Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!

 


Forgot your password?
Close
typodupeerror
×
United States Science

Was The Florida Pedestrian Bridge Collapse Triggered By Post-Tensioning? (enr.com) 138

Posted by EditorDavid from the striking-a-chord dept.
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."
This discussion has been archived. No new comments can be posted.

Was The Florida Pedestrian Bridge Collapse Triggered By Post-Tensioning? More

Was The Florida Pedestrian Bridge Collapse Triggered By Post-Tensioning?

Comments Filter:

  • 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.

    • She might. The families of the people who got pancaked probably won't.

    • Or in other words, grab $15K before the NTSB report comes out, as a hedge bet against the report saying it was unforeseeable and nobody can be sued for it.

      • Re:$15,000? (Score:4, Informative)

        by Richard_at_work ( 517087 ) on Saturday March 31, 2018 @05:20PM (#56360565)

        NTSB reports cannot be submitted as evidence into any court case, civil or criminal, in the US, by either side. They cannot be used to support a prosecution or action, and they cannot be used to defend against a prosecution or action.

        • Eh, we're both half right.

          I'll admit I confused the issue in my earlier post, but a quick bit of research straightened me out. Apparently, NTSB factual reports are admissible, but final reports are not [wilsonelser.com]. The factual reports that say who followed proper procedures would still be pretty damning to a lawsuit.

        • 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.

      • Re: (Score:3)

        by rtb61 ( 674572 )

        Obviously you have not idea how good structural engineering design works. Failure should never result in collapse but in excess deflection and cracks, at design load, at that time you should stop using the structure and then analyse the failure for possible repair, but generally due to the alteration of the materials especially steel, when it deforms https://en.wikipedia.org/wiki/... [wikipedia.org] it gets stronger due to altered crystalline structure but becomes more brittle and is likely to fail completely. So they saw

        • Obviously you have no idea how a logical claim works. What reliable evidence is there that "they saw cracks in the bridge and did nothing" or that "they took no action"? You seem to be jumping to the conclusion that the builders saw nice obvious warning signs, but to date I haven't seen any such detail in official reports, though it has been repeated in many "armchair analysis" posts by folks eager to dole out the blame.

          Fortunately, the court system does not (intend to) work that way. Sure, anyone can bring

        • Don't comment on what you clearly don't understand. Cracking is inherent in all concrete structures. It's the size and location of cracking that may be of concern to the structure. The cracking may be unrelated to the eventual failure.

  • Plastic stress strain curve (Score:5, Interesting)

    by Cassini2 ( 956052 ) on Saturday March 31, 2018 @12:53PM (#56359421)

    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.

    • Re:Plastic stress strain curve (Score:5, Interesting)

      by burtosis ( 1124179 ) on Saturday March 31, 2018 @01:51PM (#56359667)
      As a mechanical engineer, I'd just like to point out that you should place emphasis on it is crack or subsurface fracture in the tensioning member and not the concrete. The news media was making waves about some visible cracking in the concrete which would not necessarily be a concern here as the whole idea is concrete is quite strong in compression. A crack that was stable under compression, not allowing movement, would simply be compressed together and still retain structural integrity.

      • Depends on the cause of the crack...

        • Re:Plastic stress strain curve (Score:5, Informative)

          by burtosis ( 1124179 ) on Saturday March 31, 2018 @03:07PM (#56360011)
          Not directly. It depends on the shapeof the crack. If it was orthogonal to the bridge, structural strength would be just fine. It's analogous to setting one brick on another with a weight on it and still supporting the load. Now if it was a clean radius, say by two cracks in a V, so the V chunk pops out below a center tension bar, it is free to bend like a hinge. Now if it was cracked because it was dropped, and the center member damaged, this could be a failure of the tensioning member. But if they looked at it and it looked ok, I doubt the crack was a problem itself.

      • The news media was making waves about some visible cracking in the concrete which would not necessarily be a concern here as the whole idea is concrete is quite strong in compression. A crack that was stable under compression, not allowing movement, would simply be compressed together and still retain structural integrity.

        Except when you consider buckling. As far as I can make out from the accounts, videos and artist's impressions, what they were pretensioning was a concrete strut under compression. You need to keep the resultant compressive force in a concrete or masonery strut within the centre third in order to avoid tension at the outside edge and hence potential bucking, which would of course be preceeded by cracks there. The pre-tensioning may well have been meant to keep the resultant compressive force within the m

        • Those would be reasonable things to look at. The first case would be improperly tensioned members and the cracks just a sign of what was happening and not necessarily the main problem. The second case you mention the cracks would be secondary to the compressive strength of the concrete or possibly unrelated if it was a rebar issue as well. Cracks in concrete aren't always a big deal and the explanation that they were checked and fine sounds plausible. Additional evidence is that people heard an extre

    • Re:Plastic stress strain curve (Score:5, Informative)

      by Woldscum ( 1267136 ) on Saturday March 31, 2018 @02:29PM (#56359843)

      AVE on YouTube called it on March 16. Very informative visual demo.

      " I ran a test to see why the post tension rod was sticking out of the rubble. There was a problem with cracking on the pylon side of the bridge."
      https://youtu.be/KtiTm2dKLgU [youtu.be]

      • I just linked this up a ways, It was a good video, the one before it also! Best person on youtube!

      • AVE on YouTube called it on March 16.

        AVE on YouTube made a video about it on March 16. It was called by the civil engineers in the forum he linked to in his video description. It's well worth a read as well as our foul mouthed Canadian really simplifies what has turned into a very long multi page discussion on how and why.

      • Looks like that is what happened, also there were reports of a loud bang as it fell which could be the tensioning member giving out. It comes down to where exactly it went wrong. As a side note I'm putting the same note on my oscope lol. Definately has personality while being pretty accurate.

  • Post-tensioning (Score:5, Informative)

    by Megahard ( 1053072 ) on Saturday March 31, 2018 @12:56PM (#56359431)

    For those of us who are not structural engineers, here's an good easy-to-read article [dsiamerica.com] (pdf) that explains it.

    • Re:Post-tensioning (Score:4, Informative)

      by Anonymous Coward on Saturday March 31, 2018 @01:43PM (#56359633)

      The short explanation is that concrete can handle very high compression, but fails quickly under tension. For post-tensioning, there are steel rods going through the concrete between anchors at the ends of the concrete part. When these rods are tensioned, they compress the concrete, so that any loads on the concrete at most lower the compressive force on the concrete, but don't cause it to go into tension. When a rod is overtensioned, it breaks and removes that compressive bias on the concrete. This weakens the concrete immensely and it breaks.

    • Re: (Score:2)

      by kqc7011 ( 525426 )
      Here is another good video https://www.youtube.com/watch?... [youtube.com]

  • Rush to Judgement (Score:1)

    by Anonymous Coward

    Way to go, armchair engineers!

  • AvE (Score:3, Informative)

    by DrewFish ( 23138 ) <drewNO@SPAMfolta.net> on Saturday March 31, 2018 @01:13PM (#56359505) Homepage

    AvE (crusty canadian enginerd on youtube) had a couple of interesting videos on this:
    https://www.youtube.com/watch?... [youtube.com]
    https://www.youtube.com/watch?... [youtube.com]

    • Re:AvE (Score:4, Insightful)

      by DCFusor ( 1763438 ) on Saturday March 31, 2018 @04:39PM (#56360419) Homepage
      It was obvious to me he got it right, actually, and I AM an engineer. And it was most likely caused by some construction worker tightening something that was supposed to be tight already and no be messed with in the field in the absence of an engineer. When it yielded instead of getting tighter...boom. Now, why someone thought it needed that is another question. Concrete gets harder and drier for *years* after initial set. Wanna bet someone moved it too soon and too wet?

      • Could have been poor drawings or erection procedures also.

      • And it was most likely caused by some construction worker tightening something that was supposed to be tight already and no be messed with in the field in the absence of an engineer.

        Oh oh oh, you should get one of these: http://www.israellycool.com/20... [israellycool.com]

        Tensioning is not some fun cowboy activity don't by some bearded tattooed builder by his feels. You don't just make something tighter. You adjust it to the specification using calibrated hydraulic equipment. There are any number of things that could go wrong here. Time will tell if you had a really lucky guess or if it was one of the many other causes of tensioning going wrong than "some construction worker tightening something that was

      • Yea the bar popped out 6 to 8 feet and still has the hydraulic cylinder on it. Plus the extremely loud bang people reported hearing.

      • It was obvious to me he got it right, actually, and I AM an engineer. And it was most likely caused by some construction worker tightening something that was supposed to be tight already and no be messed with in the field in the absence of an engineer. ...

        This is what I see as the problem, too. Those extra two rods were added, apperently to help hold the ends when they decided to put the wheeled trucks in the wrong place. They would have been tensioned -before- the move, and the main rods undeneath loose during the move. When the bridge was placed in position, the lower rods would be tensioned and then the wheeled trucks removed. Then the two extra rods would be -detensioned- (as in loosened), since the diagonal strut would always be in compression after tha

  • 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...).

    • Re: (Score:2, Interesting)

      by burtosis ( 1124179 )
      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.

  • How is this a "baffling" mystery to solve? (Score:5, Insightful)

    by geekmux ( 1040042 ) on Saturday March 31, 2018 @01:16PM (#56359533)

    "...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.

    • The cracks do not seem like anything, the voice mail was ignored but other people looked at them.

      Adjusting the tension exactly when the span collapsed, and as you say when traffic was underneath is obviously a major screw up and there is going to be hell to pay for that.

      • The cracks were likely inconsequential because the concrete was to be used in compression and not tension.

    • Seems pretty damn clear who fucked up and who should be held accountable here.

      Then please share for those of us less enlightened. I mean in your quote you already mentioned at least two separate people, but since you've done the investigation I'm sure you can also tell us about:
      - How you've reviewed the engineering drawings and design to ensure that it was designed correctly.
      - How you've reviewed construction to ensure that construction was as per design.
      - How you've reviewed the composition of materials.
      - How you've reviewed that pre-tensioning was completed correctly.
      - How you've r

      • Seems pretty damn clear who fucked up and who should be held accountable here.

        Then please share for those of us less enlightened. I mean in your quote you already mentioned at least two separate people, but since you've done the investigation I'm sure you can also tell us about: - How you've reviewed the engineering drawings and design to ensure that it was designed correctly. - How you've reviewed construction to ensure that construction was as per design. - How you've reviewed the composition of materials. - How you've reviewed that pre-tensioning was completed correctly. - How you've reviewed that there was no damage during movement. You're clearly a structural engineer with detailed knowledge of the shape and effect of the crack on the bridge too I see, so likely you've reviewed the fact that the previous engineers were wrong when they looked at the cracks, otherwise you wouldn't have quoted that part about an "ignored" voice mail that wasn't so much ignored as it was discussed in committee for an hour. But you knew that already right?

        While we're at it, why not tell us about the process. Apparently you know the exact failure so what was it? Was the tensioning done incorrectly? Was it equipment which failed? Was the hydraulic system calibrated properly? I mean it's amazing that you know all this, last I heard people weren't even sure if they were tensioning or detensioning.

        But please enlighten us on all the above since you know the cause of it. If there's anything there you don't know then how are you certain that you actually found the cause and who's at fault? By the way you should become a super incident investigator since you've solved in mere days what normally takes months to identify. Kudos to you!

        There is no doubt an investigation to be performed, but where you are wrong is that this will not take mere "months" to investigate. It will be dragged out by insurance companies for years, which reinforces one of my main points here. NOT having the sense to clear traffic while performing an "inherently risky" operation is the most obvious fuck-up, which Common F. Sense would hope there is already a regulation on the books that can be cited to find fault quickly and efficiently. Reading about an unused b

        • NOT having the sense to clear traffic while performing an "inherently risky" operation is the most obvious fuck-up

          And is also not a root cause which is why investigations actually take months despite your claim to the contrary. The point of investigations is not to come up with what happened, but rather the reason of why it happened. So... since you know it already please share with us the detailed assessment that lead to them not closing the bridge including the risk assessment that was performed. After all I will humour you and assume you understand that post tensioning comes in a great variety of different forms, so

  • 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.

    • Re: You think? (Score:1)

      by Anonymous Coward

      As a practicing forensic structural engineer with over 35 years experience I can tell you that post tensioned concrete is generally no more risky than any other building material. An attorney asserting so is simply an posturing for the benefit of their client.

      With respect to the assertion that post tensioning requires concrete to always be in compression, this is a false statement. The post tensioning induces compression to limit the tensile stress in the concrete to acceptable levels.

      With respect to the o

  • Highlights the problem with our legal system (Score:5, Interesting)

    by Solandri ( 704621 ) on Saturday March 31, 2018 @01:44PM (#56359635)
    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.

    • 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.

      Indeed. FTFA "until the National Transportation Safety Board issues an official finding". Surely the claimants are putting their case at risk in that if the finding is something other than "a crew was post-tensioning bars", a shit design for example or crappy concrete, then won't they have to drop their case (or give their money back if they have already been awarded damages).

      If I were the judge, I would simply defer the case until the official findings have been issued.

      • NTSB findings and reports can't be used in a court of law - neither side can submit NTSB findings, so there's absolutely no benefit to waiting to file for either side.

      • Re: (Score:2)

        by Agripa ( 139780 )

        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.

        Indeed. FTFA "until the National Transportation Safety Board issues an official finding". Surely the claimants are putting their case at risk in that if the finding is something other than "a crew was post-tensioning bars", a shit design for example or crappy concrete, then won't they have to drop their case (or give their money back if they have already been awarded damages).

        The NTSB findings are inadmissible in court so irrelevant. Evidence that the NTSB finds is admissible.

    • Re: (Score:1)

      by Anonymous Coward

      You have no clue how the legal system works, I mean really and fundamentally. Stop giving people your opinion.

      The civil claim is filed to preserve the right, and all reasonably liable parties are named at that time, or you can lose your right to claim them. They reasonably know they were harmed, and that one of those specific parties is liable. There is nothing frivolous (ie. it is not brought to delay, harass or for false purpose) about the claim. It is just that you don't understand how the system wor

      • Re: (Score:1)

        by tomhath ( 637240 )

        The civil claim is filed to preserve the right, and all reasonably liable parties are named at that time, or you can lose your right to claim them

        Bullshit. The claim was filed now so if/when it becomes a class action suit that lawyer gets the big paycheck. This has nothing to do with compensation for the victims.

        • Re: (Score:1)

          by Anonymous Coward

          >Bullshit. The claim was filed now so if/when it becomes a class action suit

          You really have no idea what you are talking about. This is not a class action case. You're just randomly whining. Keep watching Erin Brockovich over and over and leave /. alone.

    • Re: (Score:2)

      by Agripa ( 139780 )

      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.

      Filing the lawsuit officially notifies the parties that evidence may not be destroyed.

  • How does the State award contracts? (Score:2, Interesting)

    by Anonymous Coward

    https://www.vdare.com/posts/th... [vdare.com]

    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.

    • Re: (Score:1)

      by Anonymous Coward

      What's wrong with not hiring the most qualified people?

  • 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.

    • The only thing i have seen seems like cell phone video. good luck getting enough FPS to be able to slow it down to anything viewable.

    • Re: (Score:2)

      by robbak ( 775424 )

      The threads on eng-tips.com (https://www.eng-tips.com/viewthread.cfm?qid=437029) are a good read. They seem to be focusing on the join at the bottom of the 11th member of the structure, at the far end where the collapse occurred, and the video seems to suggest that the 11th member broke free at the bottom. An interesting point about that junction is that both the load of the bridge and the tension of the rods were both working in the same direction, placing the same shear force on a join in the concrete.

      But

  • This picture [miamiherald.com] 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.

  • Dumbing down of Universities is the reason because of the student loan programs. Create a system with a gov bureaucracy profiting , banks profiting and school administrators more concerned with asses-on-seats rather than higher learning. Good job leftists - again.

  • 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 o

  • 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_... [cipremier.com] Does someone know more precisely what concrete was used

  • 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.

Slashdot Top Deals

Beware of all enterprises that require new clothes, and not rather a new wearer of clothes. -- Henry David Thoreau

Close