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
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[lilliput1]

Did the design engineers specify a phasing plan such or did their specification require the contractor to provide adequate temporary supports as required?

 

[hsad]

I'm guessing this has to do with construction phasing. Some lack of communication between the simple span vs continuous phases.

 

[OHIOMatt]

Reports are, that the PT tendons loosened and were being re-pulled. I am no expert on PT. Is that something that is common?

Also, video is out. The collapse was brittle.

 

[OHIOMatt]

https://www.google.com/amp/amp.miamiherald.com/news/local/community/miami-dade/article205443304.html

Video in link

[This 5 second video from a random cell phone in a building across the street captures the actual collapse of the structure]

 

[hokie66]

All the end reaction went up that first diagonal in compression, and the next diagonal couldn't take it. Maybe a failure of the anchorage of that diagonal, or maybe incidental moment in the connection. A concrete truss was a poor choice. Many of these have been done in steel, which provides the ductility necessary to cope with things a bit out of alignment. I know that Figg has done many bridges, some a bit heroic, but I think they would be wishing they had not done this one.

 

[3DDave]

From the video I think the first diagonal failed. There seems to be a wrinkle near the top just before interlace blurs between the slight deformation and the total collapse. This happens just before the video progress indicator touches the sidewalk in full screen.

I wonder if the reason so many people were standing just above the failure point was they had spotted a crack and were trying to figure that out - otherwise it's an even more horrible coincidence.

Edited - so they were apparently performing some work as noted by later posts.

 

[Ingenuity]

At the time of collapse it appears they were stressing PT bar/s to the first COMPRESSION diagonal from the top flange, at the first interior 'node' from the future stay-cable tower, as per the following photos:

Future stay-cable tower to the left of the photo:

Photo looking along the longitudinal-axis of the bridge:

General view during erection:

 

[epoxybot]

After they set the bridge in place, they de-stressed the two top deck tendons. Link

The column at the end of the bridge where the tower was to be built, was only half of its final width. Once the other half of the bridge was in place the two half columns were to be joined together & the column would have had an octagonal shape.

Here is an interesting angle Link

Couple other large scale reference photos
Link

Link

 

[haynewp]

Assuming there wasn’t a problem that was getting worse as more time passed, it would seem more likely for it to be related to what they were currently doing since it had been stable up until this point under similar gravity loads and weather.

 

[DSB123]

So much for it lasting 100 years!! Is it true the firm was being sued for a platform failure back in 2016?

 

[hokie66]

Why would they be stressing at that stage? The truss was spanning full length, and should have been complete, at least for that stage of construction.

 

[haynewp]

Report mentioned above said that cables had loosened and were being retightened.

 

[klaus.]

usually there is a combination of different 'things' when a collapse like that happens ...not just a single source....

 

[hokie66]

I don't understand the detensioning of the top deck PT.

Most of interest would be the details of the anchorages within the concrete elements. Were they castings?

An issue with a concrete truss is that it doesn't want to act only like a truss, but also like a rigid frame, so there are substantial moments at the joints. Central stressing would not cope well with these moments, so a lot of deformed bars would be required. Does anyone other than me think that the photos show very little in the way of non-prestressed reinforcement?

 

[AggieYank]

Its a cable stayed bridge, that weighed 940 tons (if we can accept that the news reporting is correct).

If the bridge was designed for 100psf of traffic on it, that is an additional 340 tons approximately. So, dead load is by far the dominant load.

Without knowing project details, it seems that the simple answer is that the bridge probably wasn't designed for simple span in construction phase, so the engineer would have assumed shored construction until cables installed. The contractor didn't understand that.

Its possible also that the strand tensioning work at bottom chord was assumed to have already been finished, prior to releasing the shoring towers.

 

[MikeHalloran]

My kids brought back souvenir statuettes from Mexico, that appeared to have been made of clay.
... that had been extended with, er, locally sourced used food, if you catch my drift.
The statuettes had broken in shipment, in a suitcase, protected by dirty laundry.
Impromptu testing caused further fractures, simply by shaking the object gently while cantilevered from a hand's grip.
Those fractures looked a lot like the TV images of the broken bridge.

So as to infinitesimally reduce my colossal ignorance of related subjects, I ask:

Does Accelerated Bridge Construction normally include moving a concrete structure before the post-tensioning strands had been fully stressed and locked?

Is 'de-stressing' a normal procedure in PT construction, ever?

Is 'de-stressing' or 're-stressing' or 'stressing' a PT tendon such a trivial undertaking that it's often carried out on a structure with civilian traffic passing beneath?




Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

Is my recollection of statics right - that the diagonal that failed would have a compressive load that would roughly be half the weight of the bridge divided by the sine of the angle (assume 30 degrees, so 0.5) it makes with the horizon? If so then this puts a 940 ton load on that small (18 inch X 18 inch) section. And then it appears they added an additional compression load via the cable tensioner which appears to feed to the diagonal?

So that puts it at 6ksi for the dead load (24 inch x 24 inch drops it to just over 3ksi.) That seems to be in the middle to a bit above middle end of the capability of typical concrete.

If the load was eccentric that would make this even worse.

 

[dik]

The loading regime of that 'truss' would be substantially different when loaded as a simple span or a cable stayed. There would likely be no similarity. As noted above, as a simple span, the end diagonal would be taking approximately half the span in compression. As a cable stayed structure, this could even be a tension member.

It will be interesting when the report on the collapse is completed, how many of the comments in this thread 'ring true'.

The bridge was under construction and it is not known what component was contractor's 'means and methods'.

Dik

 

[structuralengr89]

Ingenuity...great photos...and as Hokie66 mentioned...why would they be stressing that compression diagonal in the field? It looks like there is a single tendon running thru the center of that diagonal...Why wouldn't there be reinforcing around the perimeter of the compression diagonal to increase it's moment capacity?

I don't know how many times I've had to hear about the Hyatt Regency Failure in getting my Ethics hours...but (and I am assuming here) If there were problems with slipping of tendons...and concrete cracking observed and they were trying to re-stress a tendon...then the Traffic should have been shut down....but then there is the other side...the Pride of our design and the Fear of letting others know we made a mistake...

I also wonder what happens to Figg as a company...there are post-tensioned bridges in our state done by them...otherwise a fine, outstanding company I'm sure...but they get work through State DOTs...Do state bridge engineers hire them as consultants...the public will already be concerned about any bridge done by Figg...With the lawsuits and damage of their image...If I was working for them...I'd by looking for a new employer....my $0.02

 

[X4vier]

Does Accelerated Bridge Construction normally include moving a concrete structure before the post-tensioning strands had been fully stressed and locked?

Is 'de-stressing' a normal procedure in PT construction, ever?

Is 'de-stressing' or 're-stressing' or 'stressing' a PT tendon such a trivial undertaking that it's often carried out on a structure with civilian traffic passing beneath?

Yes, No, No.

Yes.. it's normal to under stress the strands during the construction process and tensioning more after the instalation of beams for example.
No.. Haven't heard of de-stressing... re-stressing is a common method.
No.. these are dangerous activities and must be done without traffic.