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Severely corroded bearings on the north end of the I-35W bridge in Minneapolis were a potential cause of the Aug. 1 I-35W bridge collapse that is under investigation by the National Transportation Safety Board.
Seventeen years ago, the federal government first said the bridge was structurally deficient, citing significant corrosion in its bearings. Seven years later, significant out-of-plane distortion was noted in the main trusses connected to cross girders due to resistance to motion at the connection cross bearings, according to a report released by the University of Minnesota civil engineering department. The cracks in the girders were drilled to prevent further propagation. Support struts were also added to the cross girder.
Bridge bearings, such as the multiple roller bearings used in the I-35W bridge, are used to transfer loads from the deck to the superstructure as weight burdens vary or weather conditions change. Their primary purpose is to redistribute major longitudinal shifts in load. Elastomeric and fluoropolymer materials are sometimes used in bearing systems, but often for different purposes than the steel system used on the Minneapolis Interstate bridge. It’s likely that engineers will put new emphasis on potential use of polymeric materials as a result of the calamity in Minneapolis.
The final report issued by the University of Minnesota, which was sponsored by the state Department of Transportation, said that the “bridge’s deck truss has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system.” The report, issued in 2000, also stated that the “fatigue cracking of the deck truss is not likely” and said that the Minnesota Department of Transportation “does not need to prematurely replace this bridge.”
The basic design of the bridge, however, may have made it less able to withstand the pressure of the distortions caused by the corroded roller bearing plates. The center span of the bridge consisted of a 458-ft steel arched truss over a navigation channel in the Mississippi River. No piers were built into the river bed. Also, there was no redundancy in the main truss system, meaning that a single structural failure could have catastrophic consequences. Truss-type designs date all the way back to the late 18th century, but have fallen out of favor since 1970 as better highway construction materials (pre-stressed concrete and steel plate girders) have emerged. As a result, there has been enhanced interest in studying redundancy in stress bridges. The I-35W bridge that collapsed in Minneapolis was finished in 1967.
In a redundant system, another structure would have absorbed the load if one of the deck stresses failed due to a combination of distortion and aging materials.
A detailed Federal Highway Administration (FHWA) study in 2001 concluded that "in-depth inspections are unlikely to correctly identify many of the specific types of defects for which this type of inspection is frequently prescribed."
The Minneapolis bridge failure raise questions about bridge design as well as inspection systems. According to a six-year-old federal study, less than 4 percent of weld inspections correctly identify crack propagation. One potential solution is use of piezoelectric sensors installed on potential failure points. Sensors could be wirelessly connected to monitors that would give advance warning of materials or joint degradation. Meanwhile, there are scores of deck truss bridges throughout the U.S., triggering demands for immediate inspections. New York will inspect all deck truss bridges by the end of the month. The Claude Allouez Bridge in Wisconsin is closing tomorrow for an inspection.