DYNAMICS APPROACH FOR MONITORING BRIDGE DETERIORATION

Abstract

In conjunction with a fatigue test of a full-scale in situ three-span highway bridge, an investigation was undertaken to evaluate the use of changes in dynamic properties of the bridge as a possible means of detecting structural deterioration due to fatigue cracks in the girders. Cyclic-loading tests (transient and steady-state) were conducted to determine the changes in dynamic properties. The loading was imposed by a moving-mass, closed-loop electro-hydraulic actuator system. Several different dynamic tests were employed in the investigation to determine the modal viscous damping ratios, stiffness, and mechanical impedance of the bridge at selected intervals during the fatigue loading. Acoustic emission sensors were also used to monitor the growth of fatigue cracks in the girders. The results show that changes in the bridge stiffness and vibration signatures in the form of mechanical-impedance plots are indicators of structural deterioration caused by fatigue. Stiffness coefficients were calculated from the experimental mode shapes on the basis of a multi-degree-of-freedom system that uses modified coupling. The average reduction in stiffness was approximately 20 percent. This reduction was attributed to the combined deterioration of the bridge deck and steel girders. Mechanical-impedance plots were made from frequency-sweep tests, which included five resonant modes. Early changes in the mechanical-impedance plots were related to the deterioration of the bridge deck. Subsequent changes in these plots correlated with the fatigue cracking in the steel girders. An evaluation of the acoustic emission data showed that the sensors were able to detect the rapid critical crack growth in one girder. (Authors)