1) Some reports said there was a train going by on the east (south?) bank.
The train is on the north bank, and I feel reasonably sure that the train was stopped or moving very slowly. It didn't derail, for instance, and only the cars immediately under the falling bridge seem to be damaged from the pictures. If there was a train on the south bank as well, it's not close enough to see in any of the pictures, and it doesn't go under the bridge.
2) The road work on the top may have been using vibrational tools that aren't necessarily recommended for use on bridges.
I heard this morning that one of the construction crew said that as they removed the concrete panels, the bridge began to wobble. The more they removed, the more it wobbled. This would fit with what the civil engineering report said about the truss and girder structure gaining a lot of stiffness and strength from the concrete panels. The bridge wasn't designed to take advantage of that, but apparently it was happening, and may have been the only thing keeping the bridge up, which the report describes as (in model space) being due to fall down the first week it was opened.
Also, since bridges aren't usually relying on the strength of the concrete panels to stay steady, it seems entirely reasonable that the construction teams and engineers didn't consider that circumstance. The 2001 report makes it pretty clear that something like that is going on, though.
The impromptu analysis from my civil engineering connections says that they don't believe that this was a pure "fatigue" failure. Normally when bridges fall, there is a warning, with moaning and groaning as the thing fatigues and dies, like Galloping Gertie, they knew it was going to fall long before it came apart.
The middle section going straight down is much more like the collapses you see during earthquakes, when a harmonic frequency is attained that causes a "resonance" collapse.
That's really interesting. My own theory is that the bridge fell off the south pier, which yanked the south approach span southwards off its pier, and then the main span was pulled off the pier on the north bank, letting the main span drop straight into the water. The south bearings were at the end of travel, and even tilted partly off their trays in some cases. I think that's why the southern part fell sideways rather than straight down, as the northern part did.
It is possible that the jackhammers used during the resurfacing project earlier in the day, plus the rush hour weight, water current, etc. shifted the resonance frequency of the structure, and that the train going through was the trigger so that the reasonance frequency was achieved and sudden collapse occurred.
That's a cool idea, about the resonance, but I think from reading the inspection reports that the miracle is that the bridge stayed up as long as it did. I don't know that there's any need to invoke any special resonances. Seems to me like removing the concrete weakened the structure that was already at the breaking point, and then the next full rush hour load took it out.
Of course the standard current civil engineering knowledge could be wrong and this bridge could be an educational experience that middle sections can drop out suddenly with out prior moaning and groaning "fatigue" warnings.
I don't think it was fatigue either. I think the bridge fell off its south support.
But it appears to be more of a "reasonance" collapse situation than anything else, from the early visuals on the collapse.
A really cool idea, and I'll be curious to see how it plays out. I hope we're given a full report with complete details of how they come to the conclusions they will. I think it's great that your civil sources have theories. I can't get any of mine to hazard any guesses yet. But probably it's just that they have lives, and haven't spent dozens of hours poring over all the available information like I have. Posts: 6246 | Registered: Aug 2004
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imogen, I'm sure that almost every profession is of equal or more importance than engineering. Because I'm an engineer I do feel personally involved, I think, whenever there are engineering failures. I always feel like they're avoidable and I want to learn whatever lessons are there to be learned.
Definitely the professions you've named have a lot of responsibility for public safety and welfare. We're all interdependent. I totally agree. I would also like to put in a word for some of the less glamorous professions like garbage collectors and plumbers. There are very few jobs that impact our quality of life more if they aren't done well than plumbing and garbage collection. We're so lucky in our society that we have these systems functioning properly.
Stocking grocery store shelves is another unsung but really vital task. This includes the truckers who bring the goods as well as the people who unload the trucks and fill the shelves.
And lastly I would add utility workers. The people who climb electrical poles during storms to fix fallen wires any time of the day or night are real heroes, I think. The same is true for unsung people who keep power plants going, water systems functioning, natural gas lines intact, and also roads in repair. All these systems we tend to overlook entirely unless they aren't working. So nobody ever even notices their job unless it's not done right. Then they yelp. Posts: 6246 | Registered: Aug 2004
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I was thinking about the sacred trust thing on the way home, going under a footbridge, and I could sense it again. I don't know if I will explain it right, but engineering and surgery are like games of golf. There is a standard of perfection that has to be met.
In parenting and some of the other things are more like tennis where some perfection of form is possible, but the game itself requires risk and creativity.
Posts: 11017 | Registered: Apr 2003
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I got to ask my dad about this bridge over the weekend, and how it didn't even look sturdy. He commented that in the postwar era, everyone coming off rationing, that they still liked things that were economical rather than sturdy in their design. I guess I can understand that, and that the bridge had a certain frugal elegance - with tragic results.
Posts: 11017 | Registered: Apr 2003
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