Miami Pedestrian Bridge, Part XI 32

[JAE]

A continuation of our discussion of this failure. Best to read the other threads first to avoid rehashing things already discussed.

Part I
thread815-436595

Part II
thread815-436699

Part III
thread815-436802

Part IV
thread815-436924

Part V
thread815-437029

Part VI
thread815-438451

Part VII
thread815-438966

Part VIII
thread815-440072

Part IX
thread815-451175

Part X
thread815-454618


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[Tomfh]

Why not just use it as a timeline and say the detensioing may have been the likely disturbance

That’s what is being done. The sudden dramatic worsening of the cracking occured when they began to detension, so it is likely that detensioning was a causal factor, as opposed to it just being a coincidence.

Calculations imply the rod tension was worse than no tension

And? If this saga has one thing to teach us it’s that we shouldn’t be beholden to our numbers when the structure is telling us something different.

 

[Earth314159]

And? If this saga has one thing to teach us it’s that we shouldn’t be beholden to our numbers when the structure is telling us something different.

What was the structure telling us that was different from the numbers? The detensioing is irrelevant. The calculation said that reshoring was the proper course of action. The cracks said reshoring was the proper course of action. What did we learn about the detensioing possibly initiating the crack? Both the numbers and behaviour said what the proper course of action was. The possible disturbance that started the sliding is not relevant. We can't use that to predict anything.

The detensioing wasn't the casual factor. The over-stressed joint was the causal factor. We learn a lesson about the over stressed joint. We learn to check our calculations when we see an over stressed joint. We learn to keep looking if we can't find the cause. We learn to get assistance if we can't determine the cause. We learn not to risk the public safety if we can't determine a cause. We learn to identify a joint that is over stressed in shear friction.

 

[Tomfh]

The detensioing wasn't the casual factor. The over-stressed joint was the causal factor.

They were both causal factors. The node was too weak, and was already showing signs of cracking before detensioning. The detensioning made things a lot worse.

 

[Vance Wiley]

The detensioing wasn't the casual factor. The over-stressed joint was the causal factor. We learn a lesson about the over stressed joint. We learn to check our calculations when we see an over stressed joint. We learn to keep looking if we can't find the cause. We learn to get assistance if we can't determine the cause. We learn not to risk the public safety if we can't determine a cause. We learn to identify a joint that is over stressed in shear friction.

I agree, and all very good points.
But this is a discussion forum and many points raised will not directly be causal in nature but will receive intense attention and require lengthy discussions in the legal system procedures just ahead.
And the examination of how this event developed and progressed can provide warning signs in ones future practice. This is not a common event, the firms involved represent the basic "standard of care" in their respective industries, and yet it did happen.
Hopefully it will never happen in your career. I have sat beside the Judge and addressed the jury, as an "expert witness". I have seen instances where "details matter". Even small details.
So I encourage deeper investigation. It cannot change the outcome, particularly for those who lost their lives and those injured by the event. It may further some level of understanding. That can't be a bad thing.

 

[Earth314159]

I agree, and all very good points.
But this is a discussion forum and many points raised will not directly be causal in nature but will receive intense attention and require lengthy discussions in the legal system procedures just ahead.
And the examination of how this event developed and progressed can provide warning signs in ones future practice. This is not a common event, the firms involved represent the basic "standard of care" in their respective industries, and yet it did happen.
Hopefully it will never happen in your career. I have sat beside the Judge and addressed the jury, as an "expert witness". I have seen instances where "details matter". Even small details.
So I encourage deeper investigation. It cannot change the outcome, particularly for those who lost their lives and those injured by the event. It may further some level of understanding. That can't be a bad thing.

I agree. These events also do make me think and to be careful.

 

[Eufalconimorph]

It pays to remember the "swiss cheese" analogy of catastrophe. Normally the bubbles in swiss cheese create small depressions, but they don't all line up to create holes entirely through the wheel. But sometimes a whole bunch of independent bubbles can line up and cause a whole all the way through.

A similar thing applies in most disasters. There's almost never one root cause, but instead a set of causative and exacerbatory factors. The closest to a single root cause here is that the design was insufficient to carry even the self-load of the bridge. The 11/12/deck node was particularly bad. The detensioning altered the stress patterns in and around the node, probably helping it crack. The retensioning caused further movement and the collapse of the entire structure. The decision not to close the road and shore up the bridge was also a cause. The decisions made in modeling and calculating the forces involved were causes. The regulatory oversight enforcement structure that allowed for fewer design checks than legally required is a cause.

If you dig far enough you hit the second law of thermodynamics, statistical mechanics, and the rest of the laws of physics as ultimate causes. It becomes useless in preventing future disasters. But finding the various causative factors and changing procedures to fix them can help. If any of the causes I listed (or several others) hadn't happened the structure wouldn't have collapsed when it did. It still might have collapsed, eg if the only change was to not detension the #11 rods it probably would have held up until a large live load of people was placed upon it (Murphy's law).

 

[SFCharlie]

I don't consider the detensioning the root cause, nor the retensioning, but the person who ordered the de- and re-tensioning.


SF Charlie
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[Tomfh]

Reading over the OHSA report and FIGG's actions, it seems FIGG were pretty worried despite their repeated reassurances their was no risk.

They spoke of the need to capture the node, and were urgently preparing the strapping details.

What was their rationale in insisting propping/shoring wasn't necessary? Were they concerned in may inadvertently worsen the situation? Or was it more about saving face and avoiding the embarrassment of having to shut down the road and install emergency shoring?

 

[Vance Wiley]

I'm betting it is the latter. This "Signature Bridge" came with a Prime Directive of glorifying ABC methods.

 

[Earth314159]

Reading over the OHSA report and FIGG's actions, it seems FIGG were pretty worried despite their repeated reassurances their was no risk.

They spoke of the need to capture the node, and were urgently preparing the strapping details.

What was their rationale in insisting propping/shoring wasn't necessary? Were they concerned in may inadvertently worsen the situation? Or was it more about saving face and avoiding the embarrassment of having to shut down the road and install emergency shoring?

I suspect it would be a huge embarrassment to have to close a number of the lanes after bragging about the ABC system. There is also a money issue.

Figg's comment about capturing the node is very interesting. It suggests they did know what the issue was but ignored the shear plane calculation at the pour joint in their presentation the morning of the collapse. How did they know they had to capture the node but also made the comments about not knowing where the cracks came from? It seems contradictory.

 

[Tomfh]

Figg's comment about capturing the node is very interesting. It suggests they did know what the issue was but ignored the shear plane calculation at the pour joint in their presentation the morning of the collapse. How did they know they had to capture the node but also made the comments about not knowing where the cracks came from? It seems contradictory.

I suspect they knew it was bad. Their recognition of their need to capture the node, and their haste in developing emergency strapping suggests they didn't have full confidence in their powerpoint calculations.

 

[Vance Wiley]

How did they know they had to capture the node but also made the comments about not knowing where the cracks came from? It seems contradictory.

Good point. It is contradictory. Not the first instance.
They were gambling on having time to make corrections and stated that when the backspan was complete all would be good. Unfortunately the chips on the table were the lives of 6 people. I challenge the statement about the backspan providing all the stability needed. And I do not know the schedule but it would be at least 4 months before the backspan would be constructed, cured, and have falsework removed. That is 4 months of the public being exposed to a serious risk.
Kenny Rogers sings softly in the background . . . .
You gotta know when to hold'em
Know when to fold'em
Know when to walk away
Know when to RUN

 

[Tomfh]

Unfortunately the chips on the table were the lives of 6 people. I challenge the statement about the backspan providing all the stability needed. And I do not know the schedule but it would be at least 4 months before the backspan would be constructed, cured, and have falsework removed.

They intended for the steel beams to act in the interim, prior to backspan being installed. They don't appear to have figured out how to make the backspan provide the necessary stability, but once the steel straps were in they would have some breathing room to figure it out.

That meant they only needed to worry about the period until the steel straps were installed, which is presumably where the decision to retension came in. They emphasised the need to retension ASAP.

 

[BridgeSmith]

I find some of the discussion of detensioning and retensioning of the PT rods, as if the concrete responds elastically (if detensioning widened the cracks, retensioning should have closed them), to be a bit strange. Detensioning of the rods reduces the confinement, allowing the concrete to crack more extensively and allowed the already cracked concrete to move. Retensioning the rods does not put the genie back in the bottle - you can't carry stress across the crack anymore; all it does at that point is produce new stress concentrations and local crushing (concrete shearing) in new places, speeding the demise of the connection.

 

[Tomfh]

I find some of the discussion of detensioning and retensioning of the PT rods, as if the concrete responds elastically (if detensioning widened the cracks, retensioning should have closed them), to be a bit strange.

I think most of us appreciate that it didn't work that way, and that releasing the PT allowed the cracking to dramatically worsen, and that retensioning didn't close them up but instead triggered collapse.

However it seems to have been FIGG's working assumption that retensioning would help the joint, which is why it's being discussed. Without the benefit of hindsight I don't think it was an entirely unreasonable assumption on FIGG's part. Still quite risky to not immediately shore it up, but I don't know how many of us could have readily predicted that the retensioning would cause it to fail completely.

 

[Sym P. le]

I find some of the discussion of detensioning and retensioning of the PT rods ... to be a bit strange.

I cringe everytime I hear the term used in association with this incident, especially when used by the media which lends it an air of authority. This is not a meccano set.

I prefer to think of them as having been decommisioned, having served their purpose. The fact that they ended up keeping the structure together before being used to destroy it are different matters.

 

[Sym P. le]

... I don't know how many of us could have readily predicted that the retensioning would cause it to fail completely.

There, but for the grace of God...

I disagree.

 

[Tomfh]

I disagree.

You think you would have predicted it?

 

[jrs_87]

Is it time to regroup and refocus thread?

 

[Vance Wiley]

I think I agreed with that. But much less succinctly.
See Vance Wiley (Structural)24 Jun 19 18:01