12 ms·

Experts dissect FIU bridge collapse, but can't identify cause

45 points by wglb 9y ago

deleted 9y ago
[deleted]
cryptonector 9y ago
AvE had a good proximate cause: https://www.youtube.com/watch?v=KtiTm2dKLgU https://www.youtube.com/watch?v=KtiTm2dKLgU https://www.youtube.com/watch?v=ioC61QW7SHQ https://www.youtube.com/watch?v=ioC61QW7SHQ
- tristanj 9y ago
  The article cites AvE's youtube video and brings up some skepticism about his hypothesis. His explanation is one of several and it's not yet conclusive.
  - smoyer 9y ago
    Watching that video he talks about the column and cables in the middle of the bridge supporting it after construction - The design doesn't include a pier under that tower so can someone who's more mechanically inclined describe how it even helps the strength of the bridge? From my perspective, the more you tension those support cables, the more you're going to bow the center of the span towards the roadway.
    - deleted 9y ago
      [deleted]
    - wlesieutre 9y ago
      Video doesn't show it from a good angle; there is a pier under the tower. See rendering partway down this page: https://news.fiu.edu/2018/03/community-gathers-to-watch-950-ton-bridge-move-across-southwest-8th-street/120395 https://news.fiu.edu/2018/03/community-gathers-to-watch-950-...
  - dogma1138 9y ago
    To be fair AvE is not a structural engineer he’s a very entertaining tinkerer but I would hope that there are actually qualified people investigating it and I’m not entirely sure why the media is even picking up on folks like AvE.
- nkurz 9y ago
  I thought this was a helpful video as well: https://www.youtube.com/watch?v=jOe7CvVGqz4 https://www.youtube.com/watch?v=jOe7CvVGqz4 It's a slow motion photogrammetry of the dashcam video of the collapse showing the bridge clearly failing first at the left side. At least one of the two is probably coincidence, but the point of failure on the lower deck seems to be at the same location that the transporter truck was modified to use, and on the upper canopy at the location where they were doing the tensioning. It's a nice accompaniment for the eng-tips thread linked from the article: http://www.eng-tips.com/viewthread.cfm?qid=437029 http://www.eng-tips.com/viewthread.cfm?qid=437029
  - cryptonector 9y ago
    Thanks for the link!
tristanj 9y ago
This is a really thorough article that explains the reasoning behind the design of the bridge and looks at multiple reasons why the bridge collapsed. I found the animated concept video of what the completed bridge would look like very illuminating [0]. I've never seen a bridge made of concrete/rebar designed like this before. Novel designs are more likely to fail than tried and tested ones; if they built the bridge in a more conventional style I doubt it would have collapsed. Innovation is not always free. [0] http://www.miamiherald.com/news/local/community/miami-dade/west-miami-dade/article206092509.html http://www.miamiherald.com/news/local/community/miami-dade/w...
- cryptonector 9y ago
  Per-AvE, it's not that the design was necessarily bad. AvE thinks the proximate cause was that the installation plan was changed on-site due to the sort of changed circumstance that the designers didn't know about. See the videos (I posted links already below/above).
  - function_seven 9y ago
    Right. I think you could say that the unusual design contributed to the failure, if only because the on-site changes were made using assumptions that would have worked on a more conventional design. It's kind of like batteries. It used to be that you'd want to discharge battery cells completely before recharging them. This was true for Ni-Cad cells. With newer Li-ion cells, doing this is actually dangerous. It's not necessarily the chemistry's fault when an over-discharged cell starts a fire later on, but if you substitute a new design, you'd better cover the new rules with the implementation people.
StreamBright 9y ago
I used to study mechanics during my mechanical engineering studies and one of my teacher kept saying "the bridge will collapse" as a warning to all of us to take this very seriously. Usually these bridge designs use a factor of safety 5-7. I am really curious what went wrong with this one though. Some of the videos linked in this post are pretty good explaining the basics of reinforced concrete structures.
- TheSpiceIsLife 9y ago
  Do you mean to imply he was saying this particular bridge will collapse, or that bridges in general do collapse?
  - jakeogh 9y ago
    It's like saying all hard drives fail. Kinda missing the point in this situation (HD's fail inside a lifetime, bridges like this should last multiple).
  - StreamBright 9y ago
    As a warning that if we do our jobs sloppy there will human life loss.
    - TheSpiceIsLife 9y ago
      Right, yep, gotcha, thanks for clearing that up. Yeah, I’ve had teachers who used to have similar proclivities.
- wiredfool 9y ago
  Factor of safety isn’t really uesd any more, it’s really a combination of load factors and resistance factors. However, the ‘factor of safety’ would have generally been in the range of 2, not 5-7. Now, there’s dead load, live load, wind, etc, and they’ve all got different uncertainty factors. Resistance factors are assigned based on the variability of the material, concrete more than steel, and fracture critical higher than redundant.
OrganicMSG 9y ago
We may not know the cause of the collapse, however given that it was a new construction method being tested, surely the cars should never have been allowed to be there until the span had finished being surveyed. Is a new thing, plan for it failing until you are very sure it won't.
- Piskvorrr 9y ago
  That's the thing though: "we're very sure this won't kill people" is always a probability.
morsch 9y ago
The bridge was meant as a symbolic new portal to the university, not just a way to safely cross the dangerous roadway. It would have open views, benches, planters, glass-enclosed elevators, Wi-Fi and a wide 30-foot deck for pedestrians and cyclists. As a pedestrian and a cyclist, the last thing I want to do on your new bridge over a busy six-lane road is stick around on a bench perusing the wifi.
- creaghpatr 9y ago
  Georgia Tech has this over the 85/75 connector and it works better than you would think. It's a popular tailgate spot on gamedays.
jakeogh 9y ago
Having to quibble about why it failed means the margins were _way_ too low. It's a semi-permanent structure, if it costs double (and it wont) more to make it redundant in the worst case (loss of multiple main supports for example) then that's a good thing. Skyscrapers for are built with extensive redundancy, hence the pre-weakening needed before bringing one (or 3) down. I bet when the NTSB is done we find out that this bridge wasn't actually up to code.
snarfy 9y ago
"What we can say is that when complex technological systems fail, usually no single factor is to blame. In her study of the Challenger Space Shuttle disaster, the sociologist Diane Vaughan noted a phenomenon she called the “normalization of deviance,” an acceptance of assumptions and shortcuts that over time incrementally piles on risk until, like compounding interest on debt, a kind of technological bill suddenly becomes due." [1] [1] - https://slate.com/business/2018/03/the-florida-bridge-collapse-shows-how-accelerated-bridge-construction-can-go-wrong.html https://slate.com/business/2018/03/the-florida-bridge-collap...
anonymous2381 9y ago
Not only did this modern design fail and kill people, it's ugly: https://news.fiu.edu/wp-content/uploads/bridge-model.jpg https://news.fiu.edu/wp-content/uploads/bridge-model.jpg Compare with the Millennium Bridge in London, which is also modern and ugly, and was also originally faulty, but was fixed before it could kill anyone: https://en.wikipedia.org/wiki/Millennium_Bridge,_London https://en.wikipedia.org/wiki/Millennium_Bridge,_London
coldcode 9y ago
Generally civil engineering projects fail for three reasons, incorrect or substituted materials, or incorrect or poor installation practices, or poor or fraudulent inspections of those. Rarely does something fail due to poor engineering these days. Sometimes it is hard to identify the exact cause after a failure since the failure may obliterate the cause.

45 points by wglb 9y ago

deleted 9y ago

[deleted]

AvE had a good proximate cause: https://www.youtube.com/watch?v=KtiTm2dKLgU https://www.youtube.com/watch?v=KtiTm2dKLgU https://www.youtube.com/watch?v=ioC61QW7SHQ https://www.youtube.com/watch?v=ioC61QW7SHQ

tristanj 9y ago
The article cites AvE's youtube video and brings up some skepticism about his hypothesis. His explanation is one of several and it's not yet conclusive.
- smoyer 9y ago
  Watching that video he talks about the column and cables in the middle of the bridge supporting it after construction - The design doesn't include a pier under that tower so can someone who's more mechanically inclined describe how it even helps the strength of the bridge? From my perspective, the more you tension those support cables, the more you're going to bow the center of the span towards the roadway.
  - deleted 9y ago
    [deleted]
  - wlesieutre 9y ago
    Video doesn't show it from a good angle; there is a pier under the tower. See rendering partway down this page: https://news.fiu.edu/2018/03/community-gathers-to-watch-950-ton-bridge-move-across-southwest-8th-street/120395 https://news.fiu.edu/2018/03/community-gathers-to-watch-950-...
- dogma1138 9y ago
  To be fair AvE is not a structural engineer he’s a very entertaining tinkerer but I would hope that there are actually qualified people investigating it and I’m not entirely sure why the media is even picking up on folks like AvE.
nkurz 9y ago
I thought this was a helpful video as well: https://www.youtube.com/watch?v=jOe7CvVGqz4 https://www.youtube.com/watch?v=jOe7CvVGqz4 It's a slow motion photogrammetry of the dashcam video of the collapse showing the bridge clearly failing first at the left side. At least one of the two is probably coincidence, but the point of failure on the lower deck seems to be at the same location that the transporter truck was modified to use, and on the upper canopy at the location where they were doing the tensioning. It's a nice accompaniment for the eng-tips thread linked from the article: http://www.eng-tips.com/viewthread.cfm?qid=437029 http://www.eng-tips.com/viewthread.cfm?qid=437029
- cryptonector 9y ago
  Thanks for the link!

tristanj 9y ago

This is a really thorough article that explains the reasoning behind the design of the bridge and looks at multiple reasons why the bridge collapsed. I found the animated concept video of what the completed bridge would look like very illuminating [0]. I've never seen a bridge made of concrete/rebar designed like this before. Novel designs are more likely to fail than tried and tested ones; if they built the bridge in a more conventional style I doubt it would have collapsed. Innovation is not always free. [0] http://www.miamiherald.com/news/local/community/miami-dade/west-miami-dade/article206092509.html http://www.miamiherald.com/news/local/community/miami-dade/w...

cryptonector 9y ago
Per-AvE, it's not that the design was necessarily bad. AvE thinks the proximate cause was that the installation plan was changed on-site due to the sort of changed circumstance that the designers didn't know about. See the videos (I posted links already below/above).
- function_seven 9y ago
  Right. I think you could say that the unusual design contributed to the failure, if only because the on-site changes were made using assumptions that would have worked on a more conventional design. It's kind of like batteries. It used to be that you'd want to discharge battery cells completely before recharging them. This was true for Ni-Cad cells. With newer Li-ion cells, doing this is actually dangerous. It's not necessarily the chemistry's fault when an over-discharged cell starts a fire later on, but if you substitute a new design, you'd better cover the new rules with the implementation people.

StreamBright 9y ago

I used to study mechanics during my mechanical engineering studies and one of my teacher kept saying "the bridge will collapse" as a warning to all of us to take this very seriously. Usually these bridge designs use a factor of safety 5-7. I am really curious what went wrong with this one though. Some of the videos linked in this post are pretty good explaining the basics of reinforced concrete structures.

TheSpiceIsLife 9y ago
Do you mean to imply he was saying this particular bridge will collapse, or that bridges in general do collapse?
- jakeogh 9y ago
  It's like saying all hard drives fail. Kinda missing the point in this situation (HD's fail inside a lifetime, bridges like this should last multiple).
- StreamBright 9y ago
  As a warning that if we do our jobs sloppy there will human life loss.
  - TheSpiceIsLife 9y ago
    Right, yep, gotcha, thanks for clearing that up. Yeah, I’ve had teachers who used to have similar proclivities.
wiredfool 9y ago
Factor of safety isn’t really uesd any more, it’s really a combination of load factors and resistance factors. However, the ‘factor of safety’ would have generally been in the range of 2, not 5-7. Now, there’s dead load, live load, wind, etc, and they’ve all got different uncertainty factors. Resistance factors are assigned based on the variability of the material, concrete more than steel, and fracture critical higher than redundant.

OrganicMSG 9y ago

We may not know the cause of the collapse, however given that it was a new construction method being tested, surely the cars should never have been allowed to be there until the span had finished being surveyed. Is a new thing, plan for it failing until you are very sure it won't.

Piskvorrr 9y ago
That's the thing though: "we're very sure this won't kill people" is always a probability.

morsch 9y ago

The bridge was meant as a symbolic new portal to the university, not just a way to safely cross the dangerous roadway. It would have open views, benches, planters, glass-enclosed elevators, Wi-Fi and a wide 30-foot deck for pedestrians and cyclists. As a pedestrian and a cyclist, the last thing I want to do on your new bridge over a busy six-lane road is stick around on a bench perusing the wifi.

creaghpatr 9y ago
Georgia Tech has this over the 85/75 connector and it works better than you would think. It's a popular tailgate spot on gamedays.

jakeogh 9y ago

Having to quibble about why it failed means the margins were _way_ too low. It's a semi-permanent structure, if it costs double (and it wont) more to make it redundant in the worst case (loss of multiple main supports for example) then that's a good thing. Skyscrapers for are built with extensive redundancy, hence the pre-weakening needed before bringing one (or 3) down. I bet when the NTSB is done we find out that this bridge wasn't actually up to code.

snarfy 9y ago

"What we can say is that when complex technological systems fail, usually no single factor is to blame. In her study of the Challenger Space Shuttle disaster, the sociologist Diane Vaughan noted a phenomenon she called the “normalization of deviance,” an acceptance of assumptions and shortcuts that over time incrementally piles on risk until, like compounding interest on debt, a kind of technological bill suddenly becomes due." [1] [1] - https://slate.com/business/2018/03/the-florida-bridge-collapse-shows-how-accelerated-bridge-construction-can-go-wrong.html https://slate.com/business/2018/03/the-florida-bridge-collap...

anonymous2381 9y ago

Not only did this modern design fail and kill people, it's ugly: https://news.fiu.edu/wp-content/uploads/bridge-model.jpg https://news.fiu.edu/wp-content/uploads/bridge-model.jpg Compare with the Millennium Bridge in London, which is also modern and ugly, and was also originally faulty, but was fixed before it could kill anyone: https://en.wikipedia.org/wiki/Millennium_Bridge,_London https://en.wikipedia.org/wiki/Millennium_Bridge,_London

coldcode 9y ago

Generally civil engineering projects fail for three reasons, incorrect or substituted materials, or incorrect or poor installation practices, or poor or fraudulent inspections of those. Rarely does something fail due to poor engineering these days. Sometimes it is hard to identify the exact cause after a failure since the failure may obliterate the cause.