NTSB Investigation of Bridge Collapse Continues

National Transportation Safety Board ^ | 3/21/2018 | National Transportation Safety Board Office of Public Affairs

[editor-surveyor]

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Nothing can be discerned about the mix until a core has be dissoloved in acid and titrated.

[PA Engineer]

Exactly. Watch the video. It wasn’t the engineers who designed the bridge who were at fault. It was the contractors who were building it.

We don't have all the information. In my PM/PE days, most of our work was design with Project Management and inspection. There are design build, however minimally you have design, oversight and inspection. Think Gantt chart. Normally the engineers sign off on the build and modify the drawings as-built. Changes also have to signed off on.

I already have enough issues with a "revolutionary" truss design. I'm also suspicious of A&E oversight from the firm.

[editor-surveyor]

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That would be some very expensive cosmetics!
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[BDParrish]

The tensioning rod was stretched to its “fluidity limit” during the move because it was not supported on that end. Then, when the structure was placed on its permanent supports, the rod appeared to be loose, so they tightened it, and it broke.

Here is the informative YouTube explanation which includes a “homebrew experiment” demonstration showing what happened:
https://www.youtube.com/watch?v=KtiTm2dKLgU
“Post Tension Failure Florida Bridge Collapse | Engineering EXPLAINED!” by AvE

[PA Engineer]

That’s fine. I’ll go on record (ignoring both tension and shear), the concrete will fail compression and rebar pull out requirements.

[dr_lew]

Well, from “what you read” ... sure! because they keep saying it! I can’t picture it. N.B. a “cable stay” bridge is usually built from the tower out. This case is all circumstance, and from here, it stinks.

[Alberta's Child]

If the bridge span wasn't able to support its own weight without the cables in place it would have failed in the middle, not at one end.

You can go back and find my FR posts on this subject in the immediate aftermath of the collapse. When it comes to identifying the cause, it is likely to be one or more of the following (in this order of probability):

1. shoddy construction process

2. poor materials

3. bad design

An error in the design of the bridge is not very likely. The other two are FAR more likely to be the culprits here.

[dr_lew]

I’ll go on record (ignoring both tension and shear), the concrete will fail compression and rebar pull out requirements.

Well, in this case the concrete deck was under tension, and the roof was under compression. Of course, concrete is notoriously incapable of supporting tension, and the whole thing unfolds from there, AFAICT.

Am I in a vacuum?

[dr_lew]

If the bridge span wasn't able to support its own weight without the cables in place it would have failed in the middle, not at one end.

It did have the "roof" acting as the top bar of a truss. The tension struts, designed to support the deck from the attachment points, were acting as the elements of a truss in this case, although they were asymmetrically arranged, BECAUSE THEY WERE NOT DESIGNED AS A TRUSS. Still, they connected the deck to the roof.

By happenstance, the end segments of the roof, between struts, were longer than the center segments. The "other" end, ( away from the failing segment ) was actually a little longer, but these two longer roof segments, acting as truss elements, were obviously more vulnerable to failure.

Since the struts were asymmetrical, you'd have to do an analysis. There's also the question of all this "tensioning". You know, I wondered what these "tension cables" were, but I believe they were part of the ( true ) original design, and their intended purpose was to adjust the load between the several "stays", terminating on the roof.

[Alberta's Child]

Here's a photo that helps illustrate what probably went wrong:

This is a photo of the bridge span as it was being moved into place a few days before the collapse.

If you look closely, you'll see that that there are four separate support rigs underneath the bridge section, moving it into place -- two pairs on the far (left) side, and two on the near (right) side. For the sake of this discussion, let's name them (from left to right) Rigs #1, #2, #3 and #4.

It's hard to tell this for certain because of the perspective in this photo, but Rigs #1 and #2 are closer to the far (left) end of this section than Rigs #3 and #4 are to the near (right) end. It's likely that this played a major role in the collapse.

According to one of the other sources I've linked in various threads, Rig #3 isn't in the place it was shown in the original construction plan. It was supposed to be outside Rig #4, closer to the near (right) end of the span. The contractor decided to move Rig #3 from its original position to the position shown in this photo. This was probably done because putting it in its original position would have required the removal of the concrete barriers and the installation of the same temporary steel plates on the near (right) side of the road that you see on the opposite side.

With the relocation of Rig #3, you now have a cantilevered section of the bridge span between Rig #4 and the near (right) end of the span that is longer than it would have been if Rig #3 had been in its original position.

I strongly suspect that this modified support configuration put stresses on this bridge span that it was not designed for. You can see evidence of this when you look carefully at the videos of the collapse, and at the photo of the aftermath shown below. The initial point of failure in this collapse was at the end of the span I've described here.

[Alberta's Child]

Understood. See Post #50 for my take on it (based on what we know so far).

[dr_lew]

I strongly suspect that this modified support configuration put stresses on this bridge span that it was not designed for.

Makes no sense to me. When installed, the span was resting on its two extreme ends. Now, you want to say it was compromised by being supported temporarily in some less stressful configuration ... makes no sense to me!

That's all I have to go by, and my Ph.D. in Physics, if I haven't made that known. Not making any claims, just sayin' .

I've studied this image, for sure, BTW.

[Alberta's Child]

Now, you want to say it was compromised by being supported temporarily in some less stressful configuration ...

"Less stressful" is a very relative term. This span was designed to be supported in a certain way -- both in its ultimate configuration and its interim stages where it was transported and mounted there. A cantilevered end of a structural member is subject to different stresses than a fixed end ... not "more" or "less" stress as you might think -- but DIFFERENT.

Something else to note here is that the bridge span was supposed to be mounted on the four rigs using "spreader plates" to spread the load resistance of the rig supports out over a larger area of the bridge span as it was being moved into place. I've seen information suggesting that these were not used ... which means the bridge was subject to upward forces from the rigs that were concentrated in smaller areas than expected. This may have been a major factor in the collapse, too.

That's all I have to go by ... and my BS and MS in civil engineering, plus my P.E. license. :-)

[jim_trent]

From what I have read and seen, the original plan (that came from the engineers) for the 4 trucks showed 2 on the very ends and 2 on the next lower truss joint inboard. The load went through 4 truss joints.

They changed it to have both trucks on both ends under the same lower truss joint. The load went through 3 truss joints instead of 4. That also means both ends (from the lower truss joint outward) were unsupported. They were cantilevered. That is not very good with concrete. Someone said that the post-tensioning bars in the diagonals had to be increased to handle this cantilever load. The tension bars usually are sized without huge safety margins.

They should have reduced the extra tension after it was placed on the piers, but before the supports were removed. They did not do this. They waited 4 days before reducing the load in the bars. I believe the tension bars were overstressed when the supports were removed and at least one of them broke when the tried adjusting it long after they should have done it.

[dr_lew]

It was a span designed to be supported by the “cable stays” for which it had attachment points, and lacking them, it was doomed to fail.

[William Tell]

I'd like to point out my perspective on the key complication that likely played the major role in this incident.

First it is necessary to understand that concrete does not operate effectively in tension. That is why there are cables installed in typical concrete structures to create compression that causes the cables to carry some part of the load rather than the concrete.

Second, one must realize that the loads on various parts of the structure can vary greatly during the construction phase.

Finally, let's consider how the load was to be carried at various times. In this case the load was the deck and the canopy.

While the structure was being built and transported into place, the deck was resting on its ends and the canopy with the struts was being carried by the deck. Most of the structure would be under compression.

When the bridge was to be in its final state, supported by cables from an off-center tower, the canopy would be supported by the cables and the deck would be supported by the canopy. This configuration would have caused the cables and the struts to be in tension, holding the weight of the deck.

Even without the complication of the wheeled devices used to transport the main span, it would be necessary for the struts to perform two dramatically different roles during and after construction.

What the situation tells me is that control of the tension in the struts would have been critical to the success of the project, that critical adjustments would have to be done at critical times, and that failure to appreciate the critical nature of those adjustments could be a likely cause of catastrophic failure.

[editor-surveyor]

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Seems likely

[Chode]

like i said, it was just a link

[EVO X]

“When the bridge was to be in its final state, supported by cables from an off-center tower, the canopy would be supported by the cables and the deck would be supported by the canopy. This configuration would have caused the cables and the struts to be in tension, holding the weight of the deck.”

The tower structure wasn’t designed to provide significant support. It was designed to dampen vibrations from foot traffic and provide a visual wow factor. Go to 6:30 in the video for explanation. He has the basic plans for the bridge.

https://www.youtube.com/watch?v=GxQJj8D_FE0

[EVO X]

Here is brief footage of them moving the span into place. Notice the location of the transporters.

https://www.youtube.com/watch?v=Ik1fCeo4CpQ