I have not had much time to watch the videos or see the news but heard some concrete guys here speculate that the train might play a part in the final report, if the bridge failed at that point. The live load of all the stopped traffic, the steep slope cut into the bank next to the tracks, combine with shaking from the train, and all that silt on the bottom, no bedrock there, could have made for ‘a whole lot of shaking going on’.
concrete
is my business wrote:
I have not had much time to watch the videos or see the news but heard some concrete guys here
speculate that the train might play a part in the final report, if the bridge failed at that point. The live load
of all the stopped traffic, the steep slope cut into the bank next to the tracks, combine with shaking from
the train, and all that silt on the bottom, no bedrock there, could have made for ‘a whole lot of shaking
going on’.
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I haven’t been able to determib\ne if that was a moving train or spotted cars. The railroad people I know are debating if the line under the southern approaches was active at all. Apparantly it used to extend further west, but was abandoned/torn up/not abandoned/not used/not abandoned/seldom used depending on who you ask.
A train at low speed or idle should be capable of setting up vibrations, potentially of destructive amplitude. More energy to work with at higher speeds, but the doppler component would have to remain within the structure’s resonant range to continue propagating the effect, and the dynamic loads would alter the bridge’s vibration characteristics to the point that post failure modelling may be difficult.
There is precedent for a wide range of resonant frequencies, obviously, as the Tacoma Narrows bridge tended to oscillate at widsspeeds from 25 mph winds and up before its collapse.
I see a bit more damping in this case though.
Advancing traffic can add or subtract to the resonance, and I’ve seen statements from concrete mixer operators that they have to be careful mixing at full speed on suspended structures due to vibration effects.
Mat or floating pile foundations, as opposed to foundations on bedrock could assist or impede resonance, together in concert with fixed and mobile bearing plates, and this bridge has a history of bearings that froze and had to be replaced. Tricky stuff, a lot to look into before drawing conclusions.
A bridge, traffic, cellphone, or security cam showing the reverse angle of the one we have now would be useful to watch.