Engineers look for causes of bridge collapse

Published 2 August 2007

Engineering experts puzzled by catastrophic collapse of Minneapolis bridge; may have been the result of a “prefect storm,” in which several causes combined

Engineers and scientists are already struggling to understand how the I-35W bridge spanning the Mississippi in Minneapolis collapsed so catastrophically on Wednesday, killing at least seven people, as cars and trucks were thrown into the water below. The death toll may well rise, as dozens are still missing and more than sixty are in hospital. The eight-lane bridge connected the east neighborhood of Minneapolis with the west neighborhood of the University of Minnesota (what those of us who lived in Minneapolis for a while call Dinkytown). The bridge suffered a total structural failure as the whole middle section of the bridge, along with some fifty vehicles, crashed into the river. A freight train sitting beneath the structure was also partially crushed, although this did not cause further injures.

There was little unusual about the bridge design apart from it having no supporting pillars in the water. Instead, the 580-metre-long platform was supported by a single 140-metre-long steel arch spanning the river. According to Tim Pawlenty, the governor of Minnesota, the bridge was last inspected in 2006 and no structural defects were found. “This is an amazing collapse,” Tom Foulkes, director general of the U.K. Institution of Civil Engineers, told the New Scientist. “I have never seen anything like it before. For the entire central span to collapse like this is most unusual — possibly unique.” The bridge was undergoing maintenance work when the collapse occurred, and this could be a key factor for engineers investigating the disaster. “That would be another important line of enquiry,” says Foulkes. Other causes are likely, however. “Major failures are often due to a combination of factors, rather than a single cause,” Foulkes says. “Fatigue cracks, corrosion, ship impact, bearing failure, scour or damage to the supports — all these could contribute.”

A University of Minnesota Civil Engineer report to Minnesota Department of Transportation, commissioned in 2001, noted that the bridge should be considered a “non-redundant structure.” This means that if any key structural feature fails, the bridge would collapse completely. This report found no evidence of fatigue cracking on the main truss, floor truss, or deck truss. Each truss is a combination of individual jointed beams that provides support for the overall structure. In 2003 the Department of Transportation described the condition of I-35W as “fair” on its National Bridge Inventory website, stating that it “meets currently acceptable standards.”