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Miami Pedestrian Bridge, Part I 65

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JohnRBaker

JohnRBaker

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Jun 1, 2006
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Multiple Fatalities After Pedestrian Bridge Collapses Near Florida International University


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....
Click to expand...

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
 
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It's being reported that the structural span portion of the bridge was just erected this past Sunday and that there were workers on the bridge when it collapsed.

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
 
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Looks like it used the roof structure as the top chord of the truss. I wonder if the webs broke free from the bottom chord?
 
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I wonder if the bottom section failed in tension then the top section buckled. The length to depth ratio looks very high. Were the cable reinforcement properly tensioned and were there provision for connecting and tensioning the reinforcing cables between section assemblies to unitize the assembly?
 
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This article shows a rendering of the completed, cable-supported bridge.

This may a case where the team thought that the existing deck structure was strong enough to self-span the roadway while the cables and tower were erected. I wonder if the warm Florida sun hit it, made it expand, and the temporary construction load + expansion broke loose a metal-diagonal-to-concrete-web-chord connection...





"We shape our buildings, thereafter they shape us." -WSC
 
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The bottom section looks continuous. It would be in tension, so a failure would leave a gap. The top chord is in compression and can't separate and looks continuous. This leaves the shear members and I expect this is a failure of one of the connections, either via shear or by punching though one of the decks. It's also possible the upper chord buckled.

The horrific part is that the bridge fell on a large number of cars that must have been stopped for a traffic light. It seems that at least 4 and possibly as many as 10 vehicles were under the bridge.

One witness reports that a support failed sideways, but I think that must have been in reaction to a failure elsewhere.

The lack of symmetry and the irregular columns is unsettling. See Obviously without knowing the reinforcement making a call about sufficiency is impossible, but I am unnerved by the second column on the campus side; the side that fell on the cars.

Edit:
I see that this was to be cable supported. That explains the irregular beams and columns. Someone didn't do a good job of analyzing the partial construction strength.
 
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mjb315 said:
I wonder if the warm Florida sun hit it, made it expand,
Click to expand...
I noticed the heat mirage 'waves' in the news footage... I guess we'll have to wait until some technical discussion comes out... our thoughts go out to the survivors and the families of those involved.

Dik
 
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It looks like the slab was post-tensioned. I wonder if the anchorage slipped. If so the (presumably) ungrouted conduit may have become the tragedy that some here warned was possible. Time will tell. It's certainly a horrible situation.
 
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I noticed the waterway to one side where the abutment could be on pile...

Wonder if there was lateral movement from an eccentric loading causing the abutment to shift and the bridge to lose support on that end.

Mike McCann, PE, SE (WA)


 
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MJB315 post shows link to what the completed bridge would look like. Since the cable stays and main support column is not yet installed a temporary support at the midspan of the deck (also center of roadway) should have been provided.
 
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>>>Were there some warnings or concerns expressed on this?<<<

JAE, sorry, I didn't mean to imply that there were warnings regarding this specific structure; indeed, in my case this is the first I've heard of it. Rather, I was referring to the various discussions of bonded vs. unbonded post-tensioning that have occurred over time on the structural forum. I probably should have withheld speculation, though.
 
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You do have to wonder about the scheme. If the truss was supposed to be able to span simply across the roadway without the cable supports, why were the cable supports necessary at all? In the completed structure, it looks like the truss was to be continuous, and cable stayed. So the temporary condition was critical. I know, more loading in the permanent condition, but not enough to override the construction loading.
 
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I agree and until Rapt pointed it out I didn't realize it was ultimately supposed to be a 2-span condition. And to have allowed traffic to go underneath it in the interim...?!

It will be interesting to learn what happened.
 
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