Miami pedestrian bridge collapse

[California]

Here's what I have put together as hypotheses based on what I've read so far. Surely this will be superseded by more accurate information as the picture becomes clearer.

1) I've read the future tower and cables are decorative. The structure as put in place was complete as designed to carry stress.

2) The design is experimental. It is a (theoretical) I-beam. The 'chords' (diagonals containing post-stressing threaded rods) constitute the center one-third of the I-beam.

3) The initial design proposed that the trucks to carry it into place would have platforms on top to distribute stress, and would support the bridge very near its ends as the bridge was carried into place.

4) Due to curbs or something on the street, the truck carrying the North end of the bridge instead carried it some 20 ft from its end, a point not designed for lifting stress. Additionally the platforms in the initial design weren't used between the trucks and the bottom of the bridge. These two factors concentrated stress onto a narrow location not designed to bear lifting force.

5) After it was in place the project engineer, advisor to the contractor, phoned the state DOT and described unexpected cracks.

6) Post-stressing rods, I think in the chords, were found to have less tension than spec. A crew was tightening a chord, I think up on top above that unplanned lift point and 20 out from the North end, when the structure fell.

7) I've seen conjecture that when the bridge was carried 20 ft from its end, the cantilevered last 20 ft stretched the post-tensioning rods in the chords near to breaking. So after the bridge was placed and its ends bore its weight, a rod (more?) was slack. Possibly allowing sag and the crack noticed by the project engineer.

8) The post-tensioning specialty crew was present when the bridge fell. I think up on top tightening a rod back to designed tension. But I think it snapped before it reached spec. Alternately, the crew might have been directed to apply tension to raise the bridge back up to specified height (and close the observed cracks) without regard for the strength of the rod.

9) More conjecture, unverified: I saw a photo of one tensioning rod sticking out of the roof maybe 8 ft, as if it shot out there after snapping. If it were intact it would still be in its crumpled chord. Another photo showed a different (?) post-tensioning rod sticking out of the roof several ft with the hydraulic tightening tool still attached to it. One employee of the post-stress specialty contractor died, two injured. I assume they were operating that tool.

10) A moment after it fell, above the fail point there is an empty man-lift (I think for that crew) and a heavy crane whose hook is empty. I assume the crane is there to lift that hydraulic tool and perhaps to provide hydraulic power. One witness said a heavy object fell from the crane hook causing the collapse. Possibly a trailer-mounted hydraulic pump? That was a student interviewed by tv at the site, this may be real or only what he thought he saw. I would think tensioning a weakened rod, which then snapped, is a more likely cause. Maybe the falling hydraulic lines pulled the hydraulic pump off its crane sling causing the fall that the student witnessed. Pure guessing on my part, later we shall see what really happened.

11) At the moment, I think tightening the pre-stressing rod that had stretched due to overload, caused that rod to snap leaving the bridge without the support that rod was designed to provide. The bridge fell. And there are obviously gross mis-calculations somewhere in the design and install processes given this result.

All this is my opinion. We shall see how close it is to what is found in the final report.

 

[ovrszd]

After construction, the largest mistake made was having traffic on the road below while they worked on the bridge. I don't get that at all.

 

[laurencen]

if they were indeed trying to close the cracks by tightening the rods they knew something was going on, why let traffic continue during the tightening process, had they done a stress test or trying to reduce the stress, first thing when the cracks and loose cable showed up was a center support and stop traffic

 

[Ron50]

One rod falling should not cause a collapse. Three to one saftey factor. Dropping a hydraulic power unit should not have done it either. Ever seen people jump and down on a bridge. That walk across the bay a few years ago stressed the Golden Gate Bridge to its highest level ever. Almost flattened the arch out of the bridge.
I think this is probably right in line with the Oroville dam problems. Lowest priced bid with perhaps minority contractor requirements, high level administrators that won't listen to the guys on the ground, and local officals not wanting traffic disturbed. Seriously, stress testing or tensioning over live traffic, no one works at night anymore?
Interesting comment that the cable stays were only decorative. Explains why the attachment bolts weren't inline with the angle of the stays, they were verticle.

 

[Tomtint]

Lawsuits will uncover corruption, bribery and incompetence at many levels.

 

[California]

A Miami Herald article that adds some detail about that pylon supporting the bridge:

"design documents and descriptions on FIU's website say the pylon was not meant as the principal means of support for the bridge span, which was designed to hold up by itself. Instead, the pylon system was to provide stiffness to the structure, lessen vibration for pedestrians and provide aesthetic bang."

But since the portion of the bridge beyond the center pylon hasn't been built yet, the pylon couldn't help much to support this side of the bridge.

There isn't much else in that article. It mostly says representatives of all parties are pointing the finger at one another.

 

[prichard]

Lawsuits will uncover corruption, bribery and incompetence at many levels.

Doubt it. They'll pay off lawsuits to keep the corruption covered up. I did a job in Miami years ago where inspectors, city council members and other government officials were brought up in federal charges, a few actually did time. A few years later a friend was on a job there and had to bribe inspectors to get them to pass inspections. Miami is prohibit in the top 5 most corrupt cities in the country.

 

[/pine]

I was the field engineer on a job in Miami Beach overseeing the crossing of Collins canal with a 42" force main...the municipal inspectors did not have a clue what they were doing...but the Cuban sandwiches off the lunch carts were as good as they get...!

 

[/pine]

Miami is prohibit in the top 5 most corrupt cities in the country.

There's an old joke about the longest causeway in the world being in Miami..."it goes from Israel to Cuba"...!

 

[aczlan]

Juan Brown did a video on this:

Aaron Z