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The Ordinary Engineering Behind the Horrifying Florida Bridge Collapse (wired.com) 276

An anonymous reader quotes a report from WIRED: The people of Sweetwater, Florida were supposed to wait until early 2019 for the Florida International University-Sweetwater University City Bridge to open. Instead, they will wait about that long for an official assessment from the National Transportation Safety Board of why it collapsed just five days after its installation, killing at least six people. In the immediate aftermath of the disaster, many queries have centered on the unconventional technique used to build the bridge, something called Accelerated Bridge Construction, or ABC. But ABC is more complicated than its acronym suggests -- and it's hardly brand new. ABC refers to dozens of construction methods, but at its core, it's about drastically reducing on-site construction time. Mostly, that relies on pre-fabricating things like concrete decks, abutments, walls, barriers, and concrete topped steel girders, and hauling them to the work site. There, cranes or specialized vehicles known as Self-Propelled Modular Transporter install them. A video posted online by Florida International University, which helped fund the bridge connects to its campus, showed an SPMT lifting and then lowering the span into place.

In a now-deleted press release, the university called the "largest pedestrian bridge moved via SPMT in U.S. history," but that doesn't seem to mean much, engineering-wise. SPMTs have been around since the 1970s, and have moved much heavier loads. In 2017, workers used a 600-axle SPMT to salvage the 17,000 ton ferry that sank off the coast of South Korea in 2014. The ABC technique is much more expensive than building things in place, but cities and places like FIU like it for a specific reason: Because most of the work happens far away, traffic goes mostly unperturbed. When years- or months-long construction projects can have serious effects on businesses and homes, governments might make up the money in the long run. Workers installed this collapsed span in just a few hours. These accelerated techniques are also much safer for workers, who do most their work well away from active roads.
The report goes on to note that the bridge collapse is still under investigation and the search for a culprit is ongoing. "The answers could run the gamut, from design flaws to fabrication flubs to installation issues," reports WIRED. As of publication, The Washington Post is reporting that an engineer called the state to report cracking two days before its collapse.
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The Ordinary Engineering Behind the Horrifying Florida Bridge Collapse

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  • The usual pattern (Score:5, Informative)

    by 31eq ( 29480 ) on Saturday March 17, 2018 @09:17AM (#56274659) Homepage

    Like every engineering disaster, somebody found the problem, and failed to communicate its severity. In this case, they decided it wasn't a safety issue (cracks in a brand new bridge!) and left a voice mail with somebody else who was out of the office for a few days.

    There's no substitute for risk assessments by fully qualified engineers, of course. But those engineers also need communication skills â" including persuasive skills. Engineers who can find somebody in authority and convince them to take action save lives.

    • by Anonymous Coward on Saturday March 17, 2018 @09:27AM (#56274675)

      The question I havenâ(TM)t seen asked, much less answered, is this: the final bridge was supposed to be supported by cables from a central pier. Why was safe to install the sections without that support?

    • Did the bridge crack after being assembled? Were the prefabricated parts already cracked when they arrived on site?
    • by thegarbz ( 1787294 ) on Saturday March 17, 2018 @10:24AM (#56274871)

      Like every engineering disaster

      That isn't even remotely true. The vast majority of engineering disasters happen without identification and without warning. An even larger number of infrastructure disasters have nothing to do with engineering as well.

      somebody found the problem, and failed to communicate its severity

      Nope. Somebody found *a* problem and didn't know of the severity. Cracks don't mean much at all. It may or may not have had anything to do with the collapse.

      There's no substitute for risk assessments by fully qualified engineers, of course

      You're right. But you fail to realise that this takes time.

      • Conversely, for pretty much everything that's built, someone is going to point out that it'll fail/collapse/sink/burn down/whatever. It's easy enough to fish out the 1:1 correlation after the event, but that's because you have the luxury of ignoring the 9,999 false-positive warnings given for other engineering structures.
    • Re:The usual pattern (Score:5, Informative)

      by denbesten ( 63853 ) on Saturday March 17, 2018 @12:08PM (#56275331)

      Like every engineering disaster, somebody found the problem, and failed to communicate its severity. In this case, they decided it wasn't a safety issue (cracks in a brand new bridge!) and left a voice mail with somebody else who was out of the office for a few days.

      There's no substitute for risk assessments by fully qualified engineers, of course. But those engineers also need communication skills â" including persuasive skills. Engineers who can find somebody in authority and convince them to take action save lives.

      A recent press release [fiu.edu] reports that there was a meeting the morning before the collapse in which engineers and persons in authority concluded that the cracks did not compromise the structural integrity.

      So no, I don't think there was a problem with "failure to escalate". Really, the big message here is that one needs to reserve judgement until the facts have a chance to surface. There is an official investigation underway. Amongst other things, It will determine if the analysis of the cracks was accurate and it will also determine if an appropriate escalation process was followed.

      While waiting, we should be asking if other "civilians" are at risk due to the lessons we have not yet learned from this collapse. For example, we may temporarily decide to prohibit "civilians" from being underneath active construction sites until we better understand how to protect them.

    • by elrous0 ( 869638 )

      "Who's a weak force *now*, bitch?" responded gravity.

  • by Anonymous Coward on Saturday March 17, 2018 @09:21AM (#56274667)

    The engineering drawings I have seen show a tall tower with suspension supports coming down to support the span. Was the tower complete and the suspension in place when it broke or was that for completion later? Also the suspension when to both sides of the tower so it was balanced. Are there any pictures of the bridge while it was "good" to compare with the engineering drawing?
    See here for a 3d view of the design. As far as I could tell the tower, the section over the water and the suspension lines were not in place when it collapsed.
    https://youtu.be/Q2A1wS09p0k?t=5

    • by josiahgould ( 2401420 ) on Saturday March 17, 2018 @09:30AM (#56274681)
      No, the suspension tower and cables were not in place. I don't see how they expected a suspended span to survive without the suspension cables, but then again I wasn't paid to make the plans. I of course base all my findings on Bridge Construction Set for DOS.
    • Re: (Score:2, Informative)

      by Anonymous Coward

      A true cable-stayed bridge would be built out starting from a center pier. This was a truss bridge, self-supported. The stays are pipes to stiffen structure and harmonics in high winds, as well for aesthetics. See my other post for more details.

    • by hey! ( 33014 ) on Saturday March 17, 2018 @10:03AM (#56274783) Homepage Journal

      I don't know if there are any pictures, but the situation has been confused by media reports that workers were "tightening cables" at the time it collapsed. However it turns out the pylon for the suspension cables had not been erected yet; the workers were tightening cables that ran through the structure.

      It's impossible to say whether the cable-tightening, or the cracks reportedly found in the span, had anything to do with the collapse. It's possible that neither did; it's possible that both did. Engineering disasters tend to be complex and they take time to nail down.

      • by Anonymous Coward on Saturday March 17, 2018 @11:08AM (#56275033)

        They were tightening the cables that run through the structure - it's called post tension concrete - Concrete is strong in compression but weak in tension, so the idea is to preload the structure with large cables under tension - which squeezes the concrete.
        This is extremely common with slab on grade houses these days.

        If you consider a beam supported between two supports, there's a bending moment - the top is in compression, the bottom is in tension. If you externally apply a force to squeeze the ends together then you can make it so that the bottom is in compression and the top is in even more compression, both of which are easily handled by the concrete.

        In this bridge that is done by cranking cables through the bridge. In something like, say, the Roman Arch, it's done by having massive side supports. In Gothic cathedrals, it's flying buttresses.

        This isn't exactly new engineering - although the details change over time.

        • by hey! ( 33014 ) on Saturday March 17, 2018 @11:30AM (#56275137) Homepage Journal

          I assume that's what they were up to, but of course a bridge by its nature is more extreme in one direction. It may have had nothing to do with the collapse, it may have caused the collapse.

          People in these situations want fast answers, so they jump to conclusions based on reports of cracks, or cables being tightened. But good answers take time, and it's a fair bet by the time we get good answers the public's curiosity will have waned. Even if the things drawing our attention now are involved, they're embedded in a much more complex scenario than most people have the patience for.

        • And until they tightened those cables, the truss was weak. P(ost)TC that doesn't get T'd until after the truss is installed is a disaster waiting to happen.

          P(re)TC is the norm.

          They screwed up in how they chose to make/install this truss. Pretty obvious.

        • Thanks for the informative post.
    • They slide this 950 ton slab onto the pilings and congratulated themselves. Great job of building a suspension bridge without you know the suspension part that holds it up.

  • by Anonymous Coward on Saturday March 17, 2018 @09:28AM (#56274677)

    To start, it was a truss bridge, self-supported. The stays shown in final drawings are pipes for stiffening and harmonics in high winds.

    The structure was non-redundant. A failure of any truss was near guaranteed to lead to collapse. It's theorized by some that truss member #11 at the junction of the pier was initial failure point.

    In preliminary drawings, #11 is shown with no post-tensioning bars, but the actual construction shows it with two. While those bars in #11 may have been necessary due to the move, since the ends of the bridge were cantilevered (which is different than shown in the preliminary drawings), they likely weren't needed after placement; not needed to be post-tensioned, since #11 would be in high compression.

    It appears workers were post-tensioning #11 using a crane and other equipment attached to one of the post-tension rods. It appears tensioner (blue) and part of the bar is sticking out several feet in photos of the collapse. According to some, this likely lead to the collapse.

    • by McGruber ( 1417641 ) on Saturday March 17, 2018 @11:48AM (#56275233)

      Mod AC up!

      I'm a Professional Engineer, though not licensed in Florida nor am I an expert in concrete bridges. Based upon the pictures of the debris I've seen, the bridge was made using prestressed concrete. Prestressed concrete is an amazing material - the steel reinforcement inside the concrete is used to compress the concrete, which causes the concrete member to act like high strength steel when it is loaded with tension forces.

      AC's comment above explains what happened:

      In preliminary drawings, #11 is shown with no post-tensioning bars, but the actual construction shows it with two. While those bars in #11 may have been necessary due to the move, since the ends of the bridge were cantilevered (which is different than shown in the preliminary drawings), they likely weren't needed after placement; not needed to be post-tensioned, since #11 would be in high compression.

      It appears workers were post-tensioning #11 using a crane and other equipment attached to one of the post-tension rods. It appears tensioner (blue) and part of the bar is sticking out several feet in photos of the collapse. According to some, this likely lead to the collapse.

      In its final placement as a bridge, truss member #11 would be in compression, so there was no reason to prestress it. Per AC, the preliminary design drawings did not show the two post-tensioning rods in this member.

      My guess, based upon AC's post, is that when the builders decided to assemble the bridge using the accelerated technique, the designers realized that there would be tension forces on truss member #11 during the move.... so the design drawings were changed to add the two post tensioning rods to truss member #11.

      Once the bridge was in place, the construction workers evidently began tightening the two post tensioning members even more. Member #11 was already being loaded with compression forces, from the dead weight loading of the bridge.... and tightening the post tensioners would have placed more compressive loading onto member #11. Once the combined compressive loadings (from the dead weight and the tensioning) exceeded the compressive strength of the concrete, the concrete would fail (it makes a loud popping sound when it fails in lab tests) and the single, non-redundant truss would fail.

      If my speculation is correct, it will be interesting to see whether the Figg, the engineering firm that designed the bridge, or the construction contractor gets blamed.

      • by mikael ( 484 )

        Other stories say they were loading testing the bridge at the time. Pictures in the UK Daily Mail show that there was green crane that seems to have snapped the cable holding whatever it was carrying. That cable ended up wrapped around the street light away from the mashed up side of the bridge.

        • Pictures in the UK Daily Mail show that there was green crane that seems to have snapped the cable holding whatever it was carrying.

          Thanks for pointing me to the Daily Mail story. Their first video of the collapsing bridge shows workers on top of the bridge directly above the part of the truss that seems to fail and fall first.... their second video, from the perspective of a driver approaching the bridge, more clearly shows that the side of the bridge near the crane fell first. Their article then says:

          At a news conference Friday night, officials from the National Transportation Safety Board said they have just begun their investigation, and cannot yet say whether any cracking contributed to the collapse.

          They also said workers were trying to strengthen a diagonal member on the pedestrian bridge at Florida International University when it collapsed.

          Robert Accetta, the investigator-in-charge for the NTSB, said crews were applying post-tensioning force, but investigators aren't sure if that's what caused the bridge to fall.

          I assume the "strengthen a diagonal member" description is referring to post tensioning a prestressed concrete member.

          The story and

        • by pezpunk ( 205653 )

          i think the that's just Journalist Telephone. Some engineer probably said they were "tensioning a prestressed concrete member", meaning they were inducing stress to the truss by tightening the cables that run through the concrete, and the Journalist wrote "stress test" and that got into print.

      • This sounds more like a problem with this technique in general. There should never be any confusion about whether something needs to be done. Especially when doing it twice will lead to a catastrophic failure. With car tires you can measure the current pressure and there is a label on the tire indicating the maximum pressure. Is there nothing similar for post tensioning?
        • by guruevi ( 827432 )

          Yes and no, there is no 'gauge' you can check although you can check the post tensioning being done twice. It's typically what you pay architects and engineers for, not only to design but also check and double check the work during and after construction.

          A lot of construction companies these days however rather have it done quickly and rather not deal with 'overhead'.

          I was involved in a construction project also with prefab pieces where the architect had specified a different size for some of the levels and

      • by Ryanrule ( 1657199 ) on Saturday March 17, 2018 @01:56PM (#56275727)
        He just watched the ave video is all. https://www.youtube.com/watch?... [youtube.com]
      • "the designers realized that there would be tension forces on truss member #11 during the move"

        Any "designer" (which includes the engineers) that would fail to realize that bridge components would have forces imposed on them during transport doesn't deserve to work in engineering or construction. Then again, maybe all the experienced and competent (and expensive) people have been funemployed in favor of cheaper, less-experienced software button-pushers who just think they are competent.

    • But they had CAD, 3D models and renderings and no doubt expensive, sophisticated stress analysis software. How could this have possibly happened?

    • A new youtube video [youtube.com] from a dash cam seems to support to theory that truss #11 fractured.
  • It will be interesting as one of the prime's MCM, has had issues in the past. MCM gives generously to Christie and Rubio.

  • by Applehu Akbar ( 2968043 ) on Saturday March 17, 2018 @09:42AM (#56274725)

    In my town we have a footbridge that was installed this way, several years ago. It was factory-built in Phoenix, hauled 100 miles up I-17 using one of the smaller roadable version of the SPMT, and installed overnight to cross a creek. There hasn't been a problem since.

    Offsite construction should be safer than site-built, so in this disaster let's focus on the design itself, rather than rushing to judgement on the offsite construction.

    • by c ( 8461 ) <beauregardcp@gmail.com> on Saturday March 17, 2018 @09:57AM (#56274757)

      Yeah, we have a couple where I work. Footbridges over a four lane highway. They seem solid... the newest one's been hit by dump and garbage trucks a couple times without any issue. But they're designed to be self-supporting rather than depending on external cable towers.

      My gut feeling is that if they'd just put a central support beam on that median in the center of the bridge it would've made quite a difference.

    • so in this disaster let's focus on the design itself, rather than rushing to judgement

      That is the most ironic statement I've seen in a while here.

      How about we don't pass judgement at all, and focus on the incident and all parts of it. A large majority of infrastructure failures have nothing to do with their design.

    • In my town we have a footbridge that was installed this way, several years ago. It was factory-built in Phoenix, hauled 100 miles up I-17 using one of the smaller roadable version of the SPMT, and installed overnight to cross a creek. There hasn't been a problem since.

      I'm sure your bridge was very economic, quite functional and an asset to your community.... but it is not an artistic "signature bridge" a University would demand for its main entrance.

      Here's the collapsed bridge designer's website: Figg Engineers, Creating Bridges as Art [http]

    • If 6 people died while we were inventing a new technique that improves construction for a decade, it's a sad growing pain. If 6 people died because of a fuckup in traditional design, it's horrific.

  • engineering-wise. SPMTs have been around since the 1970s, and have moved much heavier loads

    Depends what you call a SPTM. Bridges have been prefabricated and moved into place since pre-historic times, starting with carved tree trunks. It does not matter how the bridge gets put there, what matters is its strength and that of its supports. Here is a much more ambitious construction, overseen by Robert Stephenson and Brunel no less, over dangerous water too, 150 years ago :- Britannia Bridge, Menai Strait [astronet.ru]

  • The complexity and risk of the intermediate stages of a project are the most impressive things to me about civil engineering. It's one thing to design a dam, it's another to build that dam in the middle of an actual river; you need a dam to build a dam. It's one thing to draw a bridge that's perfectly stable once built, it's another to ensure the partially completed structure is in perfect equilibrium at every point from the time the moment the ground is cleared until the last cable is tightened.

    If you

    • Sort of. You don't trust a dam to hold back water until it's completed; you divert the water away from it using another method until the dam is done. You don't have cars driving over a half-completed bridge; you divert the traffic away until the cars are done. Construction sites are somewhat more dangerous than completed buildings; that's why workers wear safety gear and get safety inspections to make sure they're not doing anything too awfully stupid and risky.

      Making a half-completed project stable enoug
      • by PPH ( 736903 )

        BEFORE cars started driving on it.

        I hope not. It was a pedestrian bridge. And as such, the 'massive load' it was designed to bear was it's own weight. Pedestrians would have been insignificant load compared to the bridge's own 950 ton weight.

  • AvE explanation (Score:5, Informative)

    by CptLoRes ( 4510239 ) on Saturday March 17, 2018 @11:19AM (#56275075)
    The possible explanation to the failure (over tightening of one or more post tension rods) as detailed in this AvE video, seem to make sense. https://www.youtube.com/watch?... [youtube.com]
  • ... we hire the folks at http://11foot8.com/ [11foot8.com] to consult on building more robust bridges.

  • by McGruber ( 1417641 ) on Saturday March 17, 2018 @12:12PM (#56275355)

    The Reason Foundation has published an article about the bridge was funded by a federal program that has come under repeated fire for awarding money based on politics rather than merit:

    Collapsed FIU Bridge Was Funded by Federal Grant Program Criticized for Shoddy, Politicized Review Process - The TIGER grant program has come under fire for putting politics ahead of technical concerns. [reason.com]

  • by rush2049 ( 980385 ) <ben,raush&gmail,com> on Saturday March 17, 2018 @12:41PM (#56275453)
    AvE over on youtube did a very good commentary on how and why the bridge collapsed. Video 1 (overview): https://www.youtube.com/watch?... [youtube.com] Video 2 (technically in depth): https://www.youtube.com/watch?... [youtube.com]
  • In my opinion, it was wrong to keep the road open until the 1000 tons bridge was fully functional and well tested.

    But they could not close the road because there are basically no other modes of transportation. No bicycle routes worth mentioning, no pedestrian alleys, no trams.

    Close the road for two days and the town collapses.
    • by MS ( 18681 )

      On March 15th, the road got closed with a wall of concrete rubble. That was more than 2 days ago. Did the town collapse?

  • Were they sure EVERYONE working on the project knew it was a truss bridge?

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