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# Miami Pedestrian Bridge, Part III99

## Miami Pedestrian Bridge, Part III

### RE: Miami Pedestrian Bridge, Part III

New interesting post processed dashcam video at https://youtu.be/D6pGzgm4zZ4
Compression of member #11, before deck fall, could be pushed out PT rod, as someone suggested already.

### RE: Miami Pedestrian Bridge, Part III

2

#### Quote (dik)

Were you able to determine if the failure occurred at the ends of the web members or at the panel point?

dik....not sure. In one of the NTSB photos, it shows the anchor "blister" relatively intact but broken through the top chord. This is what led me to suggest perhaps a punching shear of the entire panel point through the top chord. There did not seem to be much, if any, lateral mild steel reinforcement across this interface.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (winelandv)

A defense of the design:

To the structurals here, who hasn't, at the behest of an architect, worked their tail off to make something superflous work? Is that not part of our job? Ok, so the "cable stays" really aren't. So?

...

Drawoh's rules of style...
1. Style costs more. There is a way to do it that is cheap and functional.
2. Keep it simple. We are techies, not artists.
3. Bad styling looks a lot worse than un-styled functional.
Okay, I am mechanical, not civil or structural. If management is determined to make it cool, ask for a budget. Maybe you will get a big one! I perceive rule 2 as an artistic issue, but maybe there is a structural component too.

What if they had designed a simple, well proportioned bridge, and added twenty tons capacity for artists to play with?

--
JHG

### RE: Miami Pedestrian Bridge, Part III

Drawoh,

I agree with your rules of style. That being said:

1) The people who decided to lay out the money for the bridge wanted something snazzy. Without actually looking in to it (I'll save that for epoxybot), I'm willing to guess that the style of the bridge contributed to the DB team getting the project. So yes, the city/state/university most-assuredly paid more for this bridge than a "standard" (whatever that is) long span bridge.

2) Unfortunetely, with bridges, as you adjust how it looks, you're adjusting how it works. You've essentially got a line with 2 or more supports. The load has to get to those somehow and there's just not much play with it. *shrug*

3) The style of this bridge was interesting enough (to me) to bring it out of the "bad styling" category. YMMV

4) A simple well-proportioned bridge with additional capacity just wouldn't have had the WOW factor that (apparently) everyone wanted.

Long story short: municipalities like fancy bridges in high-visibility areas. You'll notice that interstate bridges over no-name river in the middle of nowhere very rarely deviate from girders and a deck slab.

### RE: Miami Pedestrian Bridge, Part III

I worked for VSL as an engineer for two years several years ago, mostly monostrand commercial building design work. My understanding from some former coworkers was that they were in the process of de-stressing one of the threaded bars when it collapsed. So they were attempting to reverse the stresses within the no. 11 diagonal when the collapse occurred.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (drawoh)

Okay, I am mechanical, not civil or structural. If management is determined to make it cool, ask for a budget. Maybe you will get a big one! I perceive rule 2 as an artistic issue, but maybe there is a structural component too.
Like this ones:

### RE: Miami Pedestrian Bridge, Part III

Well if they were actually destrssing it maybe the PT rod was the only thing keeping that column straight and once they loosened it it just bent and failed in bending?

I've also been wondering if the bottom slab got damaged/ cracked during the lift due to reverse of anticipated loads / sheer.

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

### RE: Miami Pedestrian Bridge, Part III

OSUCivilEng:

Exactly my point. If concrete trusses are going to become more common, they should specifically be referenced in the code.

Also, on your point about pushing boundaries; people said the same thing about truss bridges when the transition from masonry arches to trusses was occuring, suspension bridges bridges when they first came around, steel frames in buildings instead of masonry in the late 1800s, and so on and so on. To demonize FIGG for pushing boundaries when we don’t have all the answers isn’t right. Our profession’s history is full of us pushing boundaries.

### RE: Miami Pedestrian Bridge, Part III

Hi All,

Just signed new member. I am an electronics engineer, so other than instrument and cabinet chassis, no structural experience and none with bridges or pre- and post tensioned concrete. I do have an intense interest in engineering disasters as they are virtually always caused by the "holes in the swiss cheese" lining up just so, they typically require a number of failures, and there always seems to be a huge amount of human arrogance and ignorance in play. So while I am not a structures guy, I have about 50 years of common sense engineering that has been hammered into me by various successes, failures, and guys who were at the time older and wiser than me.

Regarding this specific failure, a number of you have noted that that while trusses 11 and 2 both carry equal loads and that truss 2 had even more compression as it was more horizontally oriented than truss 11, it survived where truss 11 failed. You all got my curiosity up, so I went back to all the pictures to see if there was anything obvious, and while all the line drawings show the trusses of equal beam depth (excuse me if my terminology is incorrect as I am not a structures guy), they are most certainly not. In fact, if you see the picture of truss 2, its depth (not sure correct terminology here) is almost DOUBLE that of truss 11!

In addition to this truss having a much lower load per unit area, it will have greater torsional strength against flexing in the horizontal plane. I think the key though is take a look at how the truss is attached to the deck. As the beam depth is almost twice as great, the attachment to the deck has about double the surface area. Therefore, even if the rebar layout is poor, all the loads, including shear, will be roughly half. Last, this doubling of attachment-to-deck surface area means that the loads are transferred to the deck much further into the middle of the deck where the loads can be more effectively transferred to the deck PT cables.

So, while I believe like many of you that this was a terminally flawed design, impossible to analyze at the micro level at the nodes, blisters, and their respective connections to both chords, and that likely would have failed, possibly later killing dozens, or even hundreds of people, there does seem to be a big problem in that two trusses, both of which are the most heavily loaded trusses in the design, are of dramatically different beam sections. And one has to wonder it that last minute "demand" from FDOT, which added 11 feet to the north end, caused a fatally flawed redesign to an already weak structure.

https://res.cloudinary.com/engtips/image/fetch/w_8...
[img ]

Gary

### RE: Miami Pedestrian Bridge, Part III

I don't think anyone has posted this yet:
"Experts cite explosive joint failure as cause of Florida bridge collapse"
https://www.newcivilengineer.com/tech-excellence/e...

Their leading speculation is explosive joint failure at base of #11. Note that in the linked article they have the member numbering off by one relative to the MCM-FIGG drawings, so they refer to the failed member as #10.

### RE: Miami Pedestrian Bridge, Part III

A number of posters here have noted the awkward path by which compression in number 11 would be transmitted to the tension in the deck's torsion rods/cables.

To the limited extent we can deduce anything from the MCM-FIGG proposal drawing, and the photos posted here, Member #11's lower end pushes against an area of the deck in which forces must work around service channels for the pillar #12, and also travel laterally to the location of the innermost deck tension members. The upper portion of #11's cross section pushes substantially against vertical #12, so those forces create a shear between the base of #12 and the deck, and might meet less resistance.

Summarizing these ideas, compression in the lower end of #11 might look like this figure:

Forces distributed very unevenly over the cross section of a member could result in exceeding the member's resistance to bursting in that high compression region. This is a story which could match the newcivilengineer story, linked above.

Much caution is needed though, since it's clear that #11 and its connection to the deck were redesigned after the MCM-FIGG proposal, not least to add the two PT rods (perhaps to accommodate the transporters relocated inboard), and of course the sizeable PT end plates, that must be somewhere in or under the connection.

[Edited to add a comment about SomewhereOverChina's excellent observation]: I agree that there's a concern to have the end diagonals (#2 and #11) somewhat set back from the end of the deck so that transmission of horizontal forces from these members can be distributed to the laterally offset deck tension rods/cables. On the other hand, each of these members also presents a significant downward load, which needs to go directly down through the supporting pillar, and not create sheer in the relatively thin deck, which it would if positioned further set back from the end of the deck.

### RE: Miami Pedestrian Bridge, Part III

Good eye, SomewhereOverChina. Looking back at it, it certainly looks like several of the members are wider towards the south end, with #2 being the largest change from the initial plans. I wouldn't say they added length to the north end; the north pylon was moved and the overall length of the bridge increased. It is likely they mostly scaled the bridge to keep the same proportions.

I wonder if the root cause was making half-considered changes and not re-doing the stress work. It's like they really wanted to keep the diagonal members as thin as possible and then, for some reason, decided they could not, resulting in the drastic change to #2.

### RE: Miami Pedestrian Bridge, Part III

I know this is now moot, but I have been wondering about the fake cable stays.
I understand that there can be significant expansion and contraction of cable stays due to temperature changes.
I am thinking about the anchor bolts failing and the deck jumping up about two feet on the bridge up in Canada a while ago.
I would love to hear some comments from the bridge people about the possible issues of expansion and contraction of stays attached to a very rigid structure. Would the stays be stretching in cold weather, sagging in hot weather or both?

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

### RE: Miami Pedestrian Bridge, Part III

Continuing to look at the 11-12-deck area, which we know was redesigned relative to the MCM-FIGG proposal docs, looking at this screen cap from an NTSB video, what are the two items I've labeled with a question mark?

https://youtu.be/aeJKqojmHgY?t=129

They look to be tension rods, and within the footprint of vertical member #12. But it's not clear to me if they are something that extended vertically into #12, or were actually horizontal and bent up during collapse as the end of the deck fell adjacent to the north supporting column. They seem too inboard to be the most inboard of the deck PT rods/cables, at least as shown on the MCM-FIGG drawings, and seen exposed in photos/videos of the end of the deck.

I'm wondering if they might turn out to be part of a feature to distribute forces from member 11 to the deck.

### RE: Miami Pedestrian Bridge, Part III

gwideman,

Is it possible that those are tensioners for the future tower that was supposed to support the fake cable stays?

And regarding the same area, does it look like there is no concrete anywhere in that area? None of the tensioners, the plastic conduit, even the rebar. It is almost like that area was purposely voided for some type of service tunnel possibly for the services or other that would have been contained within the cable stay tower. And if that is truly a voided area, then truss 11 and NO SUPPORT to the edge of the deck. It seems a totally open area.

Did they figure when both sides of the bridge were done and the tower constructed that this area would be extremely strong as both truss 11 and its mirror twin on the other side would be in compression against each other? Is it possible the bridge was FEA analyzed as a complete unit? Was the shorter section, with substantially lower forces, supposed to be built first so that truss 11 would have something to work into?

Regarding this link to Civil Engineer:

https://www.newcivilengineer.com/tech-excellence/e...

while I am not a structural engineer, I was not impressed. The author obviously did little or no homework. He did not even have the truss numbers labeled properly, something that has been correct from the first post in this forum, and his "final configuration" analysis shows tension on some of the trusses like they are being supported by real cable stays. Yet if I understand everything I have read correctly, including the original proposal, those stays are almost decorative and may in fact proved more support for the tower as guy wires than as anything to hold up what is a truss bridge (apologies for the true structures guys that make the point that a true truss bridge is always hinge pinned).

This structure seems like a heavy, ugly (I am with Tomfh), costly alternative to a real cable stay bridge, virtually all of which I have seen are quite light, airy, and beautiful looking. Not sure the relative cost of a true cable stay bridge though versus a simple steel passenger bridge.

Gary

### RE: Miami Pedestrian Bridge, Part III

@gwideman, Sheet B-8 of the proposal package (linked from thread 1) shows some kind of bolt there but it looks like it's for position-keeping rather than tension.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (SomewhereOverChina)

while I am not a structural engineer, I was not impressed. The author obviously did little or no homework. He did not even have the truss numbers labeled properly, something that has been correct from the first post in this forum, and his "final configuration" analysis shows tension on some of the trusses like they are being supported by real cable stays. Yet if I understand everything I have read correctly, including the original proposal, those stays are almost decorative and may in fact proved more support for the tower as guy wires than as anything to hold up what is a truss bridge (apologies for the true structures guys that make the point that a true truss bridge is always hinge pinned).

Your assessment is correct. While I have not read the link in its entirety, the assumption that this was a cable stayed bridge shows a gross lack of research in the available information. Without the ability to tension the 'cable stays' they will not be under tension in their finished state, except for under transient loads. You can't magically turn a simply supported span into a cable stayed span without adding significant tension to the cable stays.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (Emba)

o Dik - just for interest value, on the rescue photo you can see the shirt on the lead person says "FLTF1", that's the "Florida Urban Search and Rescue Task Force 1". Same pic in wider format shows a headlamp on the helmet (https://www.cbsnews.com/pictures/deadly-miami-brid...) Rear rescuer with dog looks like she has a Sierra cup hanging from her belt. -- Emba

The comment was made because there is a movement within OSHA to replace conventional hardhats with the new style included in the link:

https://www.constructionjunkie.com/blog/2017/8/7/s...

The search and rescue helmets in the photo, likely comply to the new OSHA requirements. These are slowly making their way on to job sites. The new style hardhat is patterned off the helmets that are commonly used for rock climbing. The new OSHA conforming hardhat will likely replace the conventional hardhat; they are safer.

Dik

### RE: Miami Pedestrian Bridge, Part III

#### Quote (apster)

Don't really know except the failure was not elastic.

Even glass behaves elastically... to a point. There was, however, little or no ductility.

Dik

### RE: Miami Pedestrian Bridge, Part III

My opinion : At the time of failure the structure where it failed was beyond the elastic range and failed as a brittle material. This is equivalent to reaching the ultimate stress and strain of the material or combination of materials. A concrete cylinder can remain fairly elastic up to the point of failure, steel will remain elastic and continue to stretch i.e. increase strain without increase in load or even with some decrease in load until it ultimately fails.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (gwideman)

I don't think anyone has posted this yet:

There have been a couple, as I recall; Eng-tips is generally on top of it:

Structuraleng89, March 18
Tomfh...great video w the dash cam...i see an explosion at the bottom of the vertical at the support...maybe the strand they were pulling at the top of #11 came out at the bearing node of the truss...the tension diagonal adjacent to #11 appears to punch thru at the bottom as well

gte447f, March 18
...it looks like you can make out something shooting up vertically from the top chord first panel point and in the same frame there appears to be an "explosion" of dust at opposite end of diagonal #11

Dik, March 19
A failure at the end of a member may be different than the panel point 'crushing' or 'exploding'.

Rapt, March 20
At the 3 - 4 frame in the video you see something like crushing in this area with an explosion of material upwards from the top (not as in caused by explosives). The diagonals still seem to be intact at this point. Next frame, the top drops very quickly and you get the rotations in the 10/11 node at the top that have been noted previously as the bottom slab at this location crumbles.

Dik

### RE: Miami Pedestrian Bridge, Part III

#### Quote (dik)

There have been a couple, as I recall; Eng-tips is generally on top of it

I didn't mean to say nobody on the thread had discussed crushing at various locations.

I meant I thought nobody had yet posted a link to this article. Notable because it's from a site that purports to specialize in civil engineering, though as others have noted, a bit careless in this article.

Hmmm, and now, suddenly, I can't access the article -- it wants me to register.

### RE: Miami Pedestrian Bridge, Part III

The failure was catastrophic, and there has not been sufficient information to put the cause to rest...

Dik

### RE: Miami Pedestrian Bridge, Part III

#### Quote (Steve Nuchia)

@gwideman, Sheet B-8 of the proposal package (linked from thread 1) shows some kind of bolt there but it looks like it's for position-keeping rather than tension.

Sounds plausible, though I don't think the drawing matches the photo. On B8 "CROSS-SECTION", and "SECTION A-A", I see a couple of vertical rods, but they appear outboard of the inboard-most deck tension rods. Then in A-A and C-C there are a couple of horizontal rods at the canopy level to tie the "Main" and (not present) "Backspan" bridge canopies together. So I don't think the rods in question correspond to that drawing.

### RE: Miami Pedestrian Bridge, Part III

I was wondering why diagonal strut #11 has P.T bars, but it hasn’t P.T bars in the preliminary design.
I found a difference between the preliminary design and reality in transporting the main span. In preliminary design, the shoring supports both ends of that span therefore no need P.T bars in #11.
Maybe they narrowed the distance between two transporters to save money for site clearance, but they paid too much for that.

### RE: Miami Pedestrian Bridge, Part III

If this Oliver McGee is a licensed Professional Engineer he has likely breached the ASCE and NSPE codes of ethics for his unfounded public statements. Might also be a violation of the Texas Engineering law, but I don't know their stance on public statements. In my state, it would be a violation.

### RE: Miami Pedestrian Bridge, Part III

Leubong,

It has already been well established in earlier threads that there was a significant design change whereby the length of the span increased by 11 ft to allow for a future widening of the road. As such the pillar on the N side moved North into what is currently a grass area. This resulted in a change to the planned movement and location of the transporters further into the span and required the additional PT rods in both member no 2 and 11.

They were both supposed to be released once the span was sitting on the piers.

Now why it then took them 5 days to do it and after they re-opened the road is anyones guess and I would think will form part of the investigation into the collapse.

The PT bars in member 2 were apparently (according to the NTSB) released without incident before they started on member 11.

Please take some time to read parts I and II of this incredibly interesting and thought provoking thread, before re-introducing further items which have already been discussed.

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

### RE: Miami Pedestrian Bridge, Part III

VSL employee was tied off to horizontal safety line. The safety system became wrapped around the exposed post tension rod. See attached photos. This unfortunate worker seems to have been flung by this while bridge fell. It looks like he dangled from crane because vertical crane line held up the horizontal safety line.

The north end fell fast in part because the center of gravity falls at 1 g, but like a falling smoke stack, the free end goes faster. The marks on the pylon made by the falling deck may help with investigation. The marks begin well below the top of pylon base.

The central web truss does not look right, it looks more like a concrete canopy support system would look in a walkway/plaza on solid ground, not a support system for a 174 foot span. It looks like they were going for clear vistas from sides of walkway, so they put only one web in the middle. It's sad, but FIU/MCM put in writing (the proposal) all the constraints that added together doomed this project. I.E. ABC, a pedestrian overpass that resembles a magistic full size cable-stayed bridge, concrete to be cool in summer, wide ribbon deck...

This bridge will not be completed with this design, you can bet the new design will be highly redundant.

I will keep on open mind until report from NTSB is issued, but I must say now, I'm shocked engineers did not block this project and even more shocked rod adjustment was done while traffic was below.

This bridge failed because deck separated from joint 11-12. Half the steel of 11 went with deck, the other half went with 12. Joint burst and beam zippered, no PT-rods or anchors broke. I agree with other posters the north end depended on support from other section that was not yet in place or the stress path was bad. The "last minute" 11 foot increase in span is scary to me.

PS. There is a public FIU student dropbox with time-lapse of formwork phase. You can clearly see the rods and joints before concrete was poured.

### RE: Miami Pedestrian Bridge, Part III

7
Comparison between bridge collapse movie and theoretical simulation of truss without member #11

Deck fail between member #11 and #12 #10 and #11 is close too.
Courtesy to Zac Doyle for video and to Ivan Markov for online simulator.
Data input : (0, 100)(87, 100)(154, 100)(221, 100)(289, 100)(358, 100)(412, 100)F:(0, 139)(80, 139)(157, 139)(240, 139)(327, 139)R0:(412, 139)(0-1)(1-2)(2-3)(3-4)(4-5)(5-6)(7-8)(8-9)(9-10)(10-11)(11-12)(0-7)(7-1)(1-8)(8-2)(2-9)(9-3)(3-10)(10-4)(4-11)(11-5)(5-12)(12-6)

### RE: Miami Pedestrian Bridge, Part III

#### Quote (jrs87)

There is a public FIU student dropbox with time-lapse of formwork phase.
Care to let us in on the address?

### RE: Miami Pedestrian Bridge, Part III

Meerkat007

Can you set simulator to no connection between deck and 12?

### RE: Miami Pedestrian Bridge, Part III

#### Quote (jrs87 (Mechanical)Can you set simulator to no connection between deck and 12?)

For that simulator is same if you remove deck between #11 and #12 #10.
Delete "(11-12)" from data input.
UPDATE: You can remove member #12 entirely together with canopy.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (Jerehmy (Structural)22 Mar 18 03:20 OSUCivilEng: Exactly my point. If concrete trusses are going to become more common, they should specifically be referenced in the code. Also, on your point about pushing boundaries; people said the same thing about truss bridges when the transition from masonry arches to trusses was occuring, suspension bridges bridges when they first came around, steel frames in buildings instead of masonry in the late 1800s, and so on and so on. To demonize FIGG for pushing boundaries when we don’t have all the answers isn’t right. Our profession’s history is full of us pushing boundaries.)

I simply disagree. I think this entire design was irresponsible. Test pilots, astronauts and soldiers at war push boundaries and put their lives at risk. Innocent motorists shouldn't be put at risk with unproven designs built right over their unsuspecting head.

From the MCM-FIGG proposal:

"The superstructure shape for the new signature pedestrian bridge is innovative and one-of-a-kind. We have re-invented the traditional I-beam in a magnificant scale with a special transformation of an open truss down the middle, improving both its functionality and form for a 30’ wide path."

Somebody has been reading too much of their own press clippings.

### RE: Miami Pedestrian Bridge, Part III

OSUCivlEng just made some very good points. Jerehmy is right that new is often unfairly ridiculed. But it is never unfair to ridicule failure. Maybe Figg did not push boundary, maybe they went past it. Or the end result was such via chain of events. We all learned in engineering school the right thing. We forget as time goes by... And it's the same old story: engineers not enough power. But physics always prevails.

Anyway, no one wants to demonize anyone in this forum. Let the NTSB investigate.

### RE: Miami Pedestrian Bridge, Part III

Meerkat007, the simulation clearly showed the movement of the roller support, akin to the tore out. I am newly registered here, and found this discussion really interesting, with lots of great responses. Thanks a lot.

### RE: Miami Pedestrian Bridge, Part III

Starting enumeration of code standards:

1. Redundancy, no single plane of truss
2. Symmetry and no faux in super-structural members
3. No 30 foot wide width for 174 foot span without curve in horizontal plane or true wire stays
4. No ribbon deck so it looks like wire-stay but is box truss (wait it's not even a box truss)

This is not meant to be mean, just a thinking exercise.

PS This accident is very interesting because we know how and why this design came about, it's so obvious. But the cause of sudden collapse is very perplexing. After all it was unloaded when it failed. And there is no doubt about it, some trust of engineering as been lost in America now. There as always been accidents and failures, but this one seems different... Maybe because something so simple as a pedestrian bridge failed in the hands of those with much bigger capabilities.

After this collapse we could say big bridge designer demoted to pedestrian bridges. Oh... sorry.

I wonder too how the thin edge of deck was expected to respond to accidental strike from overhigh cargo.

### RE: Miami Pedestrian Bridge, Part III

OSUCivlEng:

How do you prove a design without building it?

FIGG shouldn't be ridiculed for trying to innovate, they SHOULD be ridiculed for not doing their due diligence in ensuring everything went correctly. I don't think they did. If you're going to do an innovative structure, you need to put extra time and effort so that everything goes correctly.

Was there a FIGG engineer on site? If not, WHY? In this book I'm reading on building/bridge history in USA, the design engineer was on-site every single day. He led the project. I feel we've gotten far away from that and I'm not sure why. Especially on a project that is innovative.

I'm also curious into their R&D in using a concrete truss. Did they do any scale models of concrete trusses? Test them to failure?

### RE: Miami Pedestrian Bridge, Part III

No single element doomed this bridge. ABC is okay but rules out wire-stay, concrete is okay but rules out thin, central web is okay but rules out no support from wire stays and ABC... Now we need a Venn Diagram and we will see I suppose this bridge will not be in any circle. Engineers are taught that "never-been done before" is a red flag, because maybe it has not been done because it is NOT an improvement.

Jerehmy, are we in the USA anymore? USA used to mean diligence. Not anymore. You make good points, but I think innovative should not be done just to be innovative in and of itself. Not everything needs to be innovative. Be innovative in the Utah bridges, that's okay. Care to share the name of the book?

I know FIU tests elements, but maybe only steel. And then only small parts. This bridge failed as a system that cannot be tested. This bridge was the lab. Right? Too many variables...

### RE: Miami Pedestrian Bridge, Part III

3

#### Quote (winelandv)

A defense of the design:

To the structurals here, who hasn't, at the behest of an architect, worked their tail off to make something superflous work? Is that not part of our job? Ok, so the "cable stays" really aren't. So?

Any of us who has come into a project halfway through has had the thought, "This, this, and this look really strange and I hope that this, this and this were accounted for." Same situation here - we're all coming in after the fact and slowly learning what the design really is. I, for one, am willing to give Figg some slack here on all the design issues: the un-symmetry, the not-really cable stays, the non-redundancy, the kinda-a-truss but kinda-a-beam-with-holes issues. That firm isn't just 2 people in a garage somewhere. They have designed many, many bridges throughout the United States (including the I-35W replacement in Minneapolis).

Maybe, when it's all said and done, we'll know one way of the other if the final design was safe (and maybe not if everything gets settled and everyone signs an NDA). Until then, all we're doing is throwing thoughts around. Yes, it failed, but there's only a couple hundred other factors that could have ultimately caused the collapse.

Yes, many people here have identified challenges with the design side. SURELY you don't actually think that FIGG didn't try and account for those challenges? To those saying that the design is completely wrong - it did stand for 5 days - surely that's at least proof of concept? Short of gaining access to the design team's notes, hand calcs, and computer models, along with the rebar shop drawings, and the constructor's construct and erection plans, there's a giant pile of unknowns as to the ultimate cause of failure. I'm not saying that there isn't a fatal flaw in the design (as I don't know either way), but let's slow down the blame game - there's enough talking heads on cable handling that. And goodness, the last thing we need is to start revamping AASTHO's spec for bridge design before we have all the facts.

-if you want to see a cable stayed bridge, go look at the conceptual pre-bid documents that TY Linn put together.

#### Quote (waross)

I believe that the root cause of the wye may be a tiny grain of sand or other foreign matter jamming the threads of the PT nut and the following cause was over tensioning the rod in an effort to free the jammed nut.

Highly unlikely. Consider than member #11 (and it PT bars) is more than 30 feet in length. So the elongation of the PT bar stressed to 70% of yieldEDIT ultimate is (70%*390 k = 273 kips) about 1-1/4". To achieve 'lift off' of the nut to the bearing plate all you need to 'daylight' a gap - let assume 1/8" (very conservative). So the bar has now elongated 1/8" above its original 1-1/4" so it elongates a total of 1-3/8" - an increment of 10% - and therefore a 10% increase in corresponding force (stress) too. 110% of 273 kips is 300 kips - way below ultimate tensile of 390 kips.

Additionally, the very design of the threads to PT bar is to handle the field conditions. It is a very robust thread - a grain of sand or cement paste splatter has little effect. Additionally, it is common for the manufacturer to supply a 'thread cleaner' nuts that you 'run' over the bar projection to clean away any deleterious materials BEFORE you install a stressing setup.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (Ingenuity)

Over stressing PT is a big deal. However, it is somewhat moot in this discussion, because the PT bar did not fail in tension, so therefore it was NOT overstressed.

Is relative strength of the components an active design consideration when designing PT members with relatively thin sections and a low number of tendons?

In other words- for the web members of this concrete truss, they are relatively slender and with a low quantity of PT tendons. We know the level of strain applied to the PT tendons was insufficient to load them to UTS, because they did not fail. But was the level of strain applied to the PT tendons high enough to fail the concrete member in compression? Does that extra 10% applied during the destressing procedure ever run the risk of cracking or otherwise damaging the PT member?

Off topic, but I'd love to see the PT procedure for a 1,000+ foot length tendon.

### RE: Miami Pedestrian Bridge, Part III

#### Quote (epoxybot)

FDOT reasonably distances itself from this project because Florida International University received Local Authority Program (LAP) Certification to oversee the project

Because they 'have big pockets', they may not be able to distance themselves far enough... we'll have to wait and find out... hopefully, their presence on site, during any part of this work, is nil or next to... If they were frequently on site, or attending meetings, they may have a problem.

Dik

### RE: Miami Pedestrian Bridge, Part III

#### Quote (jgKRI)

Off topic, but I'd love to see the PT procedure for a 1,000+ foot length tendon.

It was closer to 1,100 feet long, 31 x 0.5" dia 7-wire multistrand tendons (all 31 pulled one-time), 8 tendons total. Approx 8 feet of elongation. I may have a photo I can dig up...

### RE: Miami Pedestrian Bridge, Part III

thanks Ingenuity...finally someone with actual experience in PT design and construction....

### RE: Miami Pedestrian Bridge, Part III

bimr - I have read the entire I-35W report and still have in mind the photos of the warped gussets. I didn't feel that pasting the full contents was required to make the point that a bridge that had the same redundancies as suggested as a fix for this one still failed because a fundamental design element was never addressed.

### RE: Miami Pedestrian Bridge, Part III

4

#### Quote (jgKRI)

Is relative strength of the components an active design consideration when designing PT members with relatively thin sections and a low number of tendons?

In other words- for the web members of this concrete truss, they are relatively slender and with a low quantity of PT tendons. We know the level of strain applied to the PT tendons was insufficient to load them to UTS, because they did not fail. But was the level of strain applied to the PT tendons high enough to fail the concrete member in compression? Does that extra 10% applied during the destressing procedure ever run the risk of cracking or otherwise damaging the PT member?

Yes, the engineer would (should?) have considered all aspects of component capacity - all the PT components and the structural member with its load histories/cases and including reductions for area occupied by the tendon ducts (not yet grouted etc).

For member #11 with 2 each x 1-3/4" PT bars on a 24" x 21" concrete cross section, with PT bars stressed to about 70% of UTS - that is about 540 kips over about 500 in2 of concrete - so about 1,000 psi of axial compressive stress. Self weight support reaction of 950 kips and a member #11 angle of 36o equates to a compressive force of about 1600 kips (3,100 psi axial compressive stress) - add the self weight force to the stress induced by the PT to member #11 and you get a net of about 2,140 kips (or 4,100 psi axial compressive stress). IF (big IF) there was a 10% overstress to PT bars that would only increase the force (and stress) by 54 kips (100 psi) to the cross section, for a total of about 2,200 kips (4,400 psi). For 8,000+ psi concrete, adequately reinforced with mild steel reinforcing, (including significant confinement reinforcement) and the level of stress to the member should be okay...now the joint of #11 is another matter!

### RE: Miami Pedestrian Bridge, Part III

[quote dik][That's where I have a problem...

minimum design standards as you have noted... could have been more than AASHTO required, or, may not have been designed in conformance with AASHTO... It should have been, but, just don't know this fundamental information...

Safety factor predicated on AASHTO design... see note above.]

Yes, we do not know what the bridge was designed for - but we know for sure that it collapsed, and based on this fact, we know that there was some major error somewhere leading to the catastrophe. I have seen already another design driven by the appearance, without any understanding of the V-pier design.
And I have seen indiscriminate application of the code, without understanding of the meaning of these factors, and resulting safety of the design.
No code, or design guide are perfect - and there are many variables which should be taken into consideration, like the percentage of the particular loads in the overall design forces. And perhaps the codes should specify the limits, to which particular formulas applies. ("when DL/PL ratio is >2, use DLx1.6" - just example)
Almost anybody, with some basic computer skills, could operate Midas, or Bridge Designer, or Robot program, to name just few. One could model a bridge, and size it without even knowing how the structure works.
And I really hope, that that's not the case, and the cracking moment and shear for the span was checked, and compared with the design loads and minimum sensible safety factor achieved.
And I also hope, that somebody designing another heavy pedestrian bridge is reading this, and will alter a design to achieve minimum 1.7 SF.

### RE: Miami Pedestrian Bridge, Part III

>>>One could model a bridge, and size it without even knowing how the structure works.<<<

Given our current overly-complex codes and our reliance on software to navigate those codes I'm afraid I agree with you. I've also come to believe we may have built better in the days of the slide rule. Just my opinion.

### RE: Miami Pedestrian Bridge, Part III

wiktor, regarding this discussion of load combinations, the ratio of DL/PL concerns are exactly why bridges also have to satisfy the Strength IV combination, using DLx1.5

Maybe not the 1.7 you're shooting for, but a higher SF than has been discussed to this point.

----
The name is a long story -- just call me Lo.

### RE: Miami Pedestrian Bridge, Part III

Received this eMail at home; I've been discussing this with a couple of engineers on DesignToEurocodes...

I’m not a member of the ENG-TTPS [sic] forum, but I thought you might be interested in this:
https://facilities.fiu.edu/projects/BT_904/FIU-Ped...

Other internet finds:

RFP:
https://www.flrules.org/gateway/notice_Files.asp?I...

FDOT Pedestrian Bridge Info
http://www.fdot.gov/structures/structuresmanual/20...

The information may already be posted, but, I didn't notice it.

Dik

### RE: Miami Pedestrian Bridge, Part III

#### Quote (Archie264)

we may have built better in the days of the slide rule. Just my opinion.

Still have mine... just in case...

Dik

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