Miami Pedestrian Bridge, Part X 50

[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



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[jrs_87]

MikeW7, the housing project is under construction now.

https://www.google.com/maps/@25.761...4!1snv5vJVXkJJ9oOy6Qvhbo_g!2e0!7i16384!8i8192

 

[MikeW7]

jrs_87 - Yeah, it's still supposed to be completed in time for 2020 school year. The developer REALLY needed that bridge to be built on time, but remarkably I can't find any hard-news stories about the connection, so they must have sicced the lawyers on the news media.

Notice in your Google map (March 2019) that there is no replacement bridge construction going on - the north pier of the old bridge is still up! That crossing is going to be an absolute nightmare when Fall 2020 comes around.

EDIT: This is the walkway (2011 view) for University Bridge Residence to get to the crosswalk. Wow! - 2019 view

EDIT: this project was mentioned previously by epoxybot (Structural) 25 May 18 18:45 - my bad for not doing a forum search first.

 

[jrs_87]

MikeW7, don't worry.

 

[MikeW7]

jrs_87 - That would be one way to put the old piers to good use!

 

[saikee119]

Two photos, Fig 61 and Fig65, in the OSHA report caught my attention.

The Fig61 shows all the vertical rebar were intact after the collapse. In that the steel did not shear off like the concrete did but just debonded and separated from the spalled concrete. They were hardly bend or yielded. Most have the original 90-degree bends remain surprisingly in good shape after they came off off the concrete Member 12.

My concern is the vertical rebar of Member 12, seen from the OSHA report photo Fig61 and Fig64, did not participate much in resisting the Member 12 (together with diagonal Member 11) from being pushing outward during the collapse and none of them failed by shear. This would suggest the bulk of the resistance to prevent failure was the shear strength of the concrete alone. If any the rebar was designed to resist shear in Member 12 then the design simply did not replicate what has happened in the field.

This is most likely due to the poor structural arrangement at this location where a 8” (ID) diameter pipe sleeeve was cast horizontally for the drainage allowance. With Member 12 only 1’9” wide the 8” internal diameter sleeve would have left only about 6” concrete necking on either side of the pipe sleeve. After taking the rebar size into consideration each necking has less than 3” concrete on either side of the rebar. When Member 12&11 were pushed out of position the concrete near the pipe sleeve simply blew out and disintergrated leaving the rebar behind.

No doubt many would argue the concrete area is wider than the Member 12 as it was cast partially into the deck. However the deck has been compressed axially to deflect inward but the Member 12&11 were pushed outward so there are clear and well established shear planes between them. To make the case worse there was a construction joint, acting as a plane of weakness, between the deck and the Member 12&11. OSHA report has provided photos on the cracks and spalling at this interface prior to the collapse.

Since the majority of the rebar at the failure location remains in their original positions and suffered no shear failure the evidence would highlight a serious deficiency that the concrete member sizes there were inadequate as they were unable to produce a common strain between steel and concrete. In other word the concrete had insufficent mass to transfer its stresses into the steel reinforcement. In properly designed reinforced concrete there should be no slip in strain between reinforcing steel and concrete.

It is quite possible had the 8” ID sleeve been omitted or relocated elsewhere the FIU bridge might still exit today or the fialure pattern would have to be substantially different.

 

[Vance Wiley]

The Fig61 shows all the vertical rebar were intact after the collapse.

Good post. If one were demo'ing that concrete with a jackhammer, it would look much the same. Add the part of 12 that remained on the pier and it is an interesting failure.
One variation to what I had posted earlier was that perhaps the shear plane exited the end of the deck at the top of the 8" pipe sleeve, leaving the portion of 12 below the sleeve still attached. Then as the deck was pulled over the edge of the pier, there was 10-1/2 inches where the deck/bridge weight was being supported only by that projection, and that part separated in an upward motion relative to the top of the deck. There is spalling of the edge of the pier at the center portion only, supporting this thought.

https://res.cloudinary.com/engineering-com/image/upload/v1560323771/tips/2019-06-12_2_hrz8k2.png

from jrs_87 (Mechanical)12 Jun 19 07:23.

 

[jrs_87]

"separated in an upward motion relative"

The appearance of shattered 8" pipe pieces in the rubble at the top of the deck would support your thought as well.

Check
jrs_87 (Mechanical)8 Jun 19 17:22
jrs_87 (Mechanical)10 Jun 19 05:21
jrs_87 (Mechanical)10 Jun 19 05:36

Someone please clarify. Is not that "sleeve" the actual embedded drain pipe section for passing through diaphragm II? If so, is not "sleeve" a misnomer? The allowance of pipe at both sides of diaphragm support assertion it is actual drain. Hopefully it is not sleeve for 8" pipe to pass, because then that sleeve diameter would be greater than 8 inches. FIGG's intention was to hang drain system with no slope from the belly of the deck? There are several center line vertical passages in deck for drain system.

If the pipe is actual drain, it would be difficult to repair in the future without restricting it's diameter.

Perhaps a FEA can be made for just 11/12/deck "zoomed in" to model actual construction. NTSB has stated they laser 3D scanned the remains in 2saikee119's photo.

 

[SFCharlie]

Thanks jrs_87 via Vance Wiley
I hadn't even thought to use street view to view the aftermath.


SF Charlie
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[Vance Wiley]

Re: 8" pipe

I assumed it was a sleeve - for a smaller dia pipe thru. It would be a poor solution to cast the permanent pipe in concrete - minor structural movements would crack it someday. As to size - the roundie channel in the bottom apex was 12" dia (or 6" radius since it is hot full circle).
Perhaps that extra dim was to provide fall. Unfortunately, there was not enough time to determine the camber of the structure but I have not seen any camber instructions so I suspect it was to be flat. Kinda risky over 184 feet. Bit with the curbs and slope to center of the deck it would have only held maybe 8" of water.

 

[TheGreenLama]

Bridge was not flat. Elevations and stations are all over plans. Dwg B-04 shows slope of 0.01 ft/ft.

 

[jrs_87]

VSL shop drawings attached.

 

[jrs_87]

Updated VSL shop drawings attached.

 

[epoxybot]

FDOT's Tom Andres had comments on the drain pipe. He noted in the submittal drawings that the drain was flat, relying on hydraulic action to drain and suggested the addition of some slope would make the drain self cleaning. He was also concerned that the 2" gap (to be later grouted) between the two halves of the deck level tower/pylon would make it difficult to fit up the plumbing. I believe 1-1/2" is the amount of drop from the North pylon to the South Landing. If the had drained the main span & the back span separately, they could have eliminated the penetration all together.

The rupture stayed for the most part within the area of the original cracking.

It looks like the rebar that was in #12 also came away fully intact and that only the concrete below the drain was left to shear off when the deck finally fell.

 

[Sym P. le]

My thinking about the rebar in 12, at least that which remained part of the diaphragm, is that it was on the east/west sides of the column. There was insignificant rebar on the north/south face to resist 11 pushing through so that rebar is effectively demolished.

 

[jrs_87]

Have we seen this from Andres P.E. yet? Perhaps some pages, but not whole file?

FIU Superstructure Feedback.pdf (attached)

 

[jrs_87]

I'm trying to pin down when Pate first learned of large cracks in member 11. Would some one care to discuss this?

The reason I'm asking is because I found a FIU email from [highlight #FCE94F]3/14/18[/highlight] that mentions bridge cracks, but only the ones in the diagram. So this made me look at 3/15/18 presentation again.

I found the 3/15/18 presentation and meeting seem to address only diaphragm cracking. This begs the question - when and how was Pate made aware new cracking in the structure? We know Pate "dismissed" cracks two days prior in his voicemail to FDOT. But what cracks was he dismissing at that time? The voicemail is not specific beyond north end after move.

We are told that before the 3/15/18 meeting, he went up to bridge to see it for himself. I strongly suspect he used man-lift to view diaphragm. Was his focus on diaphragm so intense he only viewed bridge from the lift? I know this is unlikely, but It would go towards explaining the lack of reaction to the state of member 11.

The photos of major cracking are dated 3/14/18, enough time to react by 3/15/18.

I might be in left field on this one. Please check.

P.S. I'm missing something that might make part of this post moot. Looking or it now. It's about "capturing the node".

 

[SFCharlie]

jrs_87
Thank you for the shop drawings


SF Charlie
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[SFCharlie]

jrs_87
Best I remember is that the inspecting engineer wrote (emailed?) MCM with photos of member 11 cracks just after the shoring was removed, asking them to have the EOR look at them. The cracks were already concerningly large. Images of the letters and photos were in some of the first early reports.
I was wrong:
I my feeble mind, I had put 2 and 2 together and got 5
The "visual inspection of main span truss members on 02-06-18 after PT bars tendons No 2 and No 11 were stressed"
mentions cracks, be doesn't raise an alarm.

 

[jrs_87]

SFCharlie, you're welcome. Thanks for being polite. It help lessen the sting from being scolded by fellow members.

Digging documents is hard work, now I know why real forensic investigations cost so much. Please be advised there are some tiny gems in the emails preserved as attachments. I use a software I already have to extract them, but it's time intensive.

 

[SFCharlie]

Is this correct, or do they just mean groutted?
(from the OSHA report, page 27)
"March 2, 2018 Blisters of the canopy poured. Access openings for PT bars in diagonal 2 and 11, left open."

SF Charlie
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