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# Miami Pedestrian Bridge, Part XIV17

## 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 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.

### 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

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.

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.

### 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

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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