Riveted steel truss and tied arch bridges in Washington state have experienced significant cracking that is due to fatigue loading. The Lewis River and Toutle River bridges are good examples of these two bridge types. They are both on the southbound lanes of Interstate 5 and experience heavy truck traffic. This research study addressed the fatigue cracking on these two bridges.
The cracking on these two bridges was summarized and analyzed. Computer models of the two bridges were developed, and static and dynamic analyses of the bridges were performed. Instrumentation was installed on both bridges in 1996. Controlled load and weight station tests were performed on the two bridges with trucks of known weight and geometry traveling at known speeds. The results of these measurements were used to evaluate the overall behavior of the two bridges and to calibrate the instruments for further testing with trucks of unknown weight and geometry.
Upon completion of the calibration tests, uncontrolled truck traffic was measured for the two bridges over 3 to 4 weeks. Extensive data were obtained for the two bridges, and load spectra were developed. These were combined with historic truck traffic data and predictions for future traffic in the I-5 corridor, and fatigue was estimated for critical components of the two bridges.
The Toutle River bridge was found to be very sensitive to dynamic vibration under truck loading, and fatigue cracking noted in the floorbeams of this bridge was found to be deformation driven. There is also a potential for stress driven fatigue cracking in the tie chord in coming years.
The Lewis River bridge was found to have serious fatigue cracking potential in the copes of the stringers for the deck system. There is less serious potential for fatigue cracking at the cover plate terminations of the floorbeams. There appears to be little potential for fatigue cracking in the truss members, unless there is an extreme nonuniform distribution of stress through the built-up members.
The ramifications of the fatigue cracking and possible repairs and modifications were evaluated.