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Miami Pedestrian Bridge, Part XI 32

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JAE

JAE

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Jun 27, 2000
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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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From my previous experience the type/magnitude of this disaster would bring down the companies responsible. There should be virtually no prospect of getting future work to continue trading after all the facts have been revealed.

The employees would likely continue doing the same kind of work as before possibly together under new company names. The equity holders of the old companies are expected to suffer financially after the indemnity insurances have taken the first hit.
 
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This larger fragment of 11/12 node (green) is of interest to me. How it separated with this size and shape should be explored.

Turquoise marks 2 whole shims, 1 damaged shim, drain pipe fragment, and concrete fragment with impression of PVC pipe. The steel plate leaning on pier is leftover stock used to fabricate emergency steel shim?

Can rebar (red) be identified? Is it important?

nodeblock_ykvtsz.png
 
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@jr - that's a good question. It looks like the block even has a rebar mark on the front face. The steel plate is likely the shim that was used under 12.
 
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They are screenshots from OSHA report, zoomed in with added markups. The page of report and zoom level can be seen in top margin of image. Page 91/115 and 93/115. Zoom 400 or 800 percent.
 
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jrs_87 said:
... Is it important?
Click to expand...

Yes. But there comes a point of diminishing returns. Personally, I find the information to date and the touted explanations inadequate. Every piece of debris and every crack in the concrete tells part of the story. Six people died and another is permanently disabled so "it was punch out" is not much different than "it broke" or "it fell down". If the victims aren't enough of a reason, the photos and videos beg for better answers, if not just for a coherent presentation of all the facts.
 
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Damn some of that court case reports are damning.

Louis Berger cited those examples as part of its effort to persuade Judge Bailey to give it access to the draft minutes and communications FIGG and its lawyers created as it developed the “corrected minutes,” which were sent to NTSB six weeks after the collapse.

FIGG claims those records cannot be seen by others because they are privileged work product, created in consultation with its lawyers in anticipation of litigation.

Judge Bailey decided to review the underlying materials herself before deciding if FIGG made “any kind of factual alteration” in its drafts. If so, she said, it is possible she could override the privilege and allow others to see what FIGG and its attorneys were doing in preparing their version of the meeting.

Louis Berger has not yet seen the drafts and communications, but based on what it has seen in discovery, it is saying it appears FIGG did alter the facts.

“A comparison of the corrected minutes to contemporaneous handwritten meeting notes of FIGG employees and other contemporaneous documents confirm that FIGG counsel likely changed or permitted changes to the facts that resulted in corrected minutes that were not fully faithful to what transpired at the March 15 meeting regarding the cracking,” wrote Louis Berger’s attorney.
Click to expand...
 
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Another fascinating point of this collapse is seeing the party most knowledgeable with the bridge could play dumb on professional and technical matters like:

When something obviously important and wrong could be dismissed lightly with no real sense of urgency.

Knew the bridge in distress but ignored the safety and fantasised the defects could be concealed in future by building quickly its back span.

Unilaterally instigated an unsafe operation that led to the bridge collapse.

Displayed in public various attempts to conceal, alter and destroy evidence.

We all know in engineering we learn from mistakes. If one does not admits mistake then it is possible that he doesn't really know about the mistake and so is unable to correct it. Disturbingly this bridge collapse has such a hallmark.
 
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Concur with saikee119...

The bridge was screaming for help and to some extent the presentation acknowledges that.

In an instance like this with large and growing cracks at a critical location that you can not replicate with analysis, as the EOR, you have to convince yourself it is not OK until it is.

It appears they convinced themselves it was ok until it wasn't.

IC
 
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Most of you on this site have vastly more experience with real reinforced concrete than I could ever have. Please excuse my ignorance.
My simplistic understanding of the failure is:
Member 11 cracked at the cold joint and overcame the resistance of the rebar and filet.
It (11) developed enough momentum to take out member 12 and the attached portion of the deck/diaphragm due to insufficient steel connecting the thrust from 12/deck/diaphragm to the PT in the deck.
Is this so over simplified as to be wrong?
It seems to me that at least two insufficiencies were required for the bridge to collapse.

SF Charlie
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Here, in San Francisco, where we have earthquakes, we have different requirements. Reinforced concrete has to withstand repeated blows from it's foundations. (Yes, I'm still going to talk about the bridge.) First a structure of welded rebar is constructed (It looks like it could hold up the building by itself, I'm sure it can't.) The space for concrete is only a few inches between rebar. then a cage of rebar is put around the whole thing, then concrete poured. some of the retro fit columns even have a cylinder of sheet steel around the concrete to contain it if it crumbles. Now about the bridge (finely.)
Does the lower PT rod pulling out of member 11 indicate insufficient confinement? It prevented the PT rod from having any possibility of working as a dowel. Does member 12 have insufficient confinement? It seems to have "exploded" around it's rebar, leaving the rebar more or less intact.

SF Charlie
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SFCharlie (Computer),

I am convinced that 11 and 12 were moving together as one unit at the bottom. Member 12 has some additional resistance against moving outward and this is from bending its connection with the canopy. The resistance from bending is relatively small because a very small deflection at the bottom of 11/12 is enough to fail the joint and collapse the bridge.

Most people outside RC design may not realize the cast-in items, of 4 No. 4" vertical sleeves and one 8" horizontal pipe, could and have destroyed the integrity of this joint because they shortened the load paths.

You can see from my 29 Jun 21:49 posted sketches that the centrelines of 11 was able to join with 12. The deck was only to the side of 11. The one PT rod were also arranged to bit hard into the deck while the other to 12.

Member 12 is not structurally important and it together with the part of the joining canopy are omitted in a standard Warren bridge. As each joint the internal and external forces must be balanced to achieve equilibrium. The horizontal component of the diagonal 11 balances with the tension in the horizontal deck while its vertical component with the vertical reaction (50% of bridge weight) at the pier or pylon. Therefore one can chop off 12 and the bridge will still stand. However the canopy was used for anchoring the longitudinal PT tendons and 12 would have its self weight exerted on the support, other than that 12 had very little resistance against moving outward.

In conclusion 11 only needs one insufficiency from its connection with the deck and carries 12 as a piggyback. Obviously if 11/12 were to break away from the deck it could slid out of the whole CJ or part of the CJ with 12 broken off along the plane with the 8" pipe. It chose the latter.

The crack photos confirm 11 suffered some internal splitting in axial direction first slightly when it was first tensioned in the roadside and then later quite seriously, with 12 wide and 72 deep cracks, when the tension was first removed.

The pull out of the lower PT rod should be looked at a consequential damage. It was from a follow-on consequence when the deck dropped to the ground while 11/12 was caught up with the pylon. The blow out was indeed due to insufficient concrete surrounding some rebar but OSHA has reported a smaller number of rebar were sheared off at the CJ.

 
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44. “[I am] opposed to the laying down of rules or conditions to be observed in the construction of bridges lest the progress of improvement tomorrow might be embarrassed or shackled by recording or registering as law the prejudices or errors of today.”

― Isambard Kingdom Brunel

86. “Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

― Unknown

33. “Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance.”

― Dr AR Dykes​
 
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