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Inspectors, state officials and engineers alike are scrambling to find out why Minneapolis' I-35W bridge collapsed into the Mississippi River last night. When the dust settles, will the bridge structure itself be to blame or was metal fatigue the true cause of the collapse?

Kenneth Russell, professor emeritus of metallurgy and nuclear engineering at MIT and Design News contributing editor, suspects metal fatigue could be a contributing factor based on a recent report from the University of Minnesota's Center for Transportation Studies. That report analyzed the metal fatigue behavior of the bridge.

"The report concluded that fatigue shouldn't have been a problem, but pointed out that the bridge was very near to the fatigue limit and had gone through many cycles," he says. Fatigue was analyzed visually, which Russell says could mean the analysis didn't go as far as it should have. "Fatigue is very hard to see visually, and you would need to use x-rays to be absolutely certain there wasn't any," he says. The conclusions section of the report didn't say anything about using an x-ray — just that nothing was found visually.

In 2005, according to Secretary of Transportation Mary Peters, the I-35W bridge scored a 50 out of 120 rating in a federal highway inspection of structural integrity, which marked the bridge as structurally deficient. "None of those ratings indicated that there was any kind of danger here, it simply says that we need to schedule this bridge for rehabilitation and that was in the future program for MinnDOT," she says.

According to the 2005 Report Card for America's Infrastructure by the American Society of Civil Engineers, 27.1 percent of the nation's 590,750 bridges rated structurally deficient or functionally obsolete in 2003. However, the report estimates it will cost $9.4 billion a year for 20 years to eliminate all bridge deficiencies. The ASCE attributes the long-term underinvestment to a lack of a federal transportation program.

The report states that a structurally deficient bridge is closed or restricted to light vehicles because of its deteriorated structural components. While not necessarily unsafe, these bridges must have limits for speed and weight. A functionally obsolete bridge has older design features and, while it is not unsafe for all vehicles, it cannot safely accommodate current traffic volumes or all vehicle sizes and weights.

The bridge's main 458-foot span collapsed into the Mississippi River during evening rush hour, sending dozens of vehicles, tons of concrete and twisted metal into the water 64 feet below. Built in 1967, the bridge carried an average of 140,000 vehicles a day. The National Transportation Safety Board assembled a team consisting of 19 investigators representing a number of disciplines including bridge, materials, highway and survivability engineering to investigate what Mark Rosenker, Chairman of the NTSB, says will be a "complex" investigation. "The first thing we must do is recover the pieces, and after we recover these pieces of the bridge we will begin actually trying to reassemble them ... in a jigsaw puzzle way, flat, so we can look at the various parts of this bridge and understand what made it fall down."

The chairman says investigators will likely focus on two primary causes: vibration and fatigue cracking.

The I-35W bridge is the first to have collapsed like this with no impact or outside trigger since Connecticut's I-95 collapsed over the Mianus River in 1983.