Miami Pedestrian Bridge, Part I 65

[JohnRBaker]

Multiple Fatalities After Pedestrian Bridge Collapses Near Florida International University

https://www.nbcmiami.com/news/local...orida-International-University-476984523.html

As investigators continue to search the site of a deadly collapse involving a 950-ton pedestrian bridge near Florida International University in Miami Thursday, officials say the death toll has risen.

Early Friday morning, the Miami-Dade Police Department confirmed that six people have died as a result of the collapse....

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[haynewp]

FYI regarding weather this week.

https://www.wunderground.com/histor...statename=&reqdb.zip=&reqdb.magic=&reqdb.wmo=

 

[hokie66]

Nothing unusual about the weather. I think that can be discounted as a factor.

 

[oldrunner]

Here's an interesting "FINAL" design picture:

http://www.miamiherald.com/news/local/education/article168971627.html

 

[hokie66]

Was this thing a concrete truss? Hard to see from the photos if there were embedded structural steel members. Concrete trusses are rare, and for good reasons.

 

[dik]

Was that truss constructed from concrete?

Dik

 

[dik]

ya beat me by 10 sec...

Dik

 

[haynewp]

It is was a concrete truss from the pictures. Also sayas an engineering company performed a stress test in the morning?

https://www.google.com/amp/abcnews....tional-university-collapses/story?id=53774444

 

[hokie66]

An article from heavy.com...there is a time lapse of the bridge move. Hopefully, some video at the time of the collapse will turn up.

https://heavy.com/news/2018/03/florida-international-university-bridge-collapse-photos/

 

[Ron]

The collapse occurred in seconds....non-ductile.....implies perhaps a shear failure on the one side that hit the ground. As others have noted, we'll have to wait and see. The structure was designed by Figg, a reputable bridge design group. It was an attempt at "accelerated bridge construction", a concept that involves pre-constructing and setting in place. Although touted as "new", that process has been used many times, particularly with steel frames.

 

[RoopinderTara]

Cable supported bridge collapse because no cables. Duh.

What a horrible tragedy. It could it have been prevented.

I am not a civil engineer, so bear with me as restate what is being discussed here, and what I have seen in reports, photos, videos and Google street view.

Here is the artist's rendering showing the cable supported pedestrian bridge.

Here is the main bridge span resting on two supports on either side of the highway. Notice NO cable supports.

The collapsed bridge shows no evidence of the tower. No tower = no cables. So, the span crosses 8 or 9 lanes of traffic with no supporting cables.

So maybe someone had built a temporary support from median strip to the bridge span? I'm looking through the pictures and video and seeing no evidence of any temporary support structure.

How could they have opened the road to traffic when the bridge had no cables? It's a cable supported bridge, for God's sake! And without any temporary support structure?

Am I missing something here?

Roopinder Tara
Director of Content
ENGINEERING.com

 

[msquared48]

Report in the local news here that the structure was being "stress tested", whatever that means, at the time of failure. Don't know if that is true.

Mike McCann, PE, SE (WA)

 

[epoxybot]

hokie66 - That is the best of the various time lapse videos. They seem to spend an inordinate amount of time working on the bearing pads or something along those lines.

Here is a good shot of the post-tensioning Link

Notice the VSL Corp truck/van under the bridge Link
If VSL isn't working on this bridge, then they will for sure get to the bottom of what went wrong.

Notice also in the last photo the tensioning equipment. I wonder if they were tensioning some strand when something went wrong?

 

[bigengineer]

hokie66 - I have looked at several pictures and it does appear to be a concrete truss with post-tensioned bottom slab/chord. The truss web arrangement would appear strange, but looking at the final design, the webs match the angles of the permanent cable supports. Still they're not symmetrical at all for this temporary construction condition. It's a long span with certainly a lot of dead load and high truss web member forces. It's not a typical truss web arrangement, and it's a concrete truss. That's a lot of unconventional items in one large design.

I hate this situation for everyone--the injured, their families, the engineers, and our profession in general.

 

[Lomarandil]

Hokie66 -- I don't know anything about this bridge specifically. In many cases, a pedestrian bridge may be adequate to self-support in a temporary construction condition, but a secondary support (like this cable stay) is desired for vibration and serviceability performance before it sees public traffic.

TLDR: Construction workers are a lot more tolerant of L/180 deflections than the public is.

Unfortunately, getting phased construction loading right is harder than it looks, so I agree that's a likely root cause. I'm glad to see the reputable names of Figg and VSL mentioned here. I think both those companies will be motivated to iron out what happened (rather than just rushing to rebuild).

----
The name is a long story -- just call me Lo.

 

[bigengineer]

RoopinderTara - I understand your frustration. This temporary span condition would have certainly been designed for... but either an error occurred in the design process or a construction error, or combination of the two.

It's a terrible tragedy that (as you have said) could have been avoided.

 

[Lomarandil]

Colorado requires bridges in temporary conditions be inspected and signed off by a PE before public traffic is allowed underneath. Any idea if FL is the same?

If not, I bet it's coming!

----
The name is a long story -- just call me Lo.

 

[haynewp]

The reports may be confusing stressing tendons with a stress test.

 

[dik]

It was an attempt at "accelerated bridge construction", a concept that involves pre-constructing and setting in place. Although touted as "new", that process has been used many times, particularly with steel frames.

It didn't appear to have any of the cable stayed structure in place. It may be that part of the procedure for erection was missing or missed; it failed 'way too soon', and it may have required temporary support that was 'missed'. In all my years... I've never done a concrete truss. I've seen a few concrete trusses used for old railway bridges, and, that's it.

If it was designed for cable stayed, then the members should be light to reflect the closeness of the stays. That feature may have provided a weak structure if not properly supported.

Dik

 

[271828]

If I had to bet money on it at this point, I'd go with inadequate stability bracing of the top chord. What's the unbraced length of the top chord for buckling about the vertical axis? To make the unbraced length only the length of the bridge, the end columns would have to be stiff and strong to provide nodal braces, to use AISC's Appendix 6 term.

 

[bones206]

For what it's worth, someone apparently heard a loud "cracking whip" sound while stopped in his car under the bridge, several hours before the collapse: Link