Field Monitoring of Cross Frames in Composite Steel I-Girder Bridges

Many of the current fatigue design specifications for cross frames, especially those in composite steel bridge systems, have primarily been based on computational studies and component-level laboratory experiments. To fully understand the behavior of cross frames when subjected to in-service truck traffic and to critically evaluate legacy design provisions, in-service field monitoring of cross frames in various composite systems is important. Therefore, select cross frames in three bridges in the greater Houston area were instrumented with strain gage sensors—one straight bridge with normal supports, one straight bridge with skewed supports, and one horizontally curved bridge with radial supports. For each bridge, rainflow counting techniques and other postprocessing procedures were implemented for a monitoring period of one month to compare the measured cross-frame response. Among other key observations, the measured data demonstrated that load-induced force effects in the cross frames of the skewed bridge system exceeded those in the bridges with normal or radial supports and that cross-frame response was highly sensitive to the longitudinal and transverse truck positions. In all cases, though, the measured damage accumulated on all instrumented cross-frame members during their respective monitoring periods, if extrapolated to the entire service life, would likely not cause significant load-induced cracking in the critical welded gusset-to-member connections.