I believe that the triggering event in the collapse of the Minneapolis bridge was a "sympathetic resonance" induced in the bridge by the freight train which was passing below it at the exact moment of the collapse. The train created an oscillating wave of pressure on a resonant frequency with the bridge. This pressure wave was then amplified greatly by its resonance with the bridge structure, causing the bridge to shake apart, almost explosively.
Here is the support for my proposed cause . . .
In an article on "resonance," Britannica.com observes:
"RESONANCE - in physics, relatively large selective response of an object or a system that vibrates in step or phase, with an externally applied oscillatory force. Resonance was first investigated in acoustical systems such as musical instruments and the human voice. An example of acoustical resonance is the vibration induced in a violin or piano string of a given pitch when a musical note of the same pitch is sung or played nearby.
"The concept of resonance has been extended by analogy to certain mechanical and electrical phenomena. Mechanical resonance, such as that produced in bridges by wind or by marching soldiers, is known to have built up to proportions large enough to be destructive, as in the case of the destruction of the Tacoma Narrows Bridge (q.v.) in 1940. Spacecraft, aircraft, and surface vehicles must be designed so that the vibrations caused by their engines or by their movement through air are kept to a safe minimum."
The reason that freight trains never caused this problem in this bridge before is that the sympathetic resonance depends on the exact relationship between the vibrations from the train and the structure/load/forces of the bridge. The exact combination necessary for collapse had not happened before yesterday, when traffic was slowed to a standstill and weight was redistributed by lane closings for bridge surface repairs.
The two quotations below prove that a freight train was passing under the bridge at the time, and that sympathetic resonance has previously been cited as the cause of another bridge collapse.
"A freight train was passing under the bridge when it collapsed and was cut in two, witnesses on the scene reported to MSNBC." Source: http://www.msnbc.msn.com/id/20079534/?GT1=10252
"Arden Wilken on his website provides a significant example of the power of sympathetic resonance:
"The power of resonance can be seen dramatically in what occurred in 1940 to the Tacoma Narrows Bridge in the State of Washington, USA. The wind created a tone as it blew across the valley, which happened to be the natural frequency of the bridge itself. This resonance created by the wind led to the total collapse of the bridge.
"A film of the bridge in the process of collapsing showed it undulating wildly in a continuous wave, appearing to be made out of ribbon instead of concrete and steel. The wind and the bridge sang together in sympathetic resonance. In this case, unfortunately, it was the destruction of the bridge because it could not incorporate the vibration." Source: http://en.wikipedia.org/wiki/Sympathetic_resonance
Other observations by eyewitnesses are consistent with this explanation. I watched on TV as one eyewitness reported that he heard a "great roar" as the bridge began to collapse. Not a "cracking sound" or a "breaking sound" as might have been expected, but a "great roar." Another eyewitness reported that a huge cloud of dust arose from the bridge as it was collapsing. This, too, is more consistent with sympathetic resonance than with simple structural failure.
Finally, this morning I saw that the center part of the bridge collapsed first, all by itself, disconnected from the two end parts. And then the two end parts collapsed. This tells us that all three parts of the bridge were affected independently by the same forces which caused the collapse; that the center part did not drag the other two parts down with it as it collapsed.
So this is my proposed explanation of the cause of the Minneapolis bridge collapse of August 1, 2007. I hope it is helpful.
Blessings to you. May God help us all.
Rev. Bill McGinnis, Director - LoveAllPeople.org
HTML page at http://www.loveallpeople.org/bridgecollapse.html