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Miami Pedestrian Bridge, Part VII 51

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JAE

JAE

Structural
Jun 27, 2000
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A continuation of our discussion of this failure. Best to read the other threads first.

Part I
thread815-436595
Part II
thread815-436699
Part III
thread815-436802
Part IV
thread815-436924
Part V
thread815-437029
Part VI
thread815-438451




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Nothing new there. We may have to wait a year or more. I still wonder who is actually doing the design assessment, as of the 'Party Members' listed, I don't think there is one who is both technically qualified and not conflicted.
 
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Re: NTSB Preliminary Report May 23, 2018

My highlights:

the [highlight #FCE94F]propagation[/highlight] of cracks in the region of diagonal member 11

Department of Transportation Office of the [highlight #FCE94F]Inspector General[/highlight] (OIG)

Miami-Dade [highlight #FCE94F]Police[/highlight] Department (MDPD)

Lack of detail in report and OIG involvement (if not routine) indicates to me there is a strong criminal element being investigated.

 
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That NTSB Preliminary Report isn't worth the digital footprint it makes. The Pre-lim Report leads one to believe no work was carried out on the number 2 member but evidence of light chipping debris immediately outside the blister suggests otherwise, along with statements from other parties involved. Why isn't Bolton-Perez and Associates Consulting Engineers included in the list of parties assisting in the investigation? Bolton-Perez and Associates Consulting Engineers should have done a better job of documenting the cracks.
 
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Epoxybot,

I don't like the preliminary report either. NTSB can expect pressure from the families now.

Bolton Perez & Associates used photographer's thumb as scale, not a crack width meter. And a pencil mark instead of a CrackMon. If true, this is very dubious for professionals. Serious tools for monitoring cracks:



cmon_mqlxae.jpg
 
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I've read hundreds of full NTSB reports, and thousands of preliminary reports and report synopses, and this one is exactly in line with my expectations based on their past work. Their preliminary reports are never anything more than an expression of the facts available by inspection, and where practical, a statement of what resources (if any) are expected to be engaged in the ensuing investigation. They often contain substantial errors and omissions, and are rarely emended before the final report is released.

NTSB final reports are almost always of a depth commensurate with the seriousness of the accident at hand. For a non-fatal small airplane crash, the final report will usually contain only a little more than what was in the preliminary, plus a statement of probable cause. For a fatal airplane crash, the report will be quite voluminous, and may include recommendations for regulatory change in addition to the probable cause. For an accident such as the one at hand, I think we can reasonably expect that the final report will be quite thorough, will engage all necessary resources, and will evaluate a wide range of contributing factors in arriving at a probable cause.

--Bob K.
 
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Maybe we won't have to wait so long after all. "In the next [highlight #73D216]month[/highlight], the NTSB will be..." Typo maybe?

While learning the results and recommendations of the investigation will be interesting, I'd much rather see what they decide to build in place of the failed project.
 
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I think it would be sensible to put up a precast double tee bridge, as is commonly done to provide pedestrian access to properties that are otherwise isolated by new expressways. There seems to be a common or semi-standard design, supported by two or three hammerheads, terminated by spiral ramps or stairs, and completely covered by chain link fence.

That 8 foot tall 'projectile barrier' or whatever they call it in the plans is just silly. FIU no doubt has multiple students of both sexes who _could_ loft a bowling ball over it with ease. ... and at least an equal number of local nonstudents and random miscreants who _would_ do it, for no particular reason.



Mike Halloran
Pembroke Pines, FL, USA
 
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In case people didn't see this, NTSB released a 4 page preliminary report. That report confirms something important that had seemed an open question, that the PT rods in #2 and #11 were indeed de-tensioned on schedule on March 10 when the span was put into place. The activity taking place on March 15, the day of collapse, was not part of the construction plan, and involved adding tension to those rods.

 
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<tangent>
Thanks to an accidental collision with a trailer ball that I had just installed and immediately forgotten about, I spent a month in a wheelchair with a bum knee.
I found that the ADA guidelines are not comprehensive enough or strict enough.
- The allowable ADA slope is still _very_ difficult to navigate in a self-propelled wheelchair.
- Conformant bathrooms are equipped with wide doors, with very stiff closers that make it impossible to open the damn door while sitting in a wheelchair.
- I have already complained about the stutter bumps on grade crossings, that are no longer required, but were not removed, and are still occasionally placed in new installations, probably due to the glacial pace of plan reviews.

Oh. $24,000 worth of progressively fancier imaging attempts finally produced a picture of what you might call a 'spall' if you saw it on a bearing ball, and advice to wait for it to heal itself, which it eventually did. I still carry a cane, not so much for support but for reaching things on high shelves and picking stuff up off the floor.
</tangent>



Mike Halloran
Pembroke Pines, FL, USA
 
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The construction joint is irrelevant to any matter in issue. If the concrete was in compression at the construction joint, then the concrete would simply have closed up around the joint. If it was in tension, well, concrete has no strength in tension, so all the tensile forces would have to have been taken by reinforcing bars.
 
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FYE,
Tension, compression...you left out shear. The joint was not normal to the axial force. And then there is bending, which includes all three.
 
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The critical question is this: what was the amount and configuration of the steel reinforcement in the deck in the vicinity of the #11/#12 joint with the deck. That is the location of the highest shear (and hence tensile) stress in the entire structure. We are being denied any information about this aspect. No drawings shown from above of the reinforcing steel layout. Only reinforcing steel can take the shear/tensile stresses in the deck at that point - and yet, not a single image or diagram of the layout of that steel. The investigators could easily make that information available. Instead, we are being run around the mulberry bush looking at information about pre-tensioning cables which is irrelevant.
 
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