My guess is this is going to boil down to harmonics that resulted from:
1. the reduction of material on the bridge due to resurfacing,
2. jackhammering that was going on at the time
3. traffic going at a specific speed that caused a harmonic oscillation to start
4. specific temperature for natural expansion to react to 1, 2 and 3.
One of the bridges over the Ohio river, which was only used for pedestrian traffic for Reds and Bengals games had to be closed because the rhythm generated by people walking at 3 mph caused a harmonic oscillation that enhanced fatigue of the structure. That same slow driving on any bridge can cause the same effect which exacerbates any standing problems.
MINNEAPOLIS, Minnesota (CNN) -- Two reports published since 2001 pointed to structural problems with the Interstate 35W bridge that collapsed Wednesday into the Mississippi River, but both reports determined the bridge was safe despite deficiencies.
"The bridge's deck truss system has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and the floor truss system," said a report conducted for the Minnesota Department of Transportation in 2001.
Trusses are support mechanisms, and floor trusses often act as the "primary load-carrying members in a bridge superstructure," according to the American Society of Civil Engineers.
The executive summary of the report -- undertaken by the University of Minnesota's Department of Civil Engineering -- also points to fatigue problems with the bridge's approach span, the segments that connect the main span of the bridge to land. It attributed the fatigue problems primarily to "unanticipated out-of-plane distortions in the girders."
However, the report also said, "Fatigue cracking is not expected during the remaining useful life of the bridge."
In another report two years ago, the U.S. Department of Transportation's National Bridge Inventory database concluded the bridge was "structurally deficient."
Jeanne Aamodt, a spokeswoman for the Minnesota Department of Transportation, told the Star Tribune, a Minneapolis newspaper, that the department was aware of the 2005 assessment, which gave the bridge a score of four on a scale of zero to nine.
A bridge receives a four when there is "advanced section loss, deterioration, spalling or scour." Spalling is a term used to describe cracking, chipping, crumbling or fraying, while scour is a term used for erosion caused by flowing water.
During a Wednesday news conference, Minnesota Gov. Tim Pawlenty described the bridge deficiencies as "minor" and said the state was told that the bridge's deck may need to be rehabilitated or replaced in 2020 or later.
"It was last inspected both in 2005 and 2006. There were no structural deficiencies identified according to [the Minnesota Department of Transportation]," Pawlenty said.
National Transportation Safety Board investigators will begin examining the bridge Thursday, focusing on design, construction and maintenance history.
Vital clues about what caused the bridge to crumble may be lying in the Mississippi River.
One expert said that it was baffling how the bridge collapsed.
"I am totally puzzled as to why both ends of the bridge would come down all at once. When my colleague tested it, it was very low stress," said Ted Galambos, a University of Minnesota engineering professor. "I don't think it was overload, so it could have been either some fatigue, failure or some sudden buckling that would cause the failure."
The Interstate 35W span, known in federal transportation parlance as Bridge 9340, was built in 1967. It is a deck truss bridge, meaning the trusses are below the deck on which vehicles drive. Its approach spans are supported by multiple girders.
Senate Majority Leader Harry Reid, D-Nevada, said the collapse should trigger action.
"I think we should look at this tragedy that occurred as a wake-up call for us. We have -- all over the country -- crumbling infrastructure, highways, bridges, dams, and we really need to take a hard look at this," Reid said Thursday.
He said it was "the right thing to do" for the infrastructure and the economy. "For every billion dollars we spend in our crumbling infrastructure, 47,000 high-paying jobs are created," Reid said.
Most bridges are inspected every two years and receive ratings based on the conditions of various components, according to the U.S. Department of Transportation.
Bridge deficiencies are summarized as "structurally deficient" and "functionally obsolete," according to the Federal Highway Administration.
A bridge is tagged structurally deficient when significant bridge elements have deteriorated and the bridge's load-carrying capacity is reduced, according to the highway administration.
A bridge is dubbed functionally obsolete when the bridge does not meet current design standards.
Neither label indicates a bridge is unsafe for travel, the highway administration said.
As of 2003, there were about 160,570 bridges deemed structurally deficient or functionally obsolete, according to the American Society of Civil Engineers. The number represented 27.1 percent of the nation's bridges.
The American Society of Civil Engineers also reported that the number of bridge deficiencies had steadily declined from 34.6 percent in 1992 to 27.1 percent in 2003
Reports: Problems with Minnesota bridge noted twice since 2001 - CNN.com