Vibration a top suspect in collapse

Construction machinery vibration is high on the list of suspects in the Minnesota bridge collapse, according to a Chicago Tribune story.

Because of the bridge design, extraordinary care was called for in I-35W's concrete repaving work, experts told the Tribune's John Hilkevitch.

"Pavement pounding can damage a bridge truss member or the floor beams; precautions must be taken," said Karl Frank, a structural engineering professor at the University of Texas at Austin.

An anonymous engineer who was on site prior to the collapse told the Tribune that there was a "lot of redecking going on and a ton of vibrations" from pavement grinders and jackhammers. "There was also a lot of equipment on the bridge -- probably more than they should have had on there."

The vibrations from the pneumatic hammers and from the 1000-pound ramming heads could cause one weakened truss to fail and set off a cascade of structural failure, experts told Hilkevitch.

The steel support structure under the roadway had shown signs of metal fatigue in a number of places, though they seemed routine and set off no alarm bells. But, small cracks can spread under persistent heavy pounding, according to Edwin Rossow, professor emeritus of civil and environmental engineering at Northwestern University.

"If that is what happened -- and it's purely speculative at this point -- the newly weakened condition of the bridge would then be exposed to the heavy rush hour traffic on one side of the bridge, affecting how loads are delivered to the superstructure," Rossow said.