity to tolerate the additional PT force without damage. The north end did not. As I recall, the added PT load was less than the design Live Load , and the failure may have created a loading during construction that was very indicative of performance under even more perilous circumstances.
While on the subject of transport conditions, Member 10 is also under a stress reversed condition during transport. Under full span Member 10 is a tension member with tension loads supported by PT rods. Under transport conditions Member 10 is a compression member with the added benefit of PT forces adding compression. As detailed on the construction drawings I think the compression reinforcing is less than the code prescribed minimum.

Would I be correct to say in prestressing Member 11 the 11/12 joint deflects to the South or towards the inner span?

“Deflects” is probably not the best word here – it may not deflect until something fails, and given the size of the deck I doubt the elastic shortening there was significant. The final load on any element with opposing loads will be the result of the resolution of vectors. Prestressing in Member 11 will attempt to move Node 11/12 to the south as long as the joint at the top of the deck is intact because the horizontal component of the PT force is to the south at Node 11/12. Initially both PT rods were anchored in the deck. When the joint at the deck surface failed I see Member 11 and the portion of Node 11/12 above the deck surface being allowed to slide north because of the overwhelming load from truss action. So in effect, the deck will remain in place because of its mass and section properties while the base of M11 at the deck surface wants to go north – both tendencies in response to the major forces in the respective members while the truss remaind intact. When Node 11/12 fails, about 1500 kips of PT in the deck is released (no longer loaded by Member 11), and some elastic shortening will develop in the deck. This will initiate movement of the north end of the deck, but I see this as being very small compared to the dimensional change as Node 9/10 falls.


When the bridge was droped on the piers by equilibrium the vertical compenent in Member 11 axial compression (plus self weight of Member 11) would be balanced by the vertical reaction from the pier while the horizontal comonent balanced by the tension in the deck. Member 11 had to act as almost like an arch but the 11/12 joint stoped it from kicking out so its deflection is to the North or away from the span.

I find that to be a good description of conditions under full span. Member 11 carried an axial load about 50% greater than the end reaction of the entire structure. Member 11 was pushing North at loads large enough to fail itself. When cable news aired this on the afternoon of March 15 and I could see it was a truss I suspected immediately that they had lost a heel joint. That became much more certain in the months after.

Can Member 11 under two compressive cases deflect in opposite direction as I suggested above?

Again the use of the term “deflect” causes uncertainty in my mind. If we focus on elastic shortening or elongation (because it is an axially loaded member) Member 11 was under tension during transport and under compression when placed on the pier. If the concern is that the PT rods placed Member in compression and by being anchored in the bottom of the deck added a horizontal component of force directed to the south, while the structure load in Member 11 attempted to force Node 11/12 northward, it is my opinion that the effect of the PT forces in Member 11 served only to create compressive stress in Member 11 and sliding loads across construction joints. The net effect on the truss was zero because the rods were anchored in the respective ends of Member 11 and had equal and opposing force components. This is like the equilateral triagle you previously postulated – in the case of pinned joints in the triangle, the PT forces change the dimension of one leg, and the angle opposite the PT changes, but the other members are not otherwise affected. While the PT forces in Member 11 were in place and the truss was on its bearing at the pylon, the compression stress was greater and some shortening was experienced for that period. In a true truss that would not cause a lot of issues providing capacities of members and joints were adequate, but in the case of a truss with some continuity/fixity in the joints, there can be resulting secondary stresses and potential damage. I view these as being of lesser significance compared to the disastrous performance of Member 11 and Node 11/12. They no doubt contributed, however.
Your mileage may vary. I appreciate the discussion.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)7 Jul 20 23:02)

Very nice "sketches".
In sketch 5 the deck and diaphragm have slid south until the 2' wide sections of the diaphragm can just begin to fall. The 10.5" extension of Member 12 remains over the pylon. Member 12 and Member 11 are shown intact and in their original relationship to the deck.
At the point shown in sketch 5, Node 11/12 has slid 2 feet from its original location on the deck and has likely taken the 'blow out block' with it. The bottom of that block is defined by the top of the 8" sleeve, and the projection of the deck below the pipe sleeve was easily broken off the deck. But I think the important thing is Members 11 and 12 were severely damaged by their movement across the deck and the angle of Member 11 to the deck should have decreased as Node 10/11 dropped while the end of the deck had not yet cleared the pylon.
So I am of the opinion that Members 11 and 12 were damaged in their lower sections before the deck projection below the pipe sleeve was broken off.
Also the angle of the deck at the instant depicted in sketch 5 is likely a bit flatter than shown, because Node 9/10 is about 40 feet from the end of the structure, and the fall distance is roughly 18 feet. So the top of the deck at impact of Node 9/10 on the street should be less than 30 degrees below level.
Thank you,

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural)

I must admit that my schetches 5 & 6 have not been corroborated with the estimated collpase trajectory worked out by others.

I also do not have information on the order of which part of 11/12 broke off first before the others.

The purpose of my sketches was to show during the diaphragm sliding off the pylon the falling dead weight could help to tear off the rear 10.5" of Member 12 from the diaphragm's body.

In agreement with you it is entirely probable that NTSB reported blowout, to the north of Member 12, took place at the level of the 8" horizontal drain pipe, due to high concentration of stresses, and so above this drain pipe level there was no concrete left to be split from the rear 10.5" of Member 12. Nevertheless the splitting of the rear 10.5" of Member 12, commencing from the bottom upward, would be able to rip out the Member 12 north face vertical reinforcement cast in the first lift of the concrete pour.

RE: Miami Pedestrian Bridge, Part XIV

RE: Miami Pedestrian Bridge, Part XIV

RandomTaskkk (Structural)



My guess is the shearing was probably trying the direction you were suggesting but the progress might have been impeded when the shearing crack hit 7S03 & 11S03 bars. Therefater the shear changed to splitting.



On the possible extra damage to the pylon and the rear of the diaphragm my explaination is as follow:-

On the pylon we have no information of damage. Secondly the bearing area of the pylon has been designed to take half of everything the bridge got so it will not shear off a corner if that is what you expected. Beneath the bearing there should be heavily reinforced with additional anti-burst reinforcement similar to the end of the post-tension anchor.

OSHA Fig 61 & 62 show a triangular layer section of the north face of diaphragm peeling off to expose the reinforcement inside. The reason only a limit section of the surface layer came off with Member 12 is due to the proximity effect. The splitting force came from the 1'-9" wide Member 12 and the OSHA 61 below shows the influence extending about 2 to 3 times the Member 12 width, about the bottom of anchor D3/D4, on either side at the bottom gradully reducing to 1 x width at the top. This is the same agrument like when considering the second moment of area of this bridge in resisting load one cannot take the full 31'-8" width of the deck but only a short section on either side of the web as the effective width for the bottom flange. Different codes allow different effective lengths but between 2 to 3 times the web width seems to be borne out by the evidence of this bridge.

The peeling off commenced from the bottom and would have travelled upward at an angle about 45 degree. It was abruptly stop near the tendon anchor D1 because the surface layer there was in high comression resisting the splitting.


RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)

Your "sketches" are just too good to leave alone. First - the legal disclaimers.
Background sketches courtesy of saikee119 (Structural) . No copyright violation intended.
DDDDave - this is the closest thing to tracing I can get. Sorry I can't meet your standards.
A decoder ring may be needed to correlate my numbering with that of Saikee - my numbers are one less than the numbering on his sequence of sketches.
3DDave - it is the author's intent that the background sketches are detailed enough to provide sufficient relevancy to the subject of this forum.

The next 5 sketches show what I think is the probable failure sequence. Please consider the sketches as sections cut thru the center of M11 and M12 so the internal voids can be illustrated.

W01 is basically depicting my idea of conditions while re-tensioning the PT rods in Member 11. The Node 11/12 had slipped about a half inch before retensioning began. The lower PT rod is anchored in the deck, the upper PT rod is anchored in the block which is moving with the bottom of M11 and M12 and which is being pushed out of the deck at the north end. The different anchorage conditions cause cracking in M11.


Sketch W02. The angle change in the deck, M11, and M12 is shown greater than I think is correct for a sliding distance of maybe 6 inches by the deck. At 6" slide I find the angle to be about 16 degrees. Internal voids increase and cracking increases as Node 11/12 pushes farther out the end of the deck.


Sketch W03. Member 11 is just about to drop into the void left by the blowout of the block in the deck. Vertical load from M12 will likely break the end of M11, angles are changing quickly as Node 10/11 drops.


Sketch W04. Diaohragm just clears the edge of the pylon. The blow out block above the 8" pipe sleeve has cleared the end of the deck and is falling onto the top of the pylon. Member 11 is breaking up as it bends over the corner of the deck. The 10.5" projection of the deck under M12 and below the 8" sleeve is breaking off the deck.


Sketch W05. The Deck is falling beside the pylon while the bridge pancakes and Member 11 folds against the deck. The lower ends of M12 and M11 skid across the top of the pylon and stay there. The canopy dives onto the deck as the deck hits the roadway.

OOPS! Pay no attention to that screw in the upper corner - that is not the screw you were looking for.

Th - Th - Th - Thats all, Folks!!
Thank you,








RE: Miami Pedestrian Bridge, Part XIV

Sorry.
Totally wrong.
There are two triangles involved.
Triangle one is formed by member 11, member 12, and the canopy.
Triangle two is formed by member 11, member 10, and the deck.
When the collapse starts, the base of the second triangle starts to elongate.
Your sketch doesn't show this.
As the structure starts to fall, member 11 is pushed further out, hinging at the canopy.
Member 11 will continue to push out until the top of the canopy is at the level of the top of the foundation.
At that point, a little over 10% of member 11s length will be pushed out across the top of the foundation.
But don't forget the lower PT rod.
This is anchored in the deck.
As the collapse starts, the lower PT rod must break its way out of member 11.
There is no tension left on the rod but member 11 is moving north, as the structure falls the deck with the end of the PT rod is being pulled south.
In the first few inches the lower PT rod must break out of member 11.
As the structure falls the deck pivots at the lower end of member 10.
As the structure falls the deck is angled between the pivot point and the top of the foundation.
It doesn't take much fall of the structure for the deck to be pulled off of the foundation.
Member 11 is going north, the deck is going south.


This drawing is wrong.
The canopy and member 11 are shown too short.
At this stage of collapse geometry says that the 11,12 node will be pushed out past the north side of the foundation.
The deck is shown too long.
At this stage of collapse geometry says that it will have been pulled off of the foundation and be either sliding down the face of the foundation or have been pulled back enough to fall free.
I can do better than this but it will take a lot of time to first go back and find the bridge dimensions and then to set up the bridge as three pivoting triangles and show the relative positions as the structure falls.
It's grade ten geometry, but that doesn't mean that it won't take a lot of time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

A while back, I traced each member in each frame of the original dashcam video. member 11 breaks at the canopy-member 10 joint very early in the sequence.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

Quote (waross (Electrical))

that doesn't mean that it won't take a lot of time.
Not so long using a spread sheet. This is from about a week ago.
I used 134 feet from sta 0 to Node 9/10 but can change that in a sec.
Max O/O length of deck EDIT ADD measured from bottom of diaphragms and lower outside corners is at 4 ft drop of Node 9/10. That is the point where Node 9/10 passes thru a line between the lower corners of the diaphragms.
I assumed no crushing of concrete at Node 9/10 - the compression block is only about 2 inches deep.


Comments welcome.

RE: Miami Pedestrian Bridge, Part XIV

Quote (SFCharlie (Computer)10 Jul 20 15:35
A while back, I traced each member in each frame of the original dashcam video. member 11 breaks at the canopy-member 10 joint very early in the sequence.)

Were you able to determine if the deck had began 'folding' at Node 9/10 before or after your observation of Member 11? Had Node 10/11 dropped at the time?
The canopy is shorter from top of M11 to top of M12 than the deck from bottom of M10 to bottom of M12.
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural)10 Jul 20 16:19
I cannot be sure. The frame rate is 0.2 seconds per frame. I can say canopy-11-10 falls farther, sooner than deck, 10,9.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

Quote (SFCharlie (Computer))

There was discussion about a year ago about scan rates - could that affect the timing between the canopy (above) and the deck (below)? A top to bottom horizontal scan progression would seem to delay the imaging of the deck, giving it time to "catch up".
What affects frame rate in dash cams? Scan speed? Record/transmit speed?
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

To the best of my understanding, the frame rate is set so the dashcam doesn't fill up it's memory to quickly. The scan rate (top to bottom) would not be slower due to the frame rate. If the scan rate was slow, we would see bowing in the diagonals as they fell.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

The easiest access to photos is probably saikee119 (Structural)8 Jul 19 16:38 in part XI.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XIV

Anyone care to venture how much time they have spent investigating this failure? The amount of time some of you have put in to this is incredible! There is so much that wasn't included in the NTSB report, at least not in-depth, that has been hashed out here and is great information. Not just as it relates to non-redundant CIP truss bridges (we aren't designing these anymore, right?) but from a mechanics standpoint as well!

Someday, when I have the time, I would like to read this thread in it's entirety. Thus far, I have only spent ~5 hours and most of that is just catching up on the past posts, while not fully comprehending the information that is in them or working through it in my head. My only request to the moderators is if there was a way to preserve all the embedded pictures within the posts for future learning, that would be great. In some of the old threads, the embedded pictures are unavailable, but maybe that has went away now?

RE: Miami Pedestrian Bridge, Part XIV

Much talk here about Member 12 in this "truss". In a true truss, Member 12 is a zero force member. It was just intended to be part of the mast for the faux cable supports. Any benefit it had in trying to prevent sliding of Member 11 was likely just coincidence.

RE: Miami Pedestrian Bridge, Part XIV

Quote (hokie66 (Structural))

Right on.
So help me here - -
waross (Electrical)7 Jul 20 14:15
Gibbs Rule #39: There is No Such Thing as Coincidence.
Gibbs Rule #51: Sometimes You're Wrong.

Does Gibbs' Rule #51 apply to Gibbs' Rule #39?
(Love Gibbs Rules).

RE: Miami Pedestrian Bridge, Part XIV

Personally I am not bothered by which hinges of 9/10/deck and 10/11/canopy moved first and consider it irrelevant. To me the hinge that matter is 9/10/deck and the canopy hinge is just to follow it.


Let's say before the collapse the full bridge has a second moment of area of 7824 ft4 and the centroid 6.756 ft from the bottom as computed by approximation using coordiate geometry.


One the 11/12 connection failed the web would be unable to hold member forces in the canopy in balance with those in the deck. The bridge was then supported by the deck only. The second moment of area and centroid would change to 18.69 ft4 and 1.3 ft respectively.


From fundamental principle of stress = (Momentxdistance to extreme fibre)/(second moment of area) the stress in the deck-only case is about 80 times of the full deck. This huge stress increases is reslistic estimate if the postensioning was intended to cause the deck in full compression when the bridge is in ist final position

Therefore once the 11/12 connection has gone the deck would be hugely overstressed and formed a hinge at 9/10/deck. The hinge at 10/11/canopy was never a driver in the collapse.

RE: Miami Pedestrian Bridge, Part XIV

BadgerPE (Structural)

We did spent a lot of time looking into what has happened.

Apart from formal education, on-site experience we can also learn from mistakes. Luckily for all of us the mistake is someone's else.

We have a wide range of disciplines in here and so far everyone is trying to accommodate each other's view and we all learn together.

There is an immense amount of record available. The project presentation is cunning. the design is unconventional. The construction sequence is extremely interesting. The photographic records are mind-boggling. The failure is intriguing and the manner of collapse is sublime to anyone who wish to learn. If that isn't enough the FOT reviewing engineer was able to mark up the drawings the failure risks of this bridge years before they happened.

OSHA, WJE, NTSB and FIGG reports were each prepared to serve a certain purpose and they were constrained by the insurance people. What we are doing here is for the academic interest of getting really into the very bottom.

As far as I know this forum had discovered the root cause long before any of the official reports came out.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119)

If that isn't enough the FOT reviewing engineer was able to mark up the drawings the failure risks of this bridge years before they happened.

Could you point me to this information please? Sounds quite interesting!

Your post as a whole was spot on! Failures are unfortunately a great learning opportunity. Especially when they are on the edge or beyond what is considered “normal” design. As I said before, I haven’t spent as much time on this one, not for lack of interest, but more to time and the unlikelihood that I will ever design a bridge. The Hard Rock, Salesforce Transit Center, Millennium Tower, and Opal Tower issues are all more relevant to my day to day so I’ve spent significantly more time there.

RE: Miami Pedestrian Bridge, Part XIV

The FDOT markup was of localized cracking potential - there was no numerical evaluation of the eventual underlying structural failure. Every sharp transition in concrete will localize deformation into small nuisance cracks. It does not appear he had any safety concerns. I forget what the specific recommendations were but I believe the response was adding some fillets. There are markups. See document page 61, pdf page 80 of NTSB report HAR1902. From that report:

According to FDOT, the review performed on this project by the FDOT SDO—

was consistent with reviews performed on all projects; it consisted of a high-level
review only. We did not perform calculations or review EOR calculations

RE: Miami Pedestrian Bridge, Part XIV

Hi Vance.
Getting close.
A big part of the time will be to find the post with the dimensions of the bridge on a slow internet connection.
What I would look for is:
Distance between the piers.
Overall length of the deck.
Horizontal setback of the point at which the canopy hinges from the end of the deck.
The length of member 11 from the canopy "hinge" to the extreme north end.
Assuming that the deck and canopy hinge is centered on the PT bars, the height of the canopy PT bars above the deck PT bars (center to center).
I have read speculation that the rebars across the plane of failure were fully embedded in the concrete.
I have read speculation that the proximity of the 4" sleeves severely compromised the ability of the concrete to maintain a bond to the rebars.
(Was that the same poster?)
The 1/2 inch crack should have been measured horizontally.
That was the direction of displacement.
Say about 3/4 Inches.
Despite speculation to the contrary, once the 11/12 node had moved 3/4" horizontally, the rebar was now loose in fractured concrete.
The lower PT rod was all that was left holding it together.
At this point I speculate that the PT rod was crushing the corrugations in the sleeve.
High school physics: Flotation: The PT rods were rigid at the time of the concrete pour. Buoyancy in the wet concrete would have lifted the sleeves against the bottom of the PT rods.
Now, as the tension on the lower PT rod was increased, the PT rod commenced to crush its way out of member 11 in a horizontal direction.
What else?
The 11/12 node was moving north and the deck was moving south.
The PT rod was crossing the plane of separation and had to be crushing the bottom of member 11 horizontally.
As the movement continued, the top end of the PT bars would be extending out of the top of the blister.
This horizontal crushing continued until the deck cleared the foundation and started to fall.
At that point the the PT bars would be again pulled into the top of member 11
Once the slack in the PT bars was taken up, the horizontal crushing became a mostly vertical ripping.
There has been speculation that the PT sleeves were 4" in diameter.
There are some pictures of the rods and the sleeves prior to the concrete pour.
The sleeves appear to be about 150% of the PT diameter or 2.5" to 2.625".
This is moot however because buoyancy will have lifted the sleeves up into contact with the bottom of the PT rods.
The other thing that will take time is doing some auto-cad drawings of the various components as the structure fell.
My cad skills were never very good and I don't use it many times in a decade.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Miami Pedestrian Bridge, Part XIV

Quote (BadgerPE (Structural))

Quote (saikee119)
If that isn't enough the FDOT reviewing engineer was able to mark up the drawings the failure risks of this bridge years before they happened.

Could you point me to this information please? Sounds quite interesting!

The Interactive: The Path to the FIU Bridge disaster set up be NBC has recorded some basic information by FDOT Engineer Thomas Andres.

I believe FIU also have some records. There were some marked up drawings originally but later converted to typed text and replaced by computer CAD sketches.

Here are just two of the original mark-up drawings by FDOT engineer.



FDOT engineer was doing a high level review. The cracks were marked on areas whererever uneven stress areas were apparent. They were based on years of experience on postensioned concrete. As the bridge wasn't owned by FDOT so the peer review was given to a appointed party. Nevertheless FDOT offered a brief overview which proved highly relevant.

RE: Miami Pedestrian Bridge, Part XIV

Quote (waross)

That is a laundry list.
A set of drawings in PDF here:
https://cdn2.fdot.gov/fiu/13-Denney-Pate-signed-an...
Dimensions are on Drawing B37.
The changing dimensions during fall are not real sensitive to exact location of Node 9/10.
If you check the spreadsheet the "top of deck" dimensions are measured along the deck surface.

At this point I am hesitant to advance an opinion as to just where the canopy hinged. At the north end of the blister? Intersection of PT rods? Hard to tell. Easy to change in spreadsheet. By the way, to check my spreadsheet, do the math on one drop point. The same formulas are used to find the output in each column.

Flotation - a 3" sleeve displaces 7.3 pounds of concrete per foot. A 1.75" steel rod weighs 9.17 pounds per foot.
This horizontal crushing of Member 11 ? continued until the deck cleared the foundation
It is my thought that at some point the remains of Node 11/12 have departed the deck and are simply being pushed wherever, and that may have happened before the deck fell off the pylon. I attempted to convey that idea in my sketch W04. When Node 11/12 clears the deck and has no connection to the deck depends mostly on dimensions and damage.
For the most part, we are tracking together pretty well.
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

This may be crazy but could the bolt end of the PT cable have had too little concrete for compressive force and crushed the bearing concrete?

RE: Miami Pedestrian Bridge, Part XIV

Quote (ChiefInspectorJ)

could the bolt end of the PT cable have had too little concrete for compressive force
That is not crazy and thankfully not often the case.
It is not crazy because it is a concern and is in most cases the responsibility of the PT contractor. The PT is usually considered a design/construct sub who is responsible for doing it right. The contractor in this case has an excellent reputation. I have worked with their southern CA people.
The compressive stress under the anchor plate is quite high - as I recall something like 70% of specified concrete strength. To compare, working stresses (real) under the previous codes limited bending compression to 45% of specified concrete strength.
We would need access to shop drawings submitted for the PT to check the stresses.

RE: Miami Pedestrian Bridge, Part XIV

Quote (ChiefInspectorJ (Specifier/Regulator))

This may be crazy but could the bolt end of the PT cable have had too little concrete for compressive force and crushed the bearing concrete?

We should discount the fear of the PT rod overcompressing the concrete.


We know from the contract drawing the PT rod inside member 11 is 1.75" diameter, PT rod tension is 280kips, anchor plate size 12"x8" and the specified concrete strength is 6000psi.


The cross section of Member 11 is 24"x21".

If the entire 280kips bears on the concrete immediately behind the 12"x8" anchor plate the compressive stress is 2917psi. If the two 280kips PT rod tensions bear on the full cross section of Member 11 the average compressive strength is only 1111 psi.


ACI design codes, say 318, permits only 0.85x specified strength to be used in design in conjunction with other considerations. Thus below a stress of approximately 0.85x6000 = 5100 psi is a safe design.

If the concrete is about to fail the actual concrete compressive stress is of course allowed to reach to the specified strength but there is fat in the system that the actual strength is invariably higher than the specified strength. When NTSB cut cores from the collapsed bridge to test the concrete strength in a laboratory the lowest actual strength was 8,580psi while the remaining four samples registered strength all above 10,000psi.


Thus I don't think there is much mileage in proving the concrete could be overstressed by the PT rods in compression. To me the concrete in trouble is in the area outside the influence of the PT rods with not enough compression.

RE: Miami Pedestrian Bridge, Part XIV

Quote (saikee119 (Structural)11 Jul 20 13:31)


That solves that. Thank you for your detailed post and data coverage.
I do see one thing on which I will comment - as I recall viewing photos of the PT rod anchor plates which had anchored the PT in Member 11 they appear square to me - not rectangular.
EDIT ADD: I think we should look for a specific allowable compressive stress under an anchor plate at the time of tensioning. The general usage of O.85f'c is for factored loads. Under PT installation the load is real and temporary until it relaxes or anchors set, and some increase is permitted but I am not sure it reaches 0.85f'c.
Had a reference but just lost power for a few minutes so will return in a bit. Hopefully with a reference.
Thanks,

RE: Miami Pedestrian Bridge, Part XIV

Quote (Vance Wiley's (Structural))

I do see one thing on which I will comment - as I recall viewing photos of the PT rod anchor plates which had anchored the PT in Member 11 they appear square to me - not rectangular.


Many photos, like OSHA Fig 64, 65 & 67, show the PT rod anchor enembedded exposing only one ful edge. Hence measuring the two sides is not possible and I agree it does appear to be square in shape. I attached OSHA Fig 64 for example.


The best photo I could come up with for checking the PT rod anchor is from NTSB "Materials Laboratory Factual Report (Report No. 18-082) depicted above.


In its lower section of Fig 15 enclosed above you can save the image, magnify it and measure the horizontal and vertical edge. When I did that I got a aspect ratio of 1.5 so the anchor is definitely rectangular to me. The specified dimension of it is 12" by 8" for 1.75" diameter PT rod used in Member 11.

RE: Miami Pedestrian Bridge, Part XIV

Vance Wiley (Structural)

In my experience the 0.85f'c used in ACI code is to encompass a safety factor for the concrete as a material. When you do ultimate limit state design the ultimate tensile strain is 0.003 at collpase. You also have a strength reduction factor 0.9 making the net usable strength in concrete 0.9x0.85 = 0.765f'c for a ACI compliant design.

The Euro code ultimate tensile strain in concrete is 0.0035. The ultimate maximum concrete stress is equal to the specified concrete cube strength fcu divided by the material safety factor which is 1.5 for concrete. So the corresponding net usable ultimate concrete stress is 0.6fcu.

However concrete strength crushed by a cube is higher than that crushed by a cylinder. The universal conversion factor is f'c = 0.8fcu. Therefore EURO or Bristish designer would effectively use 0.6/0.8 = 0.75f'c which is close to the ACI code's 0.765f'c.

What you permitted in the design, by lowering f'c to 0.85f'c and then further reduced with a strength reduction factor, is just good engineering practice adopted universally but in slightly different formulae for different countries.

In the field the specified 6,000psi strength concrete actually has the insitu strength at least 8.580psi verified by the core samples. The acutal collapse analysis should based on the actual strength and actual stress.

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