Seismic behavior of FRP-retrofitted rectangular RC columns subjected to oblique horizontal earthquake actions

Externally wrapped fiber-reinforced polymer (FRP) confinement has proven effective in improving the seismic performance of reinforced concrete (RC) columns. However, existing seismic design recommendations are primarily based on quasi-static cyclic test results applied along the principal axis of the column section, which cannot accurately reflect the true characteristics of earthquakes. In this study, two 1/2-scale FRP-retrofitted rectangular RC columns were subjected to shaking table tests with ground motion input directions at 30° and 60° to the strong axis of the cross-section, respectively. The seismic performance, including damage mode, natural frequency, acceleration, displacement and strain response, torsional effect, and energy dissipation, was thoroughly analyzed. The result found that FRP-retrofitted rectangular RC columns exhibited completely different damage modes under shaking table tests compared to quasi-static loading, from previous ductile flexural failure to unfavorable flexural-shear-torsional failure, with more and wider cracks, more significant deformation, and more severe damages. These findings suggest that conclusions drawn from quasi-static tests tend to overestimate the retrofitting effect of FRP, and retrofitting only the plastic hinge zone of RC columns is insufficient to ensure structural safety, which should be considered in future seismic strengthening design criteria for RC structures.