Minneapolis bridge collapse investigation update—Aug. 6, 2007

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, at least five people were killed, more than 80 injured, and eight people remain missing. An estimated 50 vehicles, including construction equipment, were on the bridge at the time of its collapse and some 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.

Consulting firm URS began work in 2004 on an engineering analysis of the bridge and delivered a draft report to Mn/DOT in June 2006 that recommended retrofit measures to add redundancy and eliminate the possibility of a member fracture. USR says it also recommended further detailed inspections. A supplemental draft report in January 2007 expanded on aspects of the initial report.

National Transportation Safety Board (NTSB) Chairman Mark V. Rosenker said that the agency will be reviewing the bridge’s design—especially any unique design features—modifications, and maintenance and inspection records as part of its investigation into the collapse.

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 and finite element analysis program developed by a Federal Highways Administration employee while a Ph.D. candidate at the University of Minnesota to help in the failure analysis. The existing computer model of the bridge will allow investigators to examine numerous failure scenarios in a timely manner.

The NTSB is focusing initially on the southern end of the bridge because it says this section seemed to behave differently in the video and in how it came to rest. Most of the bridge reportedly collapsed vertically, but the southern section shifted about 50 feet sideways as it fell. However, Rosenker says, failure at the north end of the bridge could have transferred forces to the southern end.

The NTSB also is investigating whether resurfacing operations played any role in the collapse. According to news reports, construction workers removing concrete pavement claimed the bridge "wobbled" during the process. Also of concern is the extra weight of construction equipment and aggregate stockpiles on the bridge.

The Federal Bureau of Investigation was conducting a 3-D laser scan of the site, the NTSB said. In addition, within 24 hours of the bridge collapse, AERO-METRIC, Inc., Maple Grove, Minn., said it had completed a vertical aerial photography mission and an airborne LIDAR mission over the site. Collection of ground-based LIDAR data was being considered.

Inspection program reviews
Rosenker says that the NTSB will evaluate the National Bridge Inspection (NBI) Program protocols to determine if they are sufficiently robust. On Aug. 2, U.S. Transportation Secretary Mary Peters also requested the U.S. Department of Transportation’s Inspector General to conduct "a rigorous assessment" of the NBI program to determine if it delivers "the highest level of bridge safety."

Mn/DOT selected Parsons Brinkerhoff to review the state agency’s bridge inspection practices and to assist it in prioritizing bridges for inspection based on factors such as fracture critical status, sufficiency ratings, critical deficiencies, and average daily traffic. According to the Bureau of Transportation Statistics (BTS), Minnesota has 1,135 structurally deficient bridges and 451 functionally obsolete bridges, 9 percent and 3 percent, respectively, of its state bridges. On a percentage basis, Minnesota’s bridges overall appear to be in better condition than structures in most states. (See a state-by-state BTS listing here.)

In addition to evaluating the NBI program, Secretary Peters called on all states to "immediately inspect any steel deck truss bridges similar to the I-35 bridge that collapsed. According to FHWA data, there are 756 steel deck truss bridges in the United States. As of Thursday, Aug. 2, St. Louis County, Mo., announced that it was closing one bridge as a precaution.

Mn/DOT also hired consulting firms Wiss Janney Elstner and Lichtenstein & Associates to develop a plan for forensic analysis of the cause of the bridge collapse. The department expects to issue a Request for Qualifications nationally to invite contractors to bid on rebuilding the bridge under an A+B design-build contract to expedite reconstruction.

A website dedicated to the I-35W bridge collapse and rebuilding—www.dot.state.mn.us/i35wbridge—contains up-to-date information, as well as background information on the bridge, including expenditures and inspection schedules, recent inspection reports and studies, inventory reports and lists, and historical reports and drawings.