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Miami Pedestrian Bridge, Part VII 51

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JAE

JAE

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Jun 27, 2000
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A continuation of our discussion of this failure. Best to read the other threads first.

Part I
thread815-436595
Part II
thread815-436699
Part III
thread815-436802
Part IV
thread815-436924
Part V
thread815-437029
Part VI
thread815-438451




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jrs87, there’s way too many unnecessary sentences in that article , but a few comments:

[blue]”While the exact cause of this astonishing bridge collapse and horrific tragedy is not yet known” …. [/blue]
yet:
[blue]“Secondly, no new, previously unknown, engineering principle or precept will be learned or discovered here concerning the exact cause or failure mechanism.
We will only learn how known, well-understood, engineering principles or precepts were somehow violated leading to this sudden bridge collapse.”[/blue]
So the cause is not known yet – but he knows that we won’t learn anything new from it.

[blue] “The structural design concept of the bridge decks mainly involved a concrete truss reinforced by post-tensioned steel tendons. A 109-foot high central pylon, yet to be constructed, would have added more steel cables, and thus more stability, to the overall structure.”[/blue]
The cables were aesthetic – as this series of threads has revealed – so this forensic engineer is discussing something he isn’t very familiar with.

[blue] “However, it is entirely fair to begin asking some very crucial questions about what, as engineers and builders, we should be focusing on during the investigation and what things we may need to modify or change as a result of this tragedy to prevent something like this from ever happening again.”[/blue]
Tragedies won’t be “prevented” by “modifying things” – we only can attempt to minimize the future risk.

[blue] “But, the most important and crucial take-away is that this tragedy should never have been allowed to happen in the first place and cannot be allowed to occur again in the future.”[/blue]
Let’s make a law against this sort of tragedy shall we?




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chris snyder, Thanks for the post-tensioning sequence. I knew it had to be spelled out somewhere, but I was just too lazy to find it.

Before you posted it I was just assuming that the deck was poured, then all deck tendons were stressed, prior to working up and casting the diagonals. But now I see this is not the case. And it raises an interesting question: Why? Since everything was supported on falsework I don't see the need to wait until entire superstructure is cast before beginning stressing operations on the deck. Does anyone have any ideas as to why you'd want to wait?

The reason I raise this question is the following:
PT_Shear2_wnszx3.jpg


By waiting to stress deck PT, aren't you unnecessarily introducing into the system a shear force at the base region of member 11? This would be locked in, and subsequently added to the shear loads generated from dead loading when the falsework is removed. Wouldn't it, or am I missing something?

We'd expect to see the same "shear lag" effects that have been thoroughly discussed elsewhere in this thread with respect to the dead load. And the deck is much stiffer than member 11. So most PT force will stay in the deck, but some fraction will move into member 11. It's hard to say if this is a significant load. A computer model would tell the tale of the stress history.
 
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thebard3: The page you linked on MCM's website calls this a cable-stayed bridge. OOPS. Like you, I am also surprised this page is still up.

JAE: "Let’s make a law against this sort of tragedy shall we?" That's my kind of rhetorical question. Unfortunately, legislators (who are mostly lawyers) think this is actually possible. BTW, the Kellogg-Briand Pact attempted to legislate war out of existence. Except for WW2 and a few other minor skirmishes, it seems to have worked quite well. Not. Both ideas--legislating away tragedy and war--are hopelessly naive. i suspect you knew that already. :)

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
 
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jrs87 - Well the hard hat doesn't look like MCM or FIGG's logo. Structural Technologies is part of Structural Group and includes a number of different companies. Their Pompano Beach office includes VSL, Structural Technologies & Structural Preservation Systems, perhaps more. Structural Group is the product licensee of VSL/Bouygues. Structural Technologies prominent use of the VSL on much of their marketing & company property tends to give people the impression they are VSL. VSL/Bouygues does contract worldwide but mostly they license their products in various regions of the world. I don't think Structural Technologies can necessarily contract as a licensed VSL installer. Their is at least one email that mentions that the company performing the Post Tensioning is not VSL.

There is this tweet from Monique O. Madan at the Miami-Herald about an hour after the collapse. It makes me wonder if the construction workers had growing reservations about the bridge as it was getting built. It looks like they spent at least half a day of crane time trying to set the rebar cage for #12.
Link

I shudder at my past experience working with Universities & Healthcare, they turn on a dime making decisions about projects already in the works without a thought to the price already given for the work contracted; while acting like they are a poor as church mice. They really do expect you to shove 10 lbs of fudge in a 5 lb bag, with your head up in the clouds, where they can talk to you.
 
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JAE: I had a couple of exceptions to his comments, but, for preliminary comments they were a reasonable summary. In addition to your items, my two exceptions were; I would normally think that a forensic guy might be a little more constrained:

...whatever did go wrong here either could have been prevented, or at least could have been better accounted for such that loss of life and catastrophic damage did not ensue.

Although this appears to be a structural issue, with the bridge barely capable of supporting the actual dead load. It is possible that some of the fatalities could have been avoided. What if there had been a crew of 30 construction workers on the deck or canopy?

Secondly, no new, previously unknown, engineering principle or precept will be learned or discovered here concerning the exact cause or failure mechanism. We will only learn how known, well-understood, engineering principles or precepts were somehow violated leading to this sudden bridge collapse.

I would not be making that statement until all the 'dots were on the dice'.
 
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As to the MCM page linked above still being online, I imagine there is some discord between parties now. The contractor probably prefers to emphasize the role of the engineer, and the photo with the beaming dials of Linda Figg and Denney Pate front and center serves that purpose. Or am I just being cynical?
 
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How much did the length of the bridge deck change during post-tensioning? I just read several articles about PT, but didn't find anything about how much the concrete would be compressed in a typical case. I'd guess several thousandths of an inch per foot, but I'd like to know for sure.
 
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If I am reading it** right, modulus of elasticity is about 5 psi/ppm. In #11, the area is nominally 500 inch^2 and the post-tension was 560 kips, so the average pressure is about 1120 psi, so the length change 224ppm. An approximation of the beam is that it is 33 feet long - so about .0074 feet; 0.09 inches. In contrast the total delta on the tension members is 1.40 inches.

I'm not doing this for the deck because I'm lazy and don't care to find the section area, the preload, and the tension that offsets the pre-load when it's installed.

**
(Feel free to check the math, but the ratio seems about right.)
 
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Greetings to all

Regarding the stressing operations:

All stresssing. deck, canopy and diagonals, were made in 1 phase after the main span was cast on temporary supports at the side of the road.

Mr. Toomas Kaljas, whose analysis was posted in a previous thread, has revised his analysis incorporating the PT based on the existing published contract plans. The analysis show that almost all the load goes to the deck or the canopy because their stiffnesses. The diagonals get only a small load. For practical purposes, worthy of not being accounted for compared with dead load.

Good hunting...
 
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Hokie... opens up an interesting legal issue. Was the collapse in the realm of the Contractor or the Engineering Firm.

Dik
 
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There seems to be a unique osmosis regarding liability in most litigation, it tends to flow toward the deeper pockets.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
 
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ON: That's why I'm curious about the FDOT's involvement... deeper pockets.

Regarding Contractor and Engineer... I was thinking more about criminal proceedings...

Dik
 
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The worker in photo I posted May 16 is employed by Bolton Perez and Associates.

The Mad Spaniard said:
"... deck or the canopy because their stiffnesses."
Click to expand...

Mad, this has me wondering if the bridge was just a deck and nothing more, would it have been able to stand on its own unloaded? Maybe. Not trying to make an important point here, just a thought exercise. The whole idea of a truss is to replace columns under the deck with members in tension on top. The weight of the canopy and members would not be applied to deck.
 
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Greetings jr87

Given their thickness, the canopy and the deck can not span the 175 feet by themselves. The system acts mostly as a truss for dead load and live load. Now, it can also be considered as a "beam" the same way that you take a concrete beam and the shear is evaluated using "truss" behaviour having a compression flange at the top and a tension flange at the bottom.

Now, at the ends of the span, we may have a little of "beam" behaviour because the north temporary supports are distributed transversally and we do not have a support directly under member 12. For the south, we have two bearings 5 feet away from the centerline. These two details may have some effects on the "truss" behaviour of the bridge. As I have not done a 3-D analysis, I can not say where we are.

For the longitudinal PT in the canopy and the deck, the results from Mr. Kaljas indicate that the diagonals get very little load and that most of the straight tendon compression force is absorbed directly by the canopy and deck.

Typically, the canopy should not have longitudinal PT because it is in compression. However, because the back span is attached to the main span at the end of construction. the structure acts as a "two span continuous beam" and therefore there are tension forces at the canopy near the pylon due to live load. Also, because the back span is let go after being attached to the main span, this causes negative bending near the pylon with the corresponding tension forces in the canopy. This is probably, why the designer placed PT in the canopy. Also, for symplicity, they may have decided to extend the tendons all the way to the ends. At the ends, I doubt that you need so many tendons...

Best regards

Live long and prosper.
 
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Mad, thank you. I was thinking how much thicker would deck have to be with no truss or canopy at all. Would a more pronounced v-shape have helped? What kind of moments did this bridge have?

I'm beginning to suspect they really did break something while adjusting bar tension, and like a tiny pin point bursting a balloon, 11 popped. It seems to me for span to fall under dead load only, the defect must have been severe. Too bad water bottle load test was not done.
Why was T.Y. Lin's much sounder concept rejected? It seems ABC was non-negotiable. ABC as advantages, but it does not obsolete older methods. When you're holding a hammer, everything looks like a nail.
 
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jrs87: He seemed to have missed the EoR in his list; for financial reasons he may not be a target. In some jurisdictions, you can sue the engineer, the company, and, the engineer and company jointly. This adds to the permutations. Then there are party's within the group that will cross-sue other parties... it' going to be a real 'fuster cluck'.

The Spaniard: There are a few ways of looking at the framing for the bridge, but, at the end of the day, there will be a similar stress condition throughout the bridge. Treat it as a truss, treat it as a beam. A realistic model of the framing will yield similar results.

Dik
 
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