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


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[TheGreenLama]

There is no active clamping force in this zone because the transverse PT begins just south of the cut out notch zone we have defined.

If I understand your example correctly, this statement is in error. Gwideman and I were debating this a while back. I was unable to make my point clearly back then, so here's another tack.

Thought experiment: You have a bridge that is only 2'-8" long w/ one transverse PT tendon. The transverse PT force per unit length would be = 1 PT / 2'-8" . Now imagine we keep increasing the bridge length, and for every 2'-8" we add another PT tendon. Question--at what length of bridge does the PT force at the ends begin to decrease? Answer--it never does. Now our bridge is not exactly like this, but it's close.

Regarding theory, I would look at some of the references cited in ACI 318 for shear friction. Back when I was reviewing the OSHA report I seem to recall that the references cited were in turn searchable (although I may be mistaken on this). Sorry to punt on the meat of your question, but this is all I got right now.

 

[Earth314159]

Is it being suggested that by roughing up the surface of the concrete at the blue plane cold joint before pouring strut #11, this would be sufficient to eliminate the need for 25 square inches of steel pulling south?

You need to increase the shear friction area, increase the amount of vertical steel and roughen up the joint (and/or add keyways), and then you can start to make the joint work. The longitudinal PT takes the tension component (or F1 decreases the pre-compression in the deck) into the deck. There is also longitudinal mild steel which assists.

You can also have steel going through the joint opposing the force directly rather than through shear friction but it is not a necessity. You can also have part shear friction and part direct transfer.

As I stated before, there was some punching directly below #12 since there is a concentration of vertical steel that locally increased the shear friction (and some doweling action) capacity of the blue plane. From a design perspective, it is better to ignore this localized concentration of the steel and just design for the shear friction through the blue joint so that the loads can more easily be transferred to the PT and other longitudinal steel in the deck.

 

[3DDave]

Vance, I'm not sure about 'wrong' but maybe incomplete. I'm thinking that the failure process is along the lines of this:

Friction under 11 initially carried a significant part of the load, but this cannot be uniformly distributed due to compressibility of the concrete. The steel cannot carry much, if any, load out of 11 initially because it doesn't deform enough to see a load, until the friction is mostly lost, at which point 11 is bearing against the steel. 8500 psi compression vs 60ksi steel suggests that the concrete must have been crushed, allowing an S-shape to the steel until the majority of the load is transferred out of 11 to being resisted by the reinforcement in 12.

Once 11 moved more than a fraction of an inch there would be powdered cement and larger asperities would no longer function, just low amounts of friction. Perhaps turning the reinforcement into an S-shape helped enhance the friction by making a small increase in the clamp load?

 

[Earth314159]

The GreenLama,

I agree with you. The transverse PT does add a clamping force. If I put my hand between two 2x4s that are being bolted together, it will still get squeezed even if it doesn't line up with the bolts. By the time you are on the centre line of the deck, the clamping force from the PT is more or less a uniform distributed line load along the entire length of the bridge. The clamping force will only increase as the rough surface on a shear block tries to pulled out from the deck.

 

[SFCharlie]

Hey, I'm just a silly computer engineer, but since members one and two held even after it was dropped, wouldn't that same adjustment to 11 that was made to two be a first approximation to a fix?

SF Charlie
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[Earth314159]

SFCharlie,

If you build #11 and #12 like #1 and #2, the joint would have been better but still not acceptable. #1 and #2 happened not to fail but there are a lot of structures out there that are inadequate but are not (have not yet) failing. It is also like a wish bone, one side or the other will fail fist. Once one side fails, the other side isn't going to fail unless you can add a higher load.

The fact of the matter is that there is a lot of experience in dealing with these kind of force transfers and the failure should have never happened.

 

[Vance Wiley]

I recall your discussions. And I also think some transverse PT does clamp the zone I am addressing. If the zone of influence fans out at 45 degrees each side that covers the 60" notch 14 feet away. The first 4 transverse tendons probably contribute to a clamping force. It may be somewhat less than if tendons had continued at 2'-8" spacing but it is there.
What I am getting at is cannot the projecting deck at the sides of the failure zone provide as much shear friction as the reinforcing one might design for the blue area defined as blue in recent posts?
There may be indications of this being at work in the failre experienced. There is no failure under the southmost contact area of 11/12. Is that because the surface of the deck failed first because the area of the deck being "clamped" did not fail? I think we can see that the surface failure was the weak link in this south zone. Then as the failure moves farther north it begins to overcome the side friction and dive deeper below the deck surface. I think that is in part because the deck surface had already failed and offered little resistance after slipping, increasing the demand on the remaining part of the joint.
The PT tendons D1 then provided a weak plane and limited the failure laterally until the break out at the edge began to fail in diagonal tension.
Thank you for the reference to ACI background, and for your response.

 

[RCPete]

@SFCharlie at 14 Oct 19 00:09

One thing mentioned in the Part 73 analysis (section 6.3.4, third paragraph on page 87) was that not only was truss 2 36" deep (vs 24" in 11), but it also had a much larger diaphragm, 42" versus 24". It also lacked the vertical pipes used on the north end. Figure 68 on the next page implies that the south end was at nearly the same risk as the north end. Page 9 of that report mentioned that node 1-2 was showing similar, but lesser evidence of distress as 11-12.

 

[SFCharlie]

Earth314159 (Structural) RCPete (Electrical)
Thank you both for your responses. I meant no disrespect for the experience on this forum. Yes, you're both right about members one and two being on the edge of a similar failure as the NTSB crack photos point out.

SF Charlie
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[Earth314159]

SF Charlie,

No disrespect was taken. You ask good questions for a computer guy

I never got past programming beyond Fortran 77 on my XT IBM PC.

 

[Vance Wiley]

Dave - My idea was not to solve this particular failure but to explore the shear-friction issues of an idealized situation like this without the complications.
I am in agreement with your comments - I may hold a bit on your description of the steel and its S curves, but in general I am with you.
The steel that is bent into an S has likely yielded. Whether that increased clamping force is dependent on the configuration, and if yielded the elongation curve has flattened and is relatively constant for some significant elongations. But reinforcing steel is less ductile.
Thank you for responding.

 

[RCPete]

SF Charlie, EarthPi

I've been lurking since April 2018, and have found this fascinating. I had a chance to go through the full Part 73 report (recuperating from a bit of foot surgery; 3 months into a 2 month recovery [wince]), so I've had plenty of time to read.

One take I got from the report, was that there were a lot of chances for somebody to throw up a red flag before the bridge collapsed. I'd been on some projects that were going nowhere or down in flames, and I've seen the attitude of "we can make it work, we put too much into it to stop this approach. Let's just try $WHATEVER". However, in my career, catastrophic failure might cost a few tens of thousands of dollars, not lives.

I have this feeling that somebody thought "It's just a foot bridge. What can go wrong?" Not sure they changed their mind before March 15th, 2018.

 

[Vance Wiley]

I have this feeling that somebody thought "It's just a foot bridge. What can go wrong?" Not sure they changed their mind before March 15th, 2018.

The FIU Pedestrian Bridge was a "Signature Project" - to be constructed as a towering example of the brilliance of the Accelerated Bridge Construction procedure which is a banner department of the FIU educational program.
The Prime Directive was to maintain traffic on the street at all times.
Not sure but this was likely the heaviest bridge structure ever moved into place on land. The 950 ton monster was moved into place in one weekend - when commuters went home Friday it was not there and when they went to work Monday it was in place and traffic was flowing without a hitch. The accolades were mounting. Dignitaries gathered. Speeches were made. Photos were taken.
Everyone involved understood the importance of this project to the ABC program at FIU. A monument visible for miles telling the world "This is what we do. This is who we are!"
Many men with stellar careers in engineering and construction gathered at a meeting at 9:00 AM on March 15, 2018.
And no one dared to fart at the President's tea.

 

[FortyYearsExperience]

From a design perspective, it is better to ignore this localized concentration of the steel and just design for the shear friction through the blue joint . . .

Earth314159. Thanks for your comment above. Two questions arise from this statement:

1. Is it being suggested that there was in fact any actual design for shear through the blue joint ? If you look at the images on page 96, it is pretty clear there was none?

2. Is it being suggested that the collapse arose from a failure by the construction team to roughen up the concrete of the deck surface of the blue joint -- either by mechanical roughening of wet concrete, or more deliberately by cast keyways?

Thanks for your helpful comments.

 

[SFCharlie]

Vance Wiley (Structural)
I'm sure you're 100% absolutely right. ...just a note, it is the largest bridge move in the US by weight to date, but not the world by a long shot...

SF Charlie
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[SFCharlie]

FortyYearsExperience (Structural) Thanks for reminding us of this photo. Since the NTSB report came out, you are one of the members that has reminded us of the greater extent of this failure. That, together with the cracks noted at members 1,2, show that something was pervasively wrong with the understanding of the design. Thanks.

SF Charlie
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[RCPete]

Vance:

As a signature project, it now kind of makes sense. If the priority was to keep traffic flowing, minor details like the cracks spreading from marginally bad to horrific might just possibly be overlooked. Shudder.

Question: Are criminal negligence charges likely to hit anybody in this?

40 Years:

From the official report, (part 73), they mention the lack of roughening as a contributing factor, but the key takeaway from their findings was that the shear demands were underestimated, and that the capacity of the structure was overestimated. I've not seen any argument to the effect that roughening the joints per the specification would have compensated for all the other deficiencies.

 

[Vance Wiley]

No charges have been filed to my knowledge.
And I hope they will not be. Too err is human. Thankfully professionals do not err too often. We would lose thousands of doctors each year if mistakes resulted in criminal charges.
But to not recognize an impending failure or at least a structure in serious peril is beyond the pale.
My son-in-law is not an engineer but watches the documentaries about disasters and says most of the disasters have one thing in common. They are a convergence of several factors. Mistakes - sometimes multiple, combined with conditions which either conceal the impending crisis or cause people to ignore the warning signs. This project seems to fit.
The design firm says they were not informed of the rate of increase of the cracking. One look at those photos of the cracking and knowledge of its location in a truss structure clearly says it will not be long now, regardless of the speed of the progression or the time it has been developing.
I watched cable news from 3000 miles away and once I recognized it was a truss and saw the remains I suspected the truss had lost its heel joint at the north end. That happens in trusses. But it does not have to happen.
Thank you.
Hope that foot heals soon. Do not ignore a warning sign. Don't let your doctor ignore one either.

 

[3DDave]

It's possible that among many of those doctors tens of thousands of patients would not have been harmed or died.

It seems like, given all the evidence that was accumulating over the few days before the collapse, that negligence rises to criminal levels. It's not as if hidden features were failing that could not be observed, like a fatigue initiator buried in a turbine fan disk that expands over thousands of hours. This was massive chunks of concrete fracturing off over a matter of days. On an old bridge this should cause concern; how those involved accepted it in a new one is, to my mind, negligence. And that killed people.

There needs to be additional focus that can only be brought by deciding between prison and fudging a project and those unwilling to clearly chose need to face that as a possible outcome.

 

[RCPete]

Vance:

Thanks; bunion surgery with bone realignment, complete with (reinforcing pins (temporary #1 stainless "rebar"). I wasn't fully healed at the 8 week point, and the doctor had vacation (no backup, we're rural), so I had an extra month in the comfy chair. Gave me time to read the reports.

I see him tomorrow, and I'm pretty sure the bone is healed enough.