Miami Pedestrian Bridge, Part XIII 81

Per MikeW7 in the previous thread (Part XII):

MikeW7 said:

The National Transportation Safety Board announced Thursday its intention to hold a public board meeting Oct. 22, 2019, 9:30 a.m., to determine the probable cause of the March 15, 2018, FIU pedestrian bridge collapse.

Click to expand...

Check out Eng-Tips Forum's Policies here:
faq731-376

That's weird. How do you determine the cause of the collapse in a board meeting? Who is on the board? Who is invited to participate? I think we were all expecting a comprehensive report, not a meeting.

I think it is all about PR. A chance to have the spotlight for a moment.
How have they done this in the past? I can imagine a board meeting to review a proposed report and approve or modify it as board members might think proper. But in public?
That must indicate this is the final thing.
Or maybe they want some public input? That could get out of hand fast.

It is likely because there were a number of contributors and the meeting is to discuss ranking them.

At some point the draft will be circulated to all parties for more detailed comments, support, and objections. This is typical in major aircraft accidents where recommendations are being made; there is no point in the NTSB making an impossible recommendation and they recognize they can't know everything.

This is part of why it seems to take so long - the goal is to test assumptions as to what the underlying factors are and that proposed fixes will actually make for better outcomes.

October 22. There will be a webcast. Should be interesting.

NTSB posts videos of board meetings on their Youtub account

Hokie66 if you watch some of the other 'board meetings' accessible form the link posted you'll get a feel for how they are run. I watched one, and it seemed like it was simply a presentation of the facts from the investigators into the accident, with the board members of NTSB able to ask questions and probe particular things a bit further.

Actually, rather than speculate with incomplete data, I was just going to wait for the report to come out.

Is that satire?

Edit - the post was removed/deleted

Your turn. Impress me.

Click to expand...

The people who have actually studied the available failure info and have written reasons why the lower PT rod in member 11 could not have broken impress me. You ramblings don't so much, meaning I have no want to impress you back.

Back again with my collapse theory (25 Jul 19 02:18). I think it also explains the detension/retension conundrum. For this, ignore the upper PT rod, only the lower PT rod is relevant. 11 and 12 are a cohesive unit free of the slab and connected to the diaphragm only through the base of 12 (rebar between the slab or diaphragm that run through 11/12 only act as dowels which provide nothing in the way of constraint jrs_87 (Mechanical) 10 Aug 19 09:29)

When the structure was on the form work and the lower PT rod was tensioned, it served to hold the flaky joinery tight. Although the strain was already pulling at the joinery and causing the cracks to show when the formwork was removed, the full magnitude of the problem had yet to be revealed. The structure was then moved and set in place with the lower PT rod still hiding the issue. When the PT rod was detensioned, the structure settled with the slab sagging and the top of the diaphragm pulled toward the center while 12 bowed out carrying the load transmitted down 11. The structure held in this degraded form until the lower PT rod was retensioned (to the max). This activity had no leverage to pull the structure back into shape. The significant force vector from the retensioning of the lower PT rod is the horizontal component that is pulling the slab (by way of the tab) toward the center of the structure, amplifying the tearing of 11. The structure sags further building the load on 11 etc.

You might have something there alright. Add in the fact that those vertical rods adjacent to the no 12 vertical column were not connected and the slippage of the 11/12 node before they tried to apparently tighten it up and it could easily simply wrench the 11/12 node away from the deck.

your top view doesn't show the 2 or 3 tubes set into the deck which appear in some drawings and not others and have a high suspicion factor about removing the interface between 11/12 and the deck. IMHO.





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

The chevron surfaces in my model, though not fully detached are easily detached, a.k.a. tear on the dotted line. That's why I try to envision the failure as though they were detached.

I would like to expand the discussion a bit - with NO intention of being critical - we both end up at the same place.
First - you present a good description of your thoughts. Second - the graphics are very helpful. They were seen as such when first presented. I will use interrupted quotes as a means to assist my comments.
I would view this using the deck as the constant - it is the largest single element in the structure. It is 31 feet wide and 174 feet long and weighs about 1150 kips vs 16 kips kips for member 11. So I would describe the action as member 11 being forced across the deck and out the end of the deck. I am not sure there was a lot of sagging before node 11/12 went into full failure mode.
Full agreement here.
Since the lower PT rod remained anchored in the deck, and that anchor did not move (within our ability to see at this time) it seems more appropriate to view member 11 as having an unresolved force at node 10/11 from the PT rod tension, with the lower PT rod pulling node 10/11 to the north with the horizontal component of the 280 kips of rod tension. We have seen a picture of a longitudinal crack in the west side of 11 about 6 " above the bottom and maybe within a foot of the fillet between 11 and 12. I read this as the beginning of the splitting of 11 which resulted in the upper part of 11 sliding north and off the end of the deck, with the bottom of 12, while leaving the lower PT rod anchored in the deck.
Fully agree. The failure of node 11/12 to remain connected to the deck left two load paths to deliver load to the pylon - the Deck and the canopy. With both acting in bending only they failed immediately.
Thank you.

Vance, thanks for your discussion.

LittleInch said:

... it could easily simply wrench the 11/12 node away from the deck.

Click to expand...

It's ridiculous how basic the explanation is once all the clutter is removed.

FortyYearsExperience (Structural) 19 May 19 16:32 said:

There is, in reality, zero amount of steel provided to tie diagonal 11 to the deck.

Click to expand...

I would add that 12 also required being tied longitudinally into the deck as it would be a robust backstop for 11.

Berger: FIGG Is Slow To Hand Over All Bridge Collapse Data

SF Charlie
Eng-Tips.com Forum Policies

Interesting that the ENR report says the final report will be issued on 22 October, while the NTSB has scheduled a board meeting and webcast that day to "determine the probable cause". Just bureaucratic lingo, I suppose.

NTSB Docket for FIU pedestrian bridge collapse Has been made available:
Docket And Docket Items
Happy searching.


SF Charlie
Eng-Tips.com Forum Policies

Bridge Factors Factual Report Attachment 73 – FHWA Assessment of Bridge Design and Performance
Is interesting

SF Charlie
Eng-Tips.com Forum Policies