So, to sum up, can we surmise that the bridge collapsed because of the following:
overload (too many construction vehicles, poorly placed, too much traffic)
freight train vibrations/displacement (oh, please say this had something to do with it! It makes such a simple visual for me!)
You have given some great explanations, pictures even. Thanks for all your work!
blu wrote:
So, to sum up, can we surmise that the bridge collapsed because of the following:
overload (too many construction vehicles, poorly placed, too much traffic)
freight train vibrations/displacement (oh, please say this had something to do with it! It makes such a
simple visual for me!)
You have given some great explanations, pictures even. Thanks for all your work!
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We don’t know why the bridge collapsed, but we are getting a better idea of how it collapsed, the progression of the failure.
In a cantilever bridge like that one, the live load, cars and trucks, construction vehicles, etc., is a pretty small fraction of the dead load, the weight of the bridge structure itself. Sure, extra vehicles will increase the chance of failure, just as one extra straw will break the camel’s back. But it’s not the one straw that does it, it’s the total load, of which heavy traffic probably wasn’t the largest fraction.
I could see vibration from trains disturbing old concrete, but that’s why they inspect bridges periodically. Significant subsidence, or cracks from vibration, even in a sidespan pier, are unlikely to remain unnoticed.
Best guess right now, heavy live loads, train vibration, construction mistakes, and metal fatigue look good, with metal fatigue being a critical element. Something brought down the supports for the bridge deck, either at the north main span pier, the next sidespan pier north of that, or both.
I’m less likely to look for footing or concrete issues because concrete usually shows visible problems before they are fatal.
Metal fatigue shows problems too, but it can be very difficult to judge precisely how severe they are. Fatigue occurs when metal is stressed beyond it’s elastic limit. Bend or stress metal, let go, and it returns to it’s original shape, fine. Bend or stress it too far, deform it permanently, and internal issues start to cascade. Internal means “hard to see”.
Vibration from a train or construction work puts much or all of the dead load in motion. Energy calculations involving motion include a velocity squared parameter, a force magnifier which can quickly become signnifcant, either at the time of a failure, or leading to metal fatigue before a failure.
The devil’s going to be in the details on this one, either in the steel over the north mainspan pier, or the next sidespan pier and/or steel above that. Looks to me like the area around the 1st sidespan pier north of the main is hard to access right now.