FIU pedestrian bridge install left it vulnerable to collapse, WTH

[Spinnor]

"The pedestrian bridge in Miami that collapsed Thursday had only been put in place on Saturday under a process that allowed for installation of large sections — but that experts said left it vulnerable to collapse until it was complete."

From, https://www.usatoday.com/story/news/2018/03/15/fiu-pedestrian-bridge-design/429978002/

Which branch of engineering will likely own this tragedy?

Maybe some clues here, http://www.dailymail.co.uk/news/art...amis-instant-bridge-collapses-violations.html

Thanks!

 

[berkeman]

On the TV news just now, they said that the bridge was undergoing pre-opening stress tests, either when it collapsed or just before. That might be part of the final analysis...

 

[nsaspook]

On the TV news just now, they said that the bridge was undergoing pre-opening stress tests, either when it collapsed or just before. That might be part of the final analysis...

I'm not sure of the procedure for a stress test but they were working on something.

In the video you can see the crane in place before it's moved out of the way.

 

[nsaspook]

http://www.miamiherald.com/news/local/community/miami-dade/article205443304.html

 

[berkeman]

I'm not sure of the procedure for a stress test but they were working on something.

Looks like they were tightening the cables when it collapsed:

https://www.cnn.com/2018/03/16/us/bridge-collapse-florida/index.html

Florida Sen. Marco Rubio, who visited the bridge site Thursday, posted on Twitter: "The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today." Rubio has been a visiting professor at the university for the past 10 years.

 

[boneh3ad]

This is hardly the first bridge to be constructed this way. It's a relatively new technique, but it isn't truly first-of-its-kind, and I suspect the method will become more and more common in the coming years because it is pretty wonderful. My point being that I really don't think this was a problem related to the concept of how the bridge was meant to be built. Even if the bridge is technically weaker before the installation is final, this would have been expected and the initial installation would have been designed to make it plenty strong under any expected load.

This leaves two possibilities: either it was an engineering problem with this particular bridge (some kind of design error) or else it was a construction problem (a bad procedure or someone taking a shortcut). My guess is it will turn out to be a construction problem. As soon as they realized the cables were loose, they should have closed the road. Failing that, as soon as they decided to tighten the cables, they should have closed the road. This is going to almost certainly fall on the shoulders of the construction foreman who should have known better. You don't screw with a structure like that while traffic is moving beneath it.

EDIT: A manufacturing defect could also explain it. For example, perhaps one of the cables had a defect, and had, in fact stretched a bit instead of loosening as described in the article. That would imply it was past the yield stress of the cable and the act of tightening it would have rapidly led to a failure. That's pure speculation on my part, of course, but it's a plausible explanation. Again, though, they should have closed the road.

 

[nsaspook]

You would think the reinforcements in the concrete would have prevented what looked like a shear fracture. Once you strip it all down the bridge is just an I-beam. Did the vertical supports just disconnect from the upper/lower plates?

 

[Baluncore]

Once you strip it all down the bridge is just an I-beam. Did the vertical supports just disconnect from the upper/lower plates?

If I had to bet on the mode of failure, I would guess that one of the prestressed concrete diagonals was crushed slightly during the move and install. That would have slackened the enclosed diagonal tension wires. The mistake would then be to tighten the slack wires which would cause further collapse of the concrete under compression. It would also pull the top and bottom plates out of their planes and transfer compression to adjacent diagonals. The beam would then fail in the traditional way, by catastrophic buckling of the top plate under compression, with asymmetric off-axis forces.

 

[CWatters]

There is a poor quality video of one end of the bridge failing on the web.

 

[gleem]

I has been reported by an inspector that cracks appeared in the north end of the structure two days before the collapse. https://www.cnn.com/2018/03/17/us/florida-miami-bridge-collapse/index.html

It doesn't seem reasonable to me to say that a crack in a newly build bridge was not a safety concern.

 

[nsaspook]

The failure seems to happen where the crane on the left is working. No idea if that's the north end.
http://www.miamiherald.com/news/local/community/miami-dade/article205635774.html
Different types of Prestressed Beam Failure modes.

 

[boneh3ad]

It does appear that the failure began on the north end. That doesn't necessarily mean it has anything to do with the cracks. For all we know, "cracks" may have been purely cosmetic.

 

[berkeman]

I has been reported by an inspector that cracks appeared in the north end of the structure two days before the collapse. https://www.cnn.com/2018/03/17/us/florida-miami-bridge-collapse/index.html

It doesn't seem reasonable to me to say that a crack in a newly build bridge was not a safety concern.

This is the really unfortunate part of that call, IMO...

Two days before an under-construction pedestrian bridge crumbled in Miami, killing at least six people, an engineer for the company that designed it called a Florida Department of Transportation employee, warning of "some cracking."

The state employee was out on assignment that day. The call from W. Denney Pate of FIGG Bridge Engineers went unanswered, and the voice mail he recorded unheard -- until Friday.

That was the day after the span fell onto a busy eight-lane street near Florida International University, crushing people and cars.

 

[jim hardy]

There's a fellow on internet named "AVE" who postulates a tendon failed during tensioning.
Top image is a snip from his sixteen minute video. He thinks that's a hydraulic tensioner still connected to a tendon rod protruding out the end of a failed cross member joint near north end.
bottom image is my snip from a grainy TV newscast,, security cam at instant of collapse with my annotations.

We non-civil engineers who don't take advanced truss design courses are accustomed to pinned joints. But a vierendeel truss has rigid joints so stress there when it flexes must be terrific.

So this is offered as interesting and nothing more. I'm no civil engineer.

old jim

 

[jim hardy]

bigger snip still grainy.

here's that amateur video , probably not up to PF standards for a reference so don't take it to the bank.


-language warning - the guy doesn't just call a spade a spade he calls it a effin shovel...old jim

 

[Greg Bernhardt]

Stress test with the road open? Someone needs to go to prison.

 

[nsaspook]

Zoomed clip of the best video I've seen of the drop. The diagonal support just left of the crane seems to disconnect at the very start of the drop.

 

[Greg Bernhardt]

Zoomed clip of the best video I've seen of the drop. The diagonal support just left of the crane seems to disconnect at the very start of the drop.

oh dear that is hard to watch

 

[nsaspook]

oh dear that is hard to watch

Yes, it is but it also show the first instinct of people is to help.

 

[berkeman]

oh dear that is hard to watch

Does anybody know if the bridge worker who was on the very top left of the bridge near the crane arm survived that 50 foot free-fall? Ouch.

 

[boneh3ad]

Does anybody know if the bridge worker who was on the very top left of the bridge near the crane arm survived that 50 foot free-fall? Ouch.

I don't know, but I think, perhaps, your ability to estimate distances is suspect.

 

[berkeman]

I don't know, but I think, perhaps, your ability to estimate distances is suspect.

Which way? +/-? Looks to be about 10x his height (EMT estimate).

 

[boneh3ad]

Which way? +/-? Looks to be about 10x his height (EMT estimate).

I feel like that drop looks to be 20 feet at the absolute most before he lands on the roof, then another 10 to 15 of non-free-fall drop after that. Still a hell of a fall, but not 50 feet.

EDIT I pulled the photo and estimated based on the height of one of the construction workers. It was very rough on account of perspective and whatnot, but I estimated that the roof started about 40 feet above the road and that the top of the roof was roughly 20 feet above the bottom of the deck, which is in rough agreement with what I said above. That said, I am sure there is at least some degree of confirmation bias in there.

 

[jim hardy]

If I had to bet on the mode of failure, I would guess that one of the prestressed concrete diagonals was crushed slightly during the move and install. That would have slackened the enclosed diagonal tension wires. The mistake would then be to tighten the slack wires which would cause further collapse of the concrete under compression. It would also pull the top and bottom plates out of their planes and transfer compression to adjacent diagonals. The beam would then fail in the traditional way, by catastrophic buckling of the top plate under compression, with asymmetric off-axis forces.

i finally absorbed that. Sure looks plausible.

 

[nsaspook]

i finally absorbed that. Sure looks plausible.

It would also seem to make the bridge prone to single point catastrophic failure at anyone diagonal.

 

[Baluncore]

It would also seem to make the bridge prone to single point catastrophic failure at anyone diagonal.

That is true, but it is only vulnerable during the construction phase before the stays are installed.

I am not a Civil Engineer. I am just fascinated by unexpected structural failures, but only so I can avoid them.

The car-cam video is slow frame rate, but examining the surveillance video of the failure shown on the news here gives a greater frame rate. It is subtle, but the upper plate appears to shudder and rock momentarily sideways, a slow wave then travels along the T top plate and is reflected back to the trigger point where the failure then occurs and where more complex waves form. That makes me think a tension cable snapped, triggering two waves, one towards each end of the bridge. Both waves were reflected efficiently from the open plate ends, the (standing wave) sum of the deflections then causing failure of the upper plate at the weakened point.

The design plans for the bridge show a (north, river end) tower with cable stays that line up with half the diagonals. Those cable stays had not yet been installed, so the structure needed to be temporarily carried during construction by a much greater compression in the top plate. Once stayed, the upper plate would not be under such a high compression and would then function as a cover over the suspended walkway. I believe that was the reason why during installation it was left “vulnerable to collapse”.

In my opinion, the I beam design did not sufficiently constrain the top plate to prevent buckling during construction. It was probably the failure of one off-centre cable, then a doubling of the amplitude by the standing wave formation that exceeded the top plate specifications. I suspect the dynamic doubling of the amplitude due to wave reflection would not have been considered when designing the top plate for construction.

It may go back to the beginning of the design when the “open look” of one central line of stays attracted attention to the prejudice of stability. If a box form had been used with two lines of stays, one on each side, the top plate could not have rocked. During construction, the one line of cable stays along the centre line allowed one too many degrees of freedom.

A good engineer will work out the strength requirement, then double it. But when reflected waves are involved, the deflection can double and so come back to bite the engineer. After the tragedy it is more important to identify the failure mechanism than it is to prosecute and punish. The engineers involved are also victims.

The emotive term “stress test” implies a deliberate approach to the point of failure. The adjustment of cable tension after the move would have been expected. That was probably what they were doing when the cable snapped and the stored energy in the structure was released.

I may be totally wrong, but it seems to me to be the simplest explanation of how such a tragedy might happen.

Stored energy will get you everytime.

 

[jim hardy]

looking at @nsaspook 's picture from 7 there are stays i'd never noticed . So somebody was worried about at least one more degree of freedom
and the truck dashcam video shows where it broke, though a crane blocked the view so it's inexact
and if that fellow 'AVE' is right about that tendon snapping in this diagonal

Would it stay up without that first diagonal ?
dashcam view from other side...

old jim

 

[jim hardy]

better shot of that tensioner-looking apparatus, from images at http://abcnews.go.com/US/pedestrian...tional-university-collapses/story?id=53774444

broken tendon ?

 

[Spinnor]

This is a screenshot of what this bridge was to look like. Those "cable stays" when installed would have greatly increased the strength of the bridge.

Screen shot from,

On a major project like this I would hope that in all phases of the work an engineer is needed to approve every action taken by the workforce. Is that typically the case?

 

[Greg Bernhardt]

This is a screenshot of what this bridge was to look like. Those "cable stays" when installed would have greatly increased the strength of the bridge.

So it was a sort of suspension bridge and at the time there was no suspension?

 

[Baluncore]

So it was a sort of suspension bridge and at the time there was no suspension?

Yes. It was designed as a truss to survive installation, but something went wrong before the tower and final cable stays were installed.
See; https://en.wikipedia.org/wiki/Cable-stayed_bridge#Comparison_with_suspension_bridge

looking at nsaspook's picture from 7 there are stays i'd never noticed . So somebody was worried about at least one more degree of freedom

I notice those extra stay cables keep the angle between the bottom plate and the central web of diagonals perpendicular, yet there were no similar stays to keep the top plate flat. It is as if the designer of the temporary stays forgot that the fundamental weakness of the long truss structure was buckling of the top plate in compression. The end bay of the top plate looks very open and vulnerable, yet that was the area that appears from above to be most crushed in the debris.
The end diagonal would have been under high compression due to web shear. I wonder if they re-tensioned those tendons, increasing that compression, when tension was not possible in that member.

 

[jim hardy]

The end diagonal would have been under high compression due to web shear. I wonder if they re-tensioned those tendons, increasing that compression, when tension was not possible in that member.

During the move might that diagonal have been in tension? Note the moving rig was well inboard from that end to clear the concrete traffic barrier down on the road.
If it stretched and yielded the tension rod, what would happen?
Check this newscast video. https://www.cnn.com/2018/03/16/us/bridge-collapse-florida/index.html

0min:33 sec Cracking at unspecified location on north end

1min:35 sec "'..at the moment of the bridge's collapse..work was being done to strengthen the diagonal supports..."

from the dashcams only people up on top were right over that diagonal. Might somebody have tried to close up cracks by tightening the rods?

Can't conclude anything from what little we know,
but as an old troubleshooter i'd sure have a close look at that area to find out who was doing what, and why ..

old jim

 

[Dr.D]

Rather looks like it was some on-the-spot guess work that decided to support the bridge during erection other than per the intended erection plan. Thus put the supports in different places, changing all the internal loads on the structure and setting one or more tension rods up to fail. Lots of questions to be resolved and answers to be found. Look for a whole lot of finger pointing to happen. Old Jim Hardy has showed us the tensioner that seems to have driven the final failure.

 

[jim hardy]

That AVE guy on Youtube first spotted it.

Disasters are like dominoes - a whole lot of insignificant little details line up waiting for a trigger. It's how the small things of the Earth confound the mighty.

old jim

 

[jim hardy]

It gets curioser and curioser.

The end diagonal would have been under high compression due to web shear. I wonder if they re-tensioned those tendons, increasing that compression, when tension was not possible in that member.

might not have been a tensioner in it? sure looks like one in the rubble...
This is from the proposal so not an "As Built" drawing..
http://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf

so we'll have to wait on the trickle of information.

@Baluncore - that AVE youtube video pointed out when they lifted the span that end was cantilevered - wouldn't that put north-most diagonal #11 in tension?

Hopefully somebody made provision for the "As Lifted" transporter arrangement...

Sigh. Old troubleshooter just can't let go. I grew up about five miles from there.

old jim