Fatigue Assessment of Deck Truss of Bridge 9340 TH I35W over the Mississippi River

Project summary:

The deck truss of Bridge 9340 has plug welds, cover plates, and welded clips at diaphragms inside tension members among other poor fatigue details. In addition, there are only the two planes of the truss holding up eight lanes of traffic, so lack of redundancy is a major concern. A quick assessment of the details indicates that many of the details are Category D and E with threshold stress ranges of 7.0 and 5.6 ksi respectively. The analysis shows live load stress ranges in the truss members much higher than these thresholds. With 15,000 trucks per day crossing the bridge in each direction, these details should have already cracked if the stress ranges were really this high. Fortunately, when bridges are instrumented and the live-load stress ranges are measured, it is usually found that the ratio of measured stress ranges to calculated stress ranges is typically 0.5 or less. If, as suspected, this is the case with Bridge 9340, the remaining fatigue life may be much longer than anticipated, and might even be considered infinite. The bridge will be inspected and previous inspection records examined to document all poor fatigue strength details in members with fluctuating load at least partly in tension. Two tension chord members at the peak positive moment region of the middle span of the truss will be instrumented as well as a floor truss. In all, at least 24 strain gages will be installed and monitored during test runs with a truck of known weight as well as monitoring open traffic for several weeks during summer and another several weeks during the winter. A ratio of the measured to calculated stress will be estimated for the components instrumented. These ratios will be applied to the calculated stress ranges at other details, and a fatigue assessment of the worst details with the highest actual stress ranges will be performed.