
Experts dissect FIU bridge collapse, but can't identify cause - wglb
http://www.miamiherald.com/news/local/community/miami-dade/west-miami-dade/article207358659.html
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cryptonector
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)

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tristanj
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.

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smoyer
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.

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wlesieutre
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-...](https://news.fiu.edu/2018/03/community-gathers-to-
watch-950-ton-bridge-move-across-southwest-8th-street/120395)

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tristanj
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/w...](http://www.miamiherald.com/news/local/community/miami-dade/west-
miami-dade/article206092509.html)

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cryptonector
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).

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function_seven
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.

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snarfy
"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-
collap...](https://slate.com/business/2018/03/the-florida-bridge-collapse-
shows-how-accelerated-bridge-construction-can-go-wrong.html)

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StreamBright
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.

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wiredfool
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.

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jakeogh
2!? Do you have a cite?

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wiredfool
I don't have my code books any more, when I was doing this it was 20 years ago
and in a different profession. My bridge is still standing, but it was a lot
less risky than that one.

LRFD puts a 1.2-1.6 multiplier on loads (and at time less than 1 for cases
where you're relying on it for balance), and resistances are in the .8-.9
range.

I never did a whole lot with the Factor of Safety era codes, but they were
generally in the same range once you multiply out all the factors. (As you'd
expect, the newer codes tended to hit about the same design point, with some
deviations.)

There are two main things to keep in mind here:

* A factor of safety is for normal variability in loads and normal variability in material/connection strengths. It doesn't cover you for blunders.

* The loads that are calculated are the extreme loads on the system, not the normal loads. 1.4 DL + 1.6 LL + 1.2 WL is often the design envelope, and that's... a lot. Add a huge factor of safety on that and you're into physically impossible cases. Like, people are being crushed to death if you get more than 300psf of human load over a significant area, but the typical Live Load is in the range of 100psf. (Now books on a reference library moving shelf system? That's 300psf)

From what I've seen of this bridge, there was no live load at the time, so
it's unlikely that it was overloaded as designed. If I was doing failure
analysis, I'd be looking at a combination of factors, and the ones I'd start
with are the shop drawings vs. the engineer's plans and the quality of the
concrete materials, placement, and curing.

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jakeogh
Thanks. Is your bridge steel?

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wiredfool
Steel, with a concrete composite deck. I did post-tension retrofits and other
interesting concrete stuff as well.

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OrganicMSG
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.

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

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coldcode
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.

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morsch
_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.

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creaghpatr
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.

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jakeogh
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.

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anonymous2381
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)

