Minneapolis freeway bridge collapses

MINNEAPOLIS—A 1,900-foot-long, eight-lane freeway bridge across the Mississippi River near downtown Minneapolis suddenly collapsed about 6 p.m. local time, Wednesday, Aug. 1. According to news reports on Thursday, at least four people were killed, 79 injured, and as many as 30 are still missing. An estimated 50 vehicles were on the bridge at the time of its collapse and many vehicles remain trapped underwater beneath debris.

As part of the Interstate highway system (I-35W), the 14-span bridge carries both north- and south-bound traffic. Designated Bridge 9340 by the Minnesota Department of Transportation (Mn/DOT), it is a deck truss with steel multi-girder approach spans built in 1967. The most recent bridge inspection report (available here; 58.5 KB PDF)  notes several instances of poor weld details, section loss, pitting, flaking, corrosion, (including corrosion of expansion bearings), and cracks (many previously drilled out and braced) among other problems. Numerous fatigue cracks were noted in the approach spans. The bridge was described as "structurally deficient" with a rating of 50.

A research report published in March 2001, based on site studies conducted during 1999 and 2000—Fatigue Evaluation of the Deck Truss of Bridge 9340 (available here; 11.7 MB PDF)—noted that "although fatigue cracking has not occurred in the deck truss, it has many poor fatigue details on the main truss and floor truss systems." Researchers from the University of Minnesota, Department of Civil Engineering concluded at that time, "The detailed fatigue assessment … shows that fatigue cracking of the deck truss is not likely. Therefore, replacement of this bridge, and the associated very high cost, may be deferred." No timeframe for deferment is mentioned.

At a Thursday afternoon press conference, National Transportation Safety Board (NTSB) Chairman Mark V. Rosenker said that the agency will be reviewing the bridge’s design, modifications, and maintenance and inspection records as part of its investigation into the collapse. NTSB also will check with the U.S. Coast Guard to determine if the bridge had been struck at any time by a vessel on the river.

Video from a security camera that captured the bridge’s collapse also will be extremely helpful in the NTSB’s investigation, Rosenker said. (View the video on CNN’s website.) In addition, a laser survey of the collapsed structure will be integrated with a computer model of the bridge developed by a Federal Highways Administration employee while a Ph.D. candidate at the University of Minnesota to help in the failure analysis.

Rosenker also said that the NTSB will evaluate the National Bridge Inspection Program protocols to determine if they are sufficiently robust.