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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[Vance Wiley]

Since this is back on the table for discussion, the large chunk in the photo of 2 July post looks like it had a formed or finished surface. From my jigsaw puzzle days it would tell us that the piece was from the side or top of something - maybe. I held hopes that NTSB would retain and assemble the larger pieces as being important in defining the mechanism of collapse.
This piece seems quite large to be devoid of reinforcing - particularly at an important zone.

 

[MikeW7]

As the truss collapses, the canopy/#11/#12 triangle rotates the noted 80 degrees.

Thanks for the explanation of how the hinging worked, but my point was that the OSHA report states that the blow-out caused column 12 to fall over and assume its final position (with the 11-canopy end of the triangle touching the deck - see picture 52, page 91) BEFORE the bridge collapsed. This is not what happened, at all.

The dash cam video CLEARLY shows that the bottom of member 12 slipped completely off the pier, fell behind it, and was then dragged back on top of the pier by the attached canopy as the mid section of the bridge continued to fall - see dash cam frames 80-83. The triangle assumed its final position (OSHA picture 52) after the deck was pulled off the pier (frames 83-85).

ADD: After 12 slipped off the deck, member 11 was almost completely sandwiched between the canopy and deck (dash cam frame 81), which may explain why so much of 11's bottom end is missing in OSHA picture 52. After this event the "triangle" is just an "L" (member 12 and canopy) with the shredded remains of member 11 attached to the canopy.

 

[SFCharlie]

In frame 78, the angle between 12 and the canopy has decreased. Could the concrete have withstood this? Or is the rebar just holding them together????

SF Charlie
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[MikeW7]

In frame 78, the angle between 12 and the canopy has decreased.

It's hard to make out 12 behind the light pole and man lift arm, but the debris cloud appears to start in frame 77 so it's possible the base of 12 is already sliding northward, allowing the 12-canopy "L" to tip.

That 12-canopy junction was pretty strong. The "L" was pinched inward as the deck was pulled out from under 12, then it was spread outward as the canopy dragged the "L" back on top of the pier, yet all the pictures I can find show it to be intact after the collapse.

 

[SFCharlie]

MikeW7 (Electrical) Thanks!
Wow, concrete is really strong.

SF Charlie
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[Wetlander]

Six more companies are agreeing to settle lawsuits, including FIGG and Bolton Perez & Associates. [link ]

https://www.nbcmiami.com/news/local...awsuits-in-FIU-Bridge-Collapse-512530852.html

[/url]

 

[Earth314159]

Six more companies are agreeing to settle lawsuits, including FIGG and Bolton Perez & Associates

Smart move on their part. I can't see any good defense for this case.

 

[Earth314159]

Thanks for the explanation of how the hinging worked, but my point was that the OSHA report states that the blow-out caused column 12 to fall over and assume its final position (with the 11-canopy end of the triangle touching the deck - see picture 52, page 91) BEFORE the bridge collapsed. This is not what happened, at all.

I don't think the OSHA report explained the collapse very well. It could be because they were trying to write it for laypeople rather than structural engineers. There are more issues with the report besides this one. I also question the judgement of the authors based on some the things they commented on. I wasn't overly impressed.

The reason the bottom of #11 was ripped out was because the anchor for the lower PT bar was anchored into the deck and stayed with the deck. #11 stayed on the abutment. As the deck fell, the rod ripped out the back of #11. The first few ties in #11 may have (likely) been severed during the initial slide to the north. The back face of #11 was weaker than the shear capacity of the rod.

The base of #11 and #12 would have been push north as you suggest. This is consistent with the analysis of the failure mechanism.

 

[Vance Wiley]

In your post referenced above I see an opportunity to pretty well estimate the remaining length of member 12. Using a ruler and scaling the 6 foot pylon as 10 - 1/8" divisions and the length of 12 as 26 divisions my crude methods suggest the north face of 12 is 16 feet + long. (hard to pick a measuring point for the top at the canopy).
Can any 'vid gurus' improve the accuracy - or is this close enough?
If the (now) north end of 12 was the top of the deck (joint) that should tell us something. Perhaps of no importance, but something.

 

[Tomfh]

Using a ruler and scaling the 6 foot pylon as 10 - 1/8" divisions and the length of 12 as 26 divisions my crude methods suggest the north face of 12 is 16 feet + long. (hard to pick a measuring point for the top at the canopy).

Here is my attempt. The units are pixels.

If the pylon is 6 feet I measure the 205 pixel long arrow as ~205/70 = 17.6 feet


No adjustments made for parallax. Just a simple scaling

According to drawings the top of canopy to deck dimension is 17.2 feet. This would suggest the damaged 12 member we can see is approximately full length?

 

[SFCharlie]

Vance Wiley (Structural) Thanks for your probing question and estimate. Tomfh (Structural) Thanks for showing your work. It seems to show that the north face of 12 remained largely intact. I seem to remember that the rebar going up into 12 remained straight up in the top of the diaphragm. I also observed what I interpreted as a cloud of debris on the deck expanding in a couple of frames of the dashcam video (about the time I interpret 12 to have just started to tilt (top of 12 barely visibly south of its previous position)). Photos of the NTSB examining the north deck seem to show both ends of 11 damaged, so I'm having difficulty determining how much of each end of 11 disintegrated. So, I find myself asking, was the compression in 11 enough to cause it to explode, or did it require 11 to gain some momentum, or what?
Was there a cold joint at the top of 11, I think the canopy blisters were cast last.

SF Charlie
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[samwise753]

Six more companies are agreeing to settle lawsuits, including FIGG and Bolton Perez & Associates.

https://www.nbcmiami.com/news/local...awsuits-in-FIU-Bridge-Collapse-512530852.html

So what is next? I wouldn't think a settlement absolves anybody from liability.

 

[MikeW7]

I'm having difficulty determining how much of each end of 11 disintegrated.

Here's a JPEG of OSHA Figure 52 on page 91 of report. (I don't have a link to NTSB pics.) If you can reposition 11 to its original angle with the canopy that will give you a starting point. The pier width can be used a ruler, but parallax is a problem because you have to guesstimate distances from camera position.

 

[SFCharlie]

Thanks MikeW7 (Electrical)
I can use the width of 11 or 12 to calculate the remaining length of 11. What I don't know is how much of 11 was destroyed at the 10,11 joint. The first visible movement was the 10,11 joint lowering, yet the top of the blister at that joint seems to remain intact? The angle between the north section of the canopy and 11 seems to remain constant for several frames. The canopy seems to drop more than the deck in the first few frames. How is this possible, the south canopy, 10, and the south deck seem to remain intact? Also, the 12, deck joint seems to remain intact during this first few frames. My thought is that 11 can slide on the deck and the horizontal vector of the compression is focused on the fillit. Wouldn't the compression be highest in the narrower fillit?

SF Charlie
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[SFCharlie]

One more thought... What are all the white things sticking out the north end of the deck/diaphragm?

SF Charlie
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[MikeW7]

I can use the width of 11 or 12 to calculate the remaining length of 11

Ref. OSHA Figure 52 - The bottom face of 11 appears to be shredded, possibly when it was sandwiched between the deck and canopy - Frame 81, more likely due to PT rod being ripped out, as noted by Earth314159 (Structural) 11 Jul 19 16:36.

When the sandwich occurred, the top section of 11 probably moved south quite a bit (stripping it of concrete) since the end section was jammed in some way against 12. It's possible most of the lost length occured at the TOP end of 11 instead of the bottom end where the initial failure presumably occured.

 

[Earth314159]

The bottom face of 11 appears to be shredded, probably when it was sandwiched between the deck and canopy

The bottom of 11 is shredded because the PT rod rip off the bottom of the member as the deck fell to the ground.

The blistered popped off because there was a massive deceleration when the bridge hit the ground and the canopy had a punching failure at the joint.

 

[Vance Wiley]

Thank you, Tom, Charlie, and Mike. I am having a bit of a problem with my first concept of this failure.
On March 15 of 2018 the first thought was the truss lost a heel joint at the north end. I am still of that opinion.
The actual damage sequence is a bit more elusive to me. From the photo by Tomfh, west side, and the one by MikeW7, east side, the top joint of 12 to the canopy appears basically intact. And it appears from measurements (scaling display and counting pixels - wish I could do that) of the north face of 12 that the face is not blown away at the bottom, as I would expect if the joint 11/12/deck failed and moved north a foot maybe.
Is the geometry such that it 'protected' 12 throughout this bumpy ride?
The splits and separations in the lower part of 11 and across the top of the fillet zone between 11 and 12 are a clue to the condition of 11 just before the collapse.
If the cold joint is moving, that would induce angle changes in 10/11/canopy and 11/deck with subsequent stresses from bending.
Did member 11 simply explode just above the fillet (or did both 11 and the fillet to 12 explode), with the resulting shortening of 11 allowing node 10/11 to begin dropping freely? With the damage at the canopy/12 joint appearing minimal (compared), the dropping of 11/12 pulled the top of 12 to the south, prying it from the bottom joint casting, preserving the north face? There is a lot of concrete missing from the bottom of 11 and the formed joint of 11/12/deck.
If 11 "exploded", that would have been pretty sudden. I am just a bit surprised that the north face of 12 is pretty much intact.
EDIT ADD: With the concrete having departed the lower portion of 11 only the reinforcing remained, limiting any further influence of 11 primarily to tension thru what anchorage of reinforcing remained. Thus 11 was able to pull some of the remains back to the south face of the pylon.

 

[MikeW7]

Earth314159 (Structural) 11 Jul 19 16:36 - Fixed my post of 11 Jul 19 16:22 - THX!

 

[Sym P. le]

@SFCharlie - white things are various conduits running the length of the deck (both sides).