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University of Minnesota’s I-35 Bridge Fatigue Evaluation: A Closer Look
Regina Lynch, Web Editor -- Design News, August 3, 2007
Check in with our I-35W bridge collapse coverage page for the latest news, videos and photos covering the failure.
The collapse of Minneapolis’ I-35W bridge into the Mississippi River has given new visibility to a deck truss fatigue report on the bridge, performed by the University of Minnesota’s Center for Transportation Studies in 2001. Design News obtained access to a copy of the report, which resulted in a new way to assess fatigue cracking on Bridge 9340 on I-35.
According to the report, the research was conducted because the approach spans of the bridge had exhibited “several fatigue problems; primarily due to unanticipated out-of-plane distortion of the girders.” Fatigue details on the main and floor truss systems had given researchers reason to analyze any fatigue cracking of the deck truss. “Stress ranges calculated … are greater than fatigue thresholds for many of the details …[details] include intermittent fillet welds, welded longitudinal stiffeners and welded attachments at diaphragms inside tension members,” the problem statement says.
The project used strain gages at key locations on the main trusses and floor truss of the bridge to measure live-load stress ranges. Trucks with known axle weights were made to cross the bridge under “normal traffic.” Two- and three-dimensional models of the bridge were used to calculate stress ranges throughout the deck truss.
The report states the bridge’s deck truss did carry “many poor fatigue details on the main truss and floor truss system,” but research ultimately concludes fatigue cracking did not at the time require MnDOT to replace the bridge. The report says that as of 2001 fatigue cracking of the deck truss is “not likely” and “the bridge should not have any problems with fatigue cracking in the foreseeable future.”
Design analysis based on design-live-load stress ranges in the truss members shows stress ranges of up to 138 MPa, in tests where trucks passed from outside spans to the center span. But the actual stress ranges, calculated with strain gages monitoring strains under “both a known load and open traffic,” are calculated in the report as much lower than design-live-load stress ranges. This is mainly because the situation of closely spaced trucks in each lane, as the situation was in the design-live-load testing, rarely occurs on Bridge 9340 – after a year of close observation the authors never witnessed that condition. “Consequently, the fatigue life is far longer than would be predicted based on the design-live-load stress ranges,” the report says. The report later took into account daily traffic growth rates and predicted truck volume over the life of the bridge. Daily growth rates for urban interstates are observed in the report as 4.98 percent.
One major concern the report raises is that fatigue cracking in the deck truss is heightened by a “lack of redundancy in the main truss system.” The report finds that only two planes of the main trusses support the total eight lanes of traffic. The joints are “theoretically pinned” – so, the report states, if one member were severed by a fatigue crack, that plane would collapse – and if not collapse, then “require prolonged closure of the bridge and a major disruption.”
The report states its research holds implications for bridges other than the I-35 bridge. “The research verified that the behavior of this type of bridge can be deduced with a modest number of strain gages at key locations combined with detailed analyses,” says the report. “Fatigue rating [for all bridges] should be based on service-load-level analyses conducted according to these guidelines.”
Read the University of Minnesota’s fatigue evaluation report for detailed descriptions of all tests conducted on the I-35 bridge, explanations of the fatigue sensitivity of different bridge types and images and diagrams of proper gage locations on the I-35 bridge trusses.
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