Florida university bridge collapses leaving several fatalities

buka001

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Your thoughts here?
Check one of my posts above.

From the cctv footage it had an almost instantaneous brittle collapse, which indicates a catastrophic failure of one or more of the post-tensioned cables.

What led to this, it appears is a failure in the contractors responsibility to recognise the risk in changing aspects of the temporary works to the design and did not support the deck as per the design.

Gordon linked a youtube video which provides an in-depth analysis. Which I agree with.
 

crackersa

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Check one of my posts above.

From the cctv footage it had an almost instantaneous brittle collapse, which indicates a catastrophic failure of one or more of the post-tensioned cables.

What led to this, it appears is a failure in the contractors responsibility to recognise the risk in changing aspects of the temporary works to the design and did not support the deck as per the design.

Gordon linked a youtube video which provides an in-depth analysis. Which I agree with.
Great. Will do as such.
 

Spizz

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Check one of my posts above.

From the cctv footage it had an almost instantaneous brittle collapse, which indicates a catastrophic failure of one or more of the post-tensioned cables.

What led to this, it appears is a failure in the contractors responsibility to recognise the risk in changing aspects of the temporary works to the design and did not support the deck as per the design.

Gordon linked a youtube video which provides an in-depth analysis. Which I agree with.
My 2c as a civil engineer and someone who has worked with post tension cables a fair bit in my life.

First of all, looking at end of the bridge in section, it looks like the bridge has a hollow core construction so the cables spanning the road would therefore run parallel to the hollows.

Background for cracker and anyone interested - Post tension cables are used to cut down on the weight of a slab. The more cables, the less traditional reinforcement. They are fairly common in South Africa but not in the US, I've done a few in CT actually and some of you may be sitting in your office reading this above a PT cable done on one of my jobs :p

Anyway, the cables are laid out and spaced equally in bands of however many (4,5, or 6 etc), and depth wise, the depth alters between where the cable has to be pulled and where it runs along the length of the slab. So for simplicity's sake, essentially, the cables come through the shutter at about half the depth of the slab, drop down to the bottom of the slab as they run the length of the span, then rise up again the to shutter at the other end again.

So basically, the cables when pulled rely on the concrete being strong enough to stop them from straightening out. If the concrete is not at strength yet, which is generally 18MPa, then the cables will cut through it like a knife through butter. And if there is not enough concrete between the cable and the hollow core (fresh air), then it won't hold it back and it will pop through into the air.

Regarding the bridge, I'd imagine it unlikely a cable snapped but more likely if they were a factor, they were placed incorrectly and the concrete was not at the correct strength and/or with enough cover to the voids/air/outside.
 

ponder

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Miami bridge collapse: Engineer reported cracks in voicemail

http://www.bbc.com/news/world-us-canada-43440501

Miami bridge collapse: Engineer reported cracks in voicemail

An engineer working on the Miami university bridge warned of cracks in the structure, two days before it collapsed killing at least six people.

Denney Pate, lead bridge engineer for contractor FIGG, left a voicemail with the Florida transport department on Tuesday.

He warned of "cracking" but also said there was no concern "from a safety perspective".

Department employees did not hear the message until after the bridge fell.

The Florida International University (FIU) concrete bridge collapsed on to a motorway, crushing at least eight vehicles driving beneath.

The number of victims is thought likely to rise as workers clear the rubble from the 862-tonne structure, which was erected days before as a walkway for university students.

Read more...
 

buka001

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My 2c as a civil engineer and someone who has worked with post tension cables a fair bit in my life.

First of all, looking at end of the bridge in section, it looks like the bridge has a hollow core construction so the cables spanning the road would therefore run parallel to the hollows.

Background for cracker and anyone interested - Post tension cables are used to cut down on the weight of a slab. The more cables, the less traditional reinforcement. They are fairly common in South Africa but not in the US, I've done a few in CT actually and some of you may be sitting in your office reading this above a PT cable done on one of my jobs

Anyway, the cables are laid out and spaced equally in bands of however many (4,5, or 6 etc), and depth wise, the depth alters between where the cable has to be pulled and where it runs along the length of the slab. So for simplicity's sake, essentially, the cables come through the shutter at about half the depth of the slab, drop down to the bottom of the slab as they run the length of the span, then rise up again the to shutter at the other end again.

So basically, the cables when pulled rely on the concrete being strong enough to stop them from straightening out. If the concrete is not at strength yet, which is generally 18MPa, then the cables will cut through it like a knife through butter. And if there is not enough concrete between the cable and the hollow core (fresh air), then it won't hold it back and it will pop through into the air.

Regarding the bridge, I'd imagine it unlikely a cable snapped but more likely if they were a factor, they were placed incorrectly and the concrete was not at the correct strength and/or with enough cover to the voids/air/outside.
Possible, but check post above yours, the slomo footage.

Bridge collapses at the point where the guys were working. They were stressing the PT cables at that moment.

I agree with AvE video. The cable was overstressed and it failed (popped out or snapped).

Edit - contractor had poor working at height controls. The poor guys who fell would not have on ny job. Inertial reel secured to crane for fall arrest for exactly this type of a situation.
 

Spizz

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Possible, but check post above yours, the slomo footage.

Bridge collapses at the point where the guys were working. They were stressing the PT cables at that moment.

I agree with AvE video. The cable was overstressed and it failed (popped out or snapped).

Edit - contractor had poor working at height controls. The poor guys who fell would not have on ny job. Inertial reel secured to crane for fall arrest for exactly this type of a situation.
Yep, I'm surprised they allowed it and it's amazing he agreed to climb up there without an inertia reel.

But on the bridge itself, TBF I haven't read an awful lot about the incident, just the news reports and the guy talking about it in the AvE video. But surely the cables would have been pulled already before the bridge was moved into position? They would pull those cables after 3 or 4 days and certainly before striking the formwork, so certainly before moving the thing into position.

And I was assuming (and commenting on) that the cables were running the length of the bridge, not in the struts. In which case he would not be pulling the cables from up there. He'd then be on top of the supporting columns on either side of the road?

But then as I say, they would *not* be pulling cables, it doesn't make sense. And if they were, then it's no surprise the thing fell down becasue it would not be ready for placing :erm:

But yeah, I'd love to see the drawings and know the materials and the sequence of construction.
 

Spizz

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But then as I say, they would *not* be pulling cables, it doesn't make sense. And if they were, then it's no surprise the thing fell down becasue it would not be ready for placing :erm:
There are some very technical discussions on the event in this forum: http://www.eng-tips.com/viewthread.cfm?qid=436595
Reading the thread it appears it's a common practice in this type of bridge construction to partly tension some of the tendons and then fully tension in position.

Another thing is that the diagonals are actually made of concrete (I thought they were steel) and it was a tendon in the one we see the crane at that was what was being pulled at the time.
 

The_Assimilator

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If this happened in SA, the MyBB brigade would be crying BEE, ANC, Zupta blah blah blah...

Sometimes engineering just goes wrong.
Sometimes people who aren't engineers get hired to assemble or transport things, and break them.
 

Gordon_R

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Not much 'news', but some mainstream articles refer to AvE's video, and have provided some more context: http://www.stltoday.com/news/national/innovative-fiu-bridge-was-a-modern-take-on-an-old/article_cab71c19-d129-5962-9e5f-6817a29994ec.html

Associated Press said in a story Tuesday that the Florida Department of Transportation ordered the northern support pylon be moved 11 feet to make room for future expansion of the trail. That required a design change that lengthened the span — and put the support pylon in the dirt well off the edge of the roadway, which could also explain why the northernmost truck could no longer follow its original planned route.
Edit: The redesign itself was a messy affair: https://www.cbsnews.com/news/florida-bridge-collapse-fiu-florida-state-university-project-behind-schedule-over-budget-updates-2018-03-20/

Difficulties began in late 2016, when the Florida Department of Transportation emailed project officials saying they needed to move the bridge's signature pylon to allow for future widening of the road, according to the documents. The tower was to be located on the north side between the road and the canal, and was designed to have cables connecting it to the structure below.

"The first option being entertained is providing these extra travel lane (sic) on the north side of SW 8th St.," wrote Alfred Reyna, a transportation department employee working on the bridge project. "This first option places the current location of the pylon in conflict with the extra travel lane and would require bridge design modifications."

After weeks of back and forth, it was decided to move the pylon 11 feet to the north, sitting near the edge of the canal. According to documents, initial costs for the new design were $204,540, with another $402,723 for construction changes. The final cost of the change was not divulged in the documents.
 
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Gordon_R

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After months of procrastination, the NTSB has released photos showing that cracks in the bridge grew to a shocking size the day before the collapse: https://www.miamiherald.com/news/local/community/miami-dade/article216388430.html

The photos show four alarming-looking cracks — one at least 3 1/2 inches deep into the bridge’s deck — developing in precisely the section of the span that is believed to have failed on March 15, killing five motorists and one worker.

Given the size of the cracks, the photos raise new questions about why officials at FIU, the Florida Department of Transportation and the private contractors running the project did not seek to shut down the busy road underneath the bridge.
 

buka001

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After months of procrastination, the NTSB has released photos showing that cracks in the bridge grew to a shocking size the day before the collapse: https://www.miamiherald.com/news/local/community/miami-dade/article216388430.html
Holy hell.

I am shocked they did not close that bridge after they saw those cracks.

Bridge had failed by then, it was literally a matter of a day or two before final collapse.

I can't believe they did not think those cracks were serious enough.
 
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