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Miami Pedestrian Bridge, Part XIV
17

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

3

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?

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