Miami Pedestrian Bridge, Part XIII 81

[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

Part XI
thread815-454998

Part XII
thread815-455746


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

 

[Vance Wiley]

I thought it illustrated another example of how the contract drawings lacked the necessary clarity and coordination to convey the design intent of this complex structure to the construction team, and thereby contributed to a weakness in the constructed structure.
If it has been discussed previously, I apologize.

 

[TheGreenLama]

Sorry, Vance. No need to apologize. Your point is completely valid on issues with FIGG's drawings. My post was addressed at earlier posts discussing possible failure modes, some of which seem to be dealing with damage that was more secondary in nature. Based on all of the newly released pictures, the base of 11 exploding seems to be a plausible, and also the simplest, path to collapse.

 

[Vance Wiley]

Need a Link to Part XIII -
End of XII and top of forum needs a Link to XIII
Please.
Thanks.
Like Adam said to Eve -"Stand back -I don't know how big this gets".

 

[Sym P. le]

Further to my previous posts, when 11/12 detaches from the deck, the load path is realigned, roughly as indicated in the following sketch. This is consistent with the spalling on the back of the deck/diaphragm (MCM refers to it as spalling in their material) This realignment likely set in, in part, when the PT rod was detensioned.

Imagery included in the NTSB documents clearly indicate that 12 is distressed bowing to the north with numerous tension cracks on the north facing portion.

Also, the structure appears to hold together long enough that a considerable amount of the concrete is simply overwhelmed rather than being punched, scoured or otherwise abused. The following image suggests that 12 failed in keeping with the capture cone of the upper PT rod. (overlay is from my post on Part XII 24 Jul 19 23:33) If 11 failed first, it would have alleviate the stress on 12.

Edit: in the overlay, there is some excessive debris mat'l near the upper PT rod anchor plate. This is somewhat misleading and I'll repost the image once I get a chance to edit this debris out.

 

[Vance Wiley]

The photo of cracking of the north face of 11 in the report was pre-collapse. It could be due to the moment induced by some drop in node 10/11 during the early stages of slip in 11/12. I think the geometry shows 10/11 drops about twice the amount of slip at 11/12 (while member 11 is intact).
I think it is helpful to visualize the collapse sequence from the standpoint of vertical drop in node 10/11 and node 9/10.
If we accept the idea that the deck surface immediately under and in contact with member 11 has allowed slip in the cold joint, which photos taken seem to show, then any remaining resistance is under member 12 and may include any cross plane reinforcing. However, the WJE report and tests show that slip of the joint was only 0.02 to 0.025 inches at maximum resistance so it is likely that the 6 - #7 hoop bars have sheared when a bit more slip has developed.
Now if we fold in your earlier thought that member 12 has high moments causing tension in the north face, and hinges about the 3 - #11 bars in its south face, that moment would seem to be caused by the dropping of node 10/11. In observing photos of node 11/12 area of the deck, it appears the #11 bars in the south face forced the failure zone to drop to the top of the 8 inch pipe thru the diaphragm. That could indicate the reinforcing across the top of the deck at the joint with 12 provided more capacity than the deeper zones and sides, particularly since diagonal cracking had already developed in the entire zone from vertical loads in the end diaphragm.
While focused on member 12 having moment, we see member 1 has two hinges intentionally formed and has far more reinforcing than 12. Member 1 has 14 - #11 bars, totaling 21.8 sq in, while member 12 has 3 - #11 bars and 9 - #7 bars totaling 10.68 sq in. And member 12 has no formed hinges to release moments. Another indication of an apparent discontinuity in the contract drawings.
A lot is happening just after the collapse is triggered. Node 11/12 is sliding to the north, member 11 is pushing against member 12 unless 12 has already lost any ability to resist. Node 10/11 is dropping, inducing moments in member 12 and thus into its connection (if any remains) to the deck. Node 9/10 is dropping, causing the deck between 9/10 and the pylon to rotate, and slip to the south. Which one is ahead? It is a horse race, in my opinion.

 

[jrs_87]

‘Just not a discussion’: How paralyzing group-think led to six deaths at FIU bridge

https://www.miamiherald.com/news/local/community/miami-dade/article236250848.html

 

[Pete_K]

NTSB meeting occurring

http://ntsb.windrosemedia.com/

 

[JAE]

Robert L Sumwalt: "Failures up and down the line"

 

[BadgerPE]

Real quick question. Has anything definitive been posted about the reason or who the liability falls on? I assume not with NTSB hearings being conducted as we speak, but was just curious if anything has come out. A friend and I were discussing the HRH collapse in New Orleans and this one came up too. I have followed some of the earlier discussions of this from afar, but need to catch up on the later ones over the winter.

 

[JAE]

The NTSB board meeting just finished - my quick summary:
1. Lots of blame on Figg for miscalculating the demand and miscalculating the capacity of the 11/12 node. Lots of reference to shear friction, roughened surfaces, etc.
2. Reference to multiple pipe sleeves and drains creating voids in the node and reducing capacity.
3. Blame put on Figg/Lewis Burger in that Burger apparently wasn't qualified to do the peer review of this type of bridge.
4. Blame on FDOT for various things - implied lack of follow up on peer review, etc.
5. Blame on all parties for not seeing that cracks that wide were essentially the bridge "screaming at them that something was seriously wrong".
6. Recommendations for greater oversignt, peer review, etc. on complex bridges, especially non-redundant structures.
7. Clarification of, or recommended increase in, authority given for various parties to shut a bridge project down if cracks/anomalies are discovered and require immediate shoring.

 

[Kestrel42]

I’ve been following this forum since the day of the collapse but since I’m not in structural or construction, so I’ve never thrown out any comments. However, I’ve been an NTSB follower for many years and have watched at least a hundred of these board meetings. I’ve never seen anything like this. The NTSB clearly thinks that everyone involved in the design and construction passed the buck on safety. Figg, Louis Berger, FDOT, FIU, MCM and Bolton Perez are all directly responsible for the collapse and associated deaths. I’ve never seen a board meeting where the chair made such strong negative comments about so many parties to the investigation. Sumwalt even commented that there’s an insane amount of finger pointing going on.

I still don’t understand how LB got involved when they were not qualified to review the construction plans in the first place.

The most shocking thing that came out in this board meeting were the completely inappropriate (and multiple) requests to delay the release of information from the NTSB. This is huge. You just do not do this. FIGG requested that the NTSB delay the prelim report last November because they feared it would have an effect on litigation. And all 5 organizations requested that the NTSB not open the docket this past October 8th and release the background reports because it might slow down the civil settlement process. The parties even tried to play is as beneficial to the victims families to keep this info quiet for a few more weeks. Shameful and disgusting.

I’ve never seen the board use this kind of meeting to publically shame companies for their lack of transparency. Wow. Sumwalt and Homedey were clearly pissed about this sneaky request and wanted it on the record.

Now we just have to wait a few weeks to see the final report first hand.

 

[thebard3]

JAE-
Also recommentation for AASHTO to better address redundancy in concrete structures.

Brad Waybright

It's all okay as long as it's okay.

 

[Rabbit12]

The committee commented that FIGG on a couple occasions requested the NTSB to delay release of reports so that the reports didn't effect settlement talks/agreements. They made the comment that this was extraordinary. The request was denied. FIGG trying to limit their damage because they knew the results of the NTSB investigation wouldn't look good?

Ultimately, the blame rests on FIGG's shoulders. They made a mistake in the calculations and then ignored the warning signs the cracks were screaming.

 

[Rabbit12]

Another question, FIGG made the determination that this bridge was redundant. How is that even possible? Is there some weird definition of redundancy that I'm not aware of?

 

[Vance Wiley]

FIGG has found a basis for claiming redundancy due to multiple elements acting. For example, the deck had multiple PT strands, the diagonals in compression had multiple reinforcing bars, and conditions like those that provided redundancy.
I would argue that there is only one node 11/12, and there is only one member 11. While 11 has multiple reinforcing bars, it has only one piece of concrete. And the reinforcing cannot act without the concrete.
I guess one could say the fact that it is a single bridge leaves no redundancy - if if it fails, there is no backup and so you use the crosswalk. One comment I read was when only the first girder is erected, a multiple girder bridge has no redundancy. Hmmmmm
The penalty for no redundancy is a factor of 1.05 - clearly not enough.
But if it fails under dead load only and within 5 days, there are real problems - not just 5% overstressed.
I have previously made the comment that since this was a bad idea in the first place, would another bad idea provide redundancy?
And life in this world has no redundancy - there is no backup for this life.

 

[BadgerPE]

I ended up watching the last part of the board meeting, but it seemed to me that they didn't just focus on FIGG as the one to shoulder the blame for ignoring the warnings signs. Maybe I misinterpreted this and will re-watch when I have time.

 

[Rabbit12]

BadgerPE, you are correct. They did say that; the NTSB did spread the blame. However, FIGG should have known better. They had the most intimate knowledge of the design and when cracks like this developed they should have figured out what was going on. At one point, FIGG was asked if they needed to close the road and their response was no.

I don't like the dumb contractor excuse, but as engineers we have more knowledge about most technical aspects. FIGG should have known better.

 

[Rabbit12]

VanceWiley, thanks for that explanation. I agree, it's a stretch to say it's redundant based on those assumptions. My goodness.

 

[samwise753]

My company is the county engineer for a county in Mississippi; the county has no structural engineers of their own. We were alerted to an existing (40-yr old) vehicular bridge crossing a creek that lead to a power plant; an adjacent defunct railroad bridge had just collapsed and they noticed that one of the pile bents on the vehicular bridge was moving due to what was determined to be a subsurface soil slope failure. We posted for a low maximum load and had on-site constant daily observation. In a couple of days we were sent photos of the bent cap cracking around the piles and were asked what to do. We immediately closed the bridge. 24 hours later, it collapsed into the drink. The piles were tipped out above the failure surface, so the slipping slope took the bent with it.

The point is, all the parties involved looked to the structural Engineer of Record for the final say on the plan of action. I don't care who's fault it is or what the cause is, when a structure is showing signs of distress and/or imminent failure, it falls to us structural engineers to make the call especially if we designed it.

 

[Eufalconimorph]

Agreed. I think that a big part of being an engineer (even not a PE or in a licensed engineering field) is the duty of care. There's something about it in the code of ethics of every engineering professional association. EG the IEEE's precept 1:

to hold paramount the safety, health, and welfare of the public, to strive to comply with ethical design and sustainable development practices, and to disclose promptly factors that might endanger the public or the environment;

I'm not a PE, and probably never will be. But my degree is in engineering, I'm an IEEE member, so I consider myself to have an ethical duty to consider the safety impacts of the things I build.