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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[Tomfh]

but it appears to me from the video and the method of collapse that the first compression member's connection (where the alleged de-tensioning or tensioning operation was occuring) to the upper chord failed in shear.

If AVE's assertion is correct that the PT rod and ram being ejected some distance out of the conduit is evidence that the construction crew tensioned beyond the Fu of the rod, why would this cause a collapse of the entire structure? (being as it is a compression member and all)

I had the same thought. Maybe the rods clamping stress was in fact keeping the member and joint confined, keeping the member and node intact.

 

[gte447f]

Mr. "Va-Jay-O" in the youtube video linked by BAretired knows just enough to be dangerous, but he is seriously entertaining and has some nice tools at his disposal to boot.

 

[Tomfh]

There should be two rods in member #11 if they duplicated what was used elsewhere.

It appears to be a single rod hanging out the ends. That would make sense if it was just there to carry the weight of the overhangs during lifting.

 

[Tomfh]

During the bridge move the site would have been crawling with FDOT engineers. The call from FIGG was simply good conduct to the Chief FDOT Engineer for the project.

I agree.

 

[Tomfh]

A meeting about the crack a few hours before collapse:

https://www.reuters.com/article/us-...dge-crack-before-fatal-collapse-idUSKCN1GT0KG

Engineers and state and university officials met hours before a new pedestrian bridge collapsed in southern Florida, killing six people, but concluded a crack in the structure was not a safety concern, Florida International University said on Saturday.

The meeting on Thursday involved FIGG, which is the private contractor for the overall bridge design, the school, Florida Department of Transportation officials and Munilla Construction Management (MCM), which installed the $14.2 million bridge.

 

[P205]

EDIT: Tried embedding the video but I can't seem to get it. The link shows a blowup of the collapse.

<iframe width="560" height="315" src="

https://www.youtube.com/embed/Ucflj-MsJBI?rel=0"

frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>

 

[JStephen]

"The cable stayed structure, from the renderings, appears to be an attractive structure. A lot of work went in to providing these asthetics. I don’t know why it was chosen. It is likely a very costly undertaking."

Here in the Dallas area, they have built some major bridges with big arches and whatnot. There's nothing below that would prevent them from putting piers down every 10' if they chose, and the older bridges were more along that line. I deduce that whether the big arches and cable supports serve a structural function or not, they were selected primarily for aesthetics, not because that was the ideal way to build something. I suspect the Florida bridge has similar concerns going on.

 

[structuralengr89]

Tomfh (Structural)17 Mar 18 21:25

It appears to be a single rod hanging out the ends. That would make sense if it was just there to carry the weight of the overhangs during lifting.


Detail shows one pulled from the top and one pulled from the bottom....but it could have only been one in #11

Video from foreman with MOM stated that there were 2 tendons to detension (after bridge is set on abutment) and they can reopen traffic

I wonder if these 2 tendons were detensioned....traffic was reopened...cracking was observed so FIGG decided to retension the strands due to cracking...then when pulling the top one....it either snapped or the node busted

 

[Tomfh]

I wonder if these 2 tendons were detensioned....traffic was reopened...cracking was observed so FIGG decided to retension the strands due to cracking...then when pulling the top one....it either snapped or the node busted

That would make sense.

 

[dik]

It would be interesting to see the photographs of the cracks that proceeded the collapse. I assume someone took photographs...

Dik

 

[dik]

There's nothing below that would prevent them from putting piers down every 10'

For safety reasons you want supports to be well clear of the driving lanes. Clear span is good.

Dik

 

[Tomfh]

It would be interesting to see the photographs of the cracks that proceeded the collapse. I assume someone took photographs...

Yep, there would be photos. I hope someone leaks that.

 

[Ingenuity]

A possible scenario:
"Well Harry, how are we supposed to take the nut off that bar?"
"Nothing to it Joe. We use the hydraulic jack to stretch the bar a little and then spin the nut off by hand."

"Okay Joe, the nut should turn off now."
"Gosh Harry, I can't turn it. It seems to be stuck somehow."
"Okay. I'll stretch it a little more and see if it will turn."
"Oops. Too much."

That is a bit of a callous characterization, IMO. One member of the stressing crew died in the collapse, and two of his colleagues suffered injuries, meanwhile the engineer sat in his/her 'ivory tower'.

PT stressing operations are undertaken by experienced and specialized personnel, using calibrated and serviced equipment, and bridge projects even moreso (compared to building structures). VSL (the PT subcontractor on this project) have a worldwide history of post-tensioning that exceeds 50 years.

Don't run the field guys 'under the bus' - I know of several 'stressors' who have saved the engineer's a$$ a bunch of times.

 

[OSUCivlEng]

I've read through all the posts and have a few thoughts.

First, these TIGER grants are always associated with some questionable projects. For example, back in 2009 the I-44 bridge over the Arkansas River in Tulsa, Oklahoma got a TIGER Grant that funded its replacement. The reason it got the grant? Because it was a MULTIMODAL bridge. It was a double decker bridge, the top deck for cars and trucks and the bottom deck for future light rail and also a pedestrian trail on the side. That the bridge needed to be replaced had nothing to do with getting the grant, it had the ability to support light rail in the future (that doesn't exist and will never happen by the way) and people could also walk across it. Therefore, it got the Federal Grant to be built. It was all politics.

This looks to be the same way. Link
Why in the world would you spend all this money ($11.4 million) for a bridge that would have tables and chairs with ceiling fans built into it? Elevators with glass walls. Solar technology, "eco-friendly" concrete, programmable LEDs. A large center pylon with fake pipe stays that don't serve any real purpose. This one span weighed 940 tons, how does it have a vibration problem? I don't buy that the pipe stays were for vibration, they were just there for looks. All of this just added to the cost. You could have built a much simpler steel truss (with redundant members) and still used ABC with SPMTs to set it in place and saved a lot of money.

Second, why in the world would you build a non-redundant concrete truss over 6 lanes of traffic? Too dangerous in my opinion.

From what I gather in all of these posts it looks like the contractor was doing something with the post tensioning on the diagonal member (#11) when the truss failed. I've never designed anything that was post tensioned, but my hunch is that they did something that caused that diagonal member to buckle. It's pretty clear from the video that P205 posted that the source of the failure came from that member.

 

[dik]

Black humour... I thought it was funny and gave him a star for it... I can appreciate your sensitivity for the tragic collapse and loss of life, including the stressing crew member.

Dik

 

[hokie66]

The video would suggest that we are focusing on the wrong member, 11. The collapse occurred at the top and bottom of member 10. And if others are correct, member 9 which intersected with 10 at the bottom, had no prestress. But wherever the failure actually occurred, the issue here is joints in a concrete truss/frame. In these forums, we often discuss problems to do with efficiency of concrete joints, and it is widely known that they can't be 100% efficient, but I don't know of any literature about design of concrete trusses per se.

 

[dik]

The video that p205 posted was probably the best of the failure. Is there software that can show the individual frames and maybe enhance the image. The event happens so quickly I cannot determine if the failure started at the top of the diagonal member or at the base. The web member did not appear to buckle and it appeared that the panel point may have failed.

Dik

 

[Tomfh]

Here's a couple of frames from the vid. First frame is immediately before collapse. You can see #11 intersects with the node.

Following from that, something has happened to #11, and it no longer aligns with the node. Pity that crane is in the way.

Here's the relevant frames from the CCTV. Not a great video, however it appears something's happened to #11.

 

[3DDave]

There are 6 canopy tendons altogether, but they appear to have two of them tensioned only for the movement portion and then relieve tension after placement. In pictures, 4 of the six are capped, leaving access to the remaining two. I believe those are what the guy in the video was talking about when he talked about removing tension.

The work on member #11 happens after cracking is reported.

I think the construction crew was working with faulty information and tried a fix for a condition they didn't understand.

 

[Tomfh]

Dash cam video relevant bit (I "dolly zoomed" so the bridge is same size in each frame):