Resonance and bridge collapses

[reasonmclucus]

I remember the discussion of resonance from physics class a long time ago because of the illustration of the collapse of the Tacoma Narrows Bridge which was presented as a result of resonance resulting in part from the fact that the bridge's two suspended spans were the same length. http://www.ketchum.org/bridgecollapse.html
According to this site some engineers question the relevance of the resonance issue.

I noticed something interesting about the recent collapse of the I-35W bridge in Minneapolis. The west most 4 lanes were carrying traffic. The next segment of 4 lanes, of presumably equal width, were having concrete removed. Then there were another 4 lanes of traffic and the final 4 lanes were having concrete removed. The Minneapolis Star Tribune has reported that workers told a local police officer that they had noticed the bridge was wobbling and the wobble was increasing with each layer of concrete removed. http://www.startribune.com/10204/story/1343624.html

Could some type of resonance situation have occurred because of having two areas of equal width having traffic induced vibrations alternated with two equal width areas without such vibrations which were also becoming physically different? And if the bridge were close to falling apart anyway could this have reduced the bridge's stability making it more likely to fall?

Incidentally, if you think resonance is possible and have the appropriate academic credentials you might want to contact one of the news channels. You might also want to contact a trial lawyer or two about being a paid witness in one of the lawsuits that is likely to come from the collapse.


I am also curious about the degree to which the components pressing against each other might have helped keep the bridge standing. For example, the weight of the bridge might have kept the metal supports in place even though one of them may not have been actually attached to the pier upon which it was standing. The bridge fell like a house of cards and maybe that is because at the time it was held together more by physical forces than mechanical connections.

 

[mgb_phys]

A little hesitant at getting into this topic but...
Traffic vibrations generally don't cause resonance. You need some considerable power applied at a single frequency, such as the cross wind in the case of the tacoma narrows or the historical example of marching soldiers.
Removing the surface could change the mass of the deck enough to change it's resonant frequency but it doesn't seem that this had any effect.

Components pressing against each other is a standard way of making structures. There are quite a few 2000year old bridges which are held up quite nicely by stones pressing against each other - all arches work this way.
It is in fact common in large bridges for the span not to be connected to the foundations at all but to sit on rollers or pads to allow it to move accomodating thermal expansion or small ground movements.
I imagine in america the courts can find enough expert witnesses - although anyone who wants to be one should read Petrovsky's book about his experiences.