A deck truss bridge consists of two simple spans of 218 feet each. The lower lateral bracingsystem was found to exhibit excessive vibration under truck crossings. Fatigue cracks werefound at the gusset plate connecting the lower lateral bracing diagonal to the truss bottom chord.In-service monitoring was performed to measure the vibration properties of the truss and thelateral bracing members. The measurement results were correlated with the calculated naturalfrequencies of the structural system and the excitation frequency of crossing vehicles. It wasdetermined that the vehicle passages between floor beams correspond to observed vibrationfrequency of the lower lateral members. Based on results of in-service monitoring and finiteelement analysis, a retrofit was developed to increase the natural frequency of the lower lateralsystem to avoid resonance with the excitation frequency.
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