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Was The Florida Pedestrian Bridge Collapse Triggered By Post-Tensioning? (enr.com)
A new lawsuit claims post-tensioning triggered the collapse of the pedestrian bridge at Florida International University, killing five motorists and one worker. Engineering News Record reports
According to the lawsuit, the March 15, 2018 collapse occurred while a crew was post-tensioning bars in a diagonal member at the north end of the concrete truss that was the bridge's main element. The post-tensioning compressed the diagonal so that it overstressed a joint in the top chord, the lawsuit claims, triggering hinge failure at a connection in the lower chord and resulting in the catastrophic failure of the rest of the 174-ft-long structure. Post-tensioning that modifies the stresses in a structure is inherently risky and should be performed "in the absence of traffic," the lawsuit claims. [The lawsuit] draws heavily on video of the collapse, a voice message about cracks in the structure that were deemed superficial at that time by the engineer of record and design drawings in the design-build joint venture's proposal.
Slashdot reader McGruber writes: Interestingly, just two days after the collapse, an Anonymous Coward posted that post-tensioning likely led to the collapse of the bridge... A March 21, 2018 NTSB News Release said "The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed. They had done this same work earlier at the south end, moved to the north side, and had adjusted one rod. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed."
The Miami Herald reports that "how and where precisely the bridge broke apart likely won't be known for months, until the National Transportation Safety Board issues an official finding." While summarizing the leading theories, they're also calling it "the sort of baffling accident that makes structural engineers break out in sweats."
Slashdot reader McGruber writes: Interestingly, just two days after the collapse, an Anonymous Coward posted that post-tensioning likely led to the collapse of the bridge... A March 21, 2018 NTSB News Release said "The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed. They had done this same work earlier at the south end, moved to the north side, and had adjusted one rod. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed."
The Miami Herald reports that "how and where precisely the bridge broke apart likely won't be known for months, until the National Transportation Safety Board issues an official finding." While summarizing the leading theories, they're also calling it "the sort of baffling accident that makes structural engineers break out in sweats."
Is this figure correct:
Panagos, who seeks $15,000 in damages for physical and emotional injuries...
I'd imagine that any of the insurers would be happy to pay her $15K (or even more) if she'll accept that as their final liability.
In the elastic region, increasing strain increases stress / tension. When the member enters the plastic region, steel under tension starts to neck. In this region, increasing strain can result in decreased stress. Eventually, the member fails and you have lots of strain and no stress / tension.
When tensioning, the question every structural engineer must ask is: Am I in the elastic region? For sure?
Structural engineers tend to use ridiculously small assumptions for material strength to guarantee being in the elastic region. However, one good crack or subsurface fracture, and fracture can occur. High performance work tends to use fea to predict areas of stress concentration, and then eddy current, magnetic and x-ray inspections to prevent these failures. This is not common in structural applications.
For those of us who are not structural engineers, here's an good easy-to-read article (pdf) that explains it.
I eat only the real part of complex carbohydrates.
Way to go, armchair engineers!
AvE (crusty canadian enginerd on youtube) had a couple of interesting videos on this:
https://www.youtube.com/watch?...
https://www.youtube.com/watch?...
If you go back and look at the first Slashdot story on this when it happened, a number of people were thinking is was related to incorrect tension settings on the bridge (which had been pre-tensioned if I remember right, then they were trying to stress it even more...).
"There is more worth loving than we have strength to love." - Brian Jay Stanley
"...they're also calling it "the sort of baffling accident that makes structural engineers break out in sweats."
Uh, how exactly is this some kind of "baffling" mystery here? Instead of summarizing theories, let's review the facts:
"The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed...They were working on the second rod when the span failed and collapsed."
Seems pretty damn clear to me as to the cause of the collapse. Let's review the fuck-ups that lead to disaster and lives lost:
"...a[n ignored] voice message about cracks in the structure that were deemed superficial...Post-tensioning that modifies the stresses in a structure is inherently risky and should be performed "in the absence of traffic,"...The roadway was not closed while this work was being performed."
Seems pretty damn clear who fucked up and who should be held accountable here. Of course, this also happened in the United States, which means insurance companies are going to drag out pointless "investigations" for the next 2-3 years in order to keep millions in their coffers for as long as immorally possible.
Post-tensioning that modifies the stresses in a structure is inherently risky and should be performed "in the absence of traffic," the lawsuit claims.
As they say, it it was really common sense everyone would have it.
This actually highlights the problem with our legal system more than it does what caused the collapse. Lawyers have filed a lawsuit (i.e. are certain who is blame) while the investigation has barely started and is still collecting evidence, and is probably a year away from reaching a conclusion.
If you want to argue that the lawyers aren't certain, they just want be first to get their speculative lawsuit in, then that's yet another problem with our legal system. That the penalty for filing a frivolous lawsuit is so lacking that lawyers can file speculative lawsuits with impunity without a shred of evidence to back up their claim, gambling that such evidence might turn up in the future. Thereby forcing countless innocent defendants to waste money preparing a defense against lawsuits which never should have been filed in the first place.
If you know how this process works, and watch the video of the bridge falling apart like a stack of cards, it is kind of obvious.
Post tensioning is a bit of an art, especially on a non-redundant structure. I would think for a bridge with traffic running below they would be slowly tensioning the system in increments following the load path.
Maybe they set the "final" tension all at once?
https://www.vdare.com/posts/th...
There is a lot or pressure to give contracts to minority owned firms. If you ever enter into one of these government bidding competitions, the number of questions devoted to the diversity of your business is greater than questions related to your ability to fulfill the contract.
Possibly. However, from my understanding, the 2 PT bars in truss #2 at the other end of the bridge had been DEtensioned to some degree along with the upper PT bar in truss #11. Leaving the lower #11 PT bar remaining to be adjusted. May seem counter-intutive that DEtensioning both PT bars in #11 could have led to collapse. However, if there was already cracking in truss #11 and/or in lower chord (deck) and/or at base of the pier (vertical #12), the PT bars could have been what was helping to hold it together. Another theory to consider.
While some have questioned the choice of a large concrete truss-like structure for a pedestrian bridge, it's notable that truss #2 (which was enlarged from the preliminary drawings) on the other side of the bridge along with nearby support members remained relatively intact after collapse. Why was #2 enlarged, but not #11 is a question some are asking.
Also, it's important to note the bridge was shifted over 11 feet from its original position, as per Florida Department of Transportation to allow for a future lane, placing the pier on the north side closer to the canal necessitating last minute changes in late 2016. Could the soil there have compromised the pier on that side. Another aspect to consider.
In short, it appears the tensioning adjustment trigger the collapse, but likely not the underlying reason for failure.
Bad engineering - from office to execution.
If Post-tensioning is needed... it is easy execute but not trivial to be managed by non-experienced people.
I have been looking for a slow motion video of the failure to see where the first break was and the progression of the failure. Also was the bridge subject to a any loading at the time. The best I can tell is that left side failed away from the left end and might have been a shear failure. Concrete is generally stronger in compression in static loading than either tension or shear.
This picture from the Miami Herald article shows a design with no redundancy. If any one of those diagonal elements fails, the whole thing comes down.
And one did fail.
It seems strange to me that the pylon and its cables were not in place to provide support for the span. ..... You don't build the suspension part of a suspension bridge just for kicks...
From what I have heard, they did exactly that. The tower and "stays" were for decoration. Incredible. It was not just meant to be a bridge, it was meant to be a statement of ... er ... I'm not sure what it was meant to be a statement of (some have suggested of feminism or diversity), but anyway it was. It is certainly a statement of something now, but not the intended one.
My thoughts were, Why the FUCK wasn't the road closed during all post-tensioning. That would have been the smart thing to do.
No. Wait what! Betteridge was wrong? Yes actually the answer is clear that post-tensioning activities were quite likely what the collapse. That is pretty evident from the video footage showing small explosions either side of support member 11 followed about 2 frames later with the bridge starting to fall, along with the fact that crews were working on top of the member (and the only thing up there are the ends of the tensioning rods).
But that's the end of what we know. There's nothing about the root cause of the incident. Nothing about if this was a routine exercise that exposed some other flaw or that they screwed up the process itself, or maybe the designer screwed it up, who knows. That's what investigation teams are for and the many variables are why investigations generally take a long time.
I would say it's a bit premature for a lawsuit, but this is America. I'm surprised they don't just pre-emptively sue all engineering companies at this point and then just drop lawsuits if nothing goes wrong.
We dont need no steenking roads!
I've seen little focus on the concrete, other than the observation of some "minor cracking," prior to the failure.
One notable feature of the bridge is that it was THE FIRST IN THE WORLD built with "self cleaning concrete," most frequently done by adding TiO2 (Titanium dioxide) to the mixture. A little research suggests that concrete strength is negatively affected, on the order of 10% weaker with 6% TiO2, according to http://www.cipremier.com/e107_... Does someone know more precisely what concrete was used?
While most are focusing on a failure of the tensioning members, another cause may be a hinge failure at the point where the posts meet, where the concrete strength may be an important factor. In addition to the self-cleaning mix being weaker in general, other problems in the concrete pour or the concrete mix could create even weaker points, and as the hinge failure is at the top of the structure, presumably at the end of the pour, using the "bottom of the barrel" of the concrete mix which may differ from majority of the concrete produced.
The finding of facts in the report can be used. It is the interpretation of those facts which is out of bound of the court. E.g. if NSTB find out there was tension work of the rod that day, that there was a 21 inch V form crack on member 11/member 12, and that the road was not closed contrary to best practice, it can be used as a fact in the court. But any interpretation of those facts ("it was likely involved in the failure" / "it was not likely involved in the failure") will not be admissible.
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I wholeheartedly agree. The people who allowed traffic near the bridge while post-tensioning activities should (as a minimum) have their licenses revoked and be put on trial for manslaughter.
Running up to final tension on one joint before moving to another joint is a sure recipe for stress cracking. But, it is so much faster than going to an intermediate spec, removing hydraulic tensioners and moving to the next, and taking each point up incrementally. If you don't get it, check "torque pattern" for the lugs on your car tires for the basic concept.
NRRPT/RCT
I see, you mean youre a Clueless right wing nutjob spouting an irrelevant fantasy. Sad.
Making a personal attack on the opposition, is what the left is being accused of these days... ;-)
Are you sure you want to perpetuate the stereotype?
I think that there is a good reason why concrete truss bridges of this type are rare. The connections are fundamentally difficult to analyse correctly, although the member forces can be simply calculated by hand in a few minutes. I believe that an over-reliance on computer generated outputs may be an underlying cause of this failure. And a desire for "innovation" without wanting to pay the price in terms of thorough design reviews.