Miami Pedestrian Bridge, Part III 99

[JStephen]

"Site management has requested that we limit the length of the other thread by forming a new one. This subject may require III, IV...."
So here's Part III. Please don't post any more in Parts 1 and 2.

Part I thread815-436595
Part II thread815-436699

 

[dik]

But reality doesn't seem to fit those drawings.

What was constructed may differ considerably from the proposal... it will be nice if they release the crack photographs and shop drawing records.

Dik

 

[SwinnyGG]

It doesn't make much sense that releasing compression from the strand would cause failure when the member is acting in compression anyways - technically you should be making the situation better (less compressive stress).

There's a couple of ways that removing compression load from one PT strand could create a failure-prone condition:

1) Removing load from one strand and not the other would create an internal force imbalance inside the member being destressed. Concrete experts (ie not me) would be better suited to say whether or not it's possible that this force imbalance could possibly create a large enough internal moment that the member could not handle, but on first principles it seems logical to me.

2) In order to reduce tension on a PT strand, tension must first be added- the jack and stool are attached and the tendon is stretched so that the nut on one end can be loosened by hand. So, for a short period, the PT strand being loosened is under significantly more tension, and is applying significantly more load to the member, than the designed installed load. If there's a mistake during the procedure for reducing strand tension, the tendon could fail or the member could be grossly overstressed.

3) Compression load from the PT tendons could be providing confinement to the nodes/joints at the ends of the members, and possibly transferring shear loads across a deeper section than would otherwise be developed in shear. Removal of the tension could reduce or eliminate that 'bonus' joint confinement and make one or both joints at the ends of the destressed member unstable.

 

[12345abc6ttyui67]

jgKRI - 1) Good point, I hadn't considered that.

2) I'd thought about this, but (in my naivety) I find it hard to believe this was what caused failure. Surely you just need a teeny tiny bit more tension to back the nut off by hand. All you need to do is overcome the original pretension by a very small margin in order to free up the nut? .... But I could be wrong!

3) Is exactly what I was trying to suggest in my post, although somewhat less eloquently...

 

[dik]

I guess, like most, that the failure occurred with member #11, or the node, or the slab at member #11. I'm not certain, but from the pictures something happened in this area. If you can imagine a portion of the structure, associated with this area 'simply disappearing', the collapse would be similar to what was shown. I have several ideas based on information from this thread, but, have not 'wrapped my ears' around the actual cause of collapse. It's elusive due to lack of information.

This visualisation would be in keeping with the sudden catastrophic collapse. We have very limited information on how the truss was designed, detailed and construction. Hopefully some of this information will be forthcoming. There are numerous things that could have been done to prevent the loss of life, maybe not the collapse.

Hopefully new information will be divulged, and, maybe identify the actual mechanism of collapse.

My $.02US, $.03C.

Dik

 

[SwinnyGG]

I'd thought about this, but (in my naivety) I find it hard to believe this was what caused failure. Surely you just need a teeny tiny bit more tension to back the nut off by hand. All you need to do is overcome the original pretension by a very small margin in order to free up the nut? .... But I could be wrong!

You'd have to add enough tension to make up for any creep or force-induced shortening of the member, as a percentage of total preload already applied, this could be a LOT of force. We're talking about 140 tons of preload on one of these tendons, and a process performed manually. Yes the crews performing the work were allegedly experienced and highly skilled, but mistakes can still happen.

I'm not a PT concrete expert either, so I would have to defer to more experienced engineers on how realistic this possibility is or isn't as a potential root cause.

 

[Archie264]

MortenA,

In general you are correct. However, on *this* bridge the pylon and stays were there largely for looks with little, if any, structural purpose. Yes, you read that correctly.

 

[Ron]

jgKRI....there is some possible evidence of such an imbalance if you look at one of the photos of member 11 where the concrete had spalled along one side.

 

[gwideman]

there is some possible evidence of such an imbalance if you look at one of the photos of member 11 where the concrete had spalled along one side.

Maybe, but not necessarily. Member 11's lower PT rod was pulled down by the falling deck, while the rest of member 11 stayed attached to 12 up at the top of the pedestal. This may have created a zipper effect, pulling off the bottom of member 11.

 

[JAE]

There is a whole bunch of 'amateur' conjecturing and speculation going on in this thread but at least there is virtually instantaneous peer review. Plenty of new sign ups to this forum too (myself included) so who knows what credibility so many of these posts have. But that is the nature of community discussion and in the right contexts good conclusions can emerge.
In contrast Oliver McGee is an idiot, who reporters have no idea that they should ignore.

I'm not sure "amateur" accurately characterizes those participating in this thread, or in Eng-Tips in general.
I think the 400+ comments are all a representation of engineers natural inquisitiveness and DNA compelling pure problem-solving.

As hokie66 suggests, [blue]"I don't think we know exactly where the failure initiated. Perhaps we never will."[/blue] - probably very true until more study/factfinding and analysis is performed by, hopefully, individuals who can successfully get to the bottom of this disaster.

But I think generally that your statement reveals just how great Eng-Tips is as an engineering forum.



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[OSUCivlEng]

Sadly Bridgebuster in the consulting world that seems spot on. Especially with what I see with regards to state DOT marketing.

 

[dik]

there is some possible evidence of such an imbalance if you look at one of the photos of member 11 where the concrete had spalled along one side.

I noticed there was a lot of small particle debris, possibly from the concrete spalling from the chord members... I would not normally expect to find such relatively small parts. I've never encountered a failure so catastrophic and sudden.

Dik

 

[dik]

I'm not sure "amateur" accurately characterizes those participating in this thread

There's probably a kazillion years of experience on Eng-tips.

Dik

 

[JAE]

kazillion....let me get my slide rule out to check that...

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[TehMightyEngineer]

There's probably a kazillion years of experience on Eng-tips.

I wonder what would happen if eng-tips designed something collaboratively. Replacement bridge design anyone?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[WinelandV]

I wonder what would happen if eng-tips designed something collaboratively. Replacement bridge design anyone?

Some 400 engineers trying to design something via forum? It would certainly be entertaining to watch.

 

[SwinnyGG]

I have a great deal of respect for pretty much everyone I've interacted with on this board.

With that said... there is such a thing as too many chiefs

 

[VolsCE84]

After watching the video that Meerkat 007 supplied. Around 0:17 seconds we see something protrude out from back side of the interface of member 12, 11, & the deck (bottom chord). My thoughts earlier were this was the end of the PT Bar that possibly broke while being de-stressed. In the roll-in photos we see no openings for the PT Bars in end of the deck. They had to have been dead ended in this area. When looking at the piece that popped out. It came out horizontally, NOT in line with member 11's angle, but in line with the bottom chord and it stays there until the deck (bottom chord) starts to fold downward and we see a puff of dust around this connection. The piece then pulls inward with the collapse.

In the photos gwideman provided it looks as though the PT bars did not break. It appears they did zipper member 11 as gwideman proposed. The top bar stayed top with parts of 12 on the top of the pier and the bottom with deck on the ground. Thus, ripping member 11 down the center. It is concerning that we see in gwideman last photo (from the NTSB)the amount of voids in very close proximity to the high amount of SHEAR in this area. The preliminary plans show only one anchor bolt on each side. The photo show at least 4 Pipe with vertical bars that have not been grouted, plus the duct vents, reducing the area to resist shear. There is a heck of a lot "stuff" in this area. There is a Heck of a lot of SHEAR in this area. Notice the near perfect square that gwideman boxed out in blue, looks like a square piece of concrete is gone. Could it possibly be what we see PUNCH out in the video supplied by MeerKat 007. Could the PT Bar that was being de-tensioned been the force that was keeping this joint from shearing?

 

[TehMightyEngineer]

I noticed there was a lot of small particle debris, possibly from the concrete spalling from the chord members... I would not normally expect to find such relatively small parts. I've never encountered a failure so catastrophic and sudden.

You usually find small particles and debris during extreme failure with concrete, especially if there was high compression involved.

For reference, here's a precast arch structure I designed that was unfortunately damaged during shipping (no injuries except to the drivers pride, thank goodness). It was not prestressed but just look under the fracture in the shadows; lots of concrete debris in various sizes. If this was prestressed I imagine it would look even more destructive.

https://res.cloudinary.com/engineering-com/image/upload/v1521644731/tips/IMG_2243_aqs9j7.jpg

Concrete just doesn't like being thrown at the ground I guess.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Spartan5]

For what it’s worth regarding small debris, I recall the following reporting immediately after the accident. I do not have a link. But the owner of the car that was half crushed (the entire rear end) stated that she had heard what sounded like rocks falling on the trunk and turned or was turning her head when the rest of the bridge came down and flattened her car. So maybe half a second to register and react to something like that?

Not sure if that information with the location of her car helps with any of this. Just thought I’d throw it out there.

 

[mibro]

JAE,
That Oliver McGee character was on TV news, espousing that rubbish. I linked it in the first thread. He is some sort of professor at Texas Tech. He needs stopping.

If you check his LinkedIn page you'll see that won't be possible. The best part is he appears to me to be commenting on his own engineering word salad article himself using sockpuppet accounts.