Cumulative seismic damage of circular bridge columns: Variable amplitude tests

The findings from an experimental study to investigate cumulative seismic d amage in reinforced concrete circular bridge piers is presented. Twelve ide ntical quarter-scale bridge columns, designed in accordance with current AA SHTO specifications, were fabricated and tested to failure. Results from Ph ase I testing, which included constant amplitude tests to determine the low -cycle fatigue characteristics of the bridge column, were presented in a co mpanion paper. This paper summarizes results of variable amplitude testing that focused on the effects of load pth on cumulative damage. The imposed d isplacement histories were obtained form analytical simulations of the mode l column subjected to a sequence of earthquakes of varying duration and mag nitude. Test observations indicate that failure is generally initiated by c onfinement inadequacy and the rupture of the transverse spiral reinforcemen t. The tests also demonstrated the potential for low-cycle fatigue fracture of the main longitudinal steel when the specimen was subjected to relative ly larger displacement amplitudes, typically in excess of 4 percent lateral drift. A fatigue-based damage model, derived from the constant-amplitude t ests completed in Phase I testing, was applied to the observed response of the six specimens tested in this phase. Findings from the study indicate th at the energy capacity of members is ductility-dependent and that fatigue-b ased damage models offer a reliable means of assessing seismic structural p erformance.