MINNEAPOLIS—At press time, two weeks after the catastrophic collapse of a 1,900-foot-long, eight-lane freeway bridge across the Mississippi River near downtown Minneapolis, recovery operations and on-site investigations continued, even as the Minnesota Department of Transportation (Mn/DOT) pushed forward with rebuilding plans. Condition of the 14-span bridge, a deck truss with steel multi-girder approach spans built in 1967, has been a concern to Mn/DOT for several years.
The most recent bridge inspection report noted 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 out of 100.
A research report published in March 2001 based on site studies conducted during 1999 and 2000, 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 said it also recommended further detailed inspections. A supplemental draft report in January 2007 expanded on aspects of the initial report.
The National Transportation Safety Board (NTSB) is working with the Federal Highway Administration (FHWA) to conduct a structural analysis of the bridge using computational finite element analysis methods. Data collected at the accident scene, including a 3-D laser scan, will be used in the computer model to further refine the model. 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.
Initially, the NTSB focused on the southern end of the bridge because it says this section seemed to behave differently 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, NTSB Chairman Mark V. Rosenker said failure at the north end of the bridge could have transferred forces to the southern end.
Using a gyro stabilizer, high-resolution camera mounted on a state police helicopter, the NTSB then looked at the superstructure on the north side of the bridge where it found several tensile fractures. However, nothing looked to be the initiating location, the Board said. NTSB investigators plan to take a closer look at the superstructure on the north end when it becomes more accessible.
NTSB investigators noted a design issue at particular locations with gusset plates that tie steel beams together. They are verifying the loads and stresses on the gusset plates at these locations, as well as the materials used in constructing the plates.
The NTSB also is investigating whether resurfacing operations played any role in the collapse, in particular, the extra weight of construction equipment and materials stockpiles on the bridge. NTSB is reviewing construction records to determine the location of construction equipment and construction materials on the bridge at the time of the collapse, and to verify the weights of those vehicles and materials. The Board also obtained core samples of the bridge deck material to get a better picture of the deck thickness to help make an assessment about the amount of concrete on the bridge at the time of the accident.
Inspection program reviews
Rosenker says that the NTSB will evaluate the National Bridge Inspection (NBI) Program protocols to determine if they are sufficiently robust. 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, 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.
In addition to evaluating the NBI program, 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. St. Louis County, Mo., closed one such bridge as a precaution.
Mn/DOT 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 also issued 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.