Seismic behavior of square spiral-confined high-strength concrete-filled steel tube columns under high axial load ratio

Abstract

Adding internal spiral reinforcement is an effective and practical approach to enhancing the ductility of square concrete-filled steel tube (CFST) columns with high-strength concrete (HSC). To investigate the seismic behavior of CFST and spiral-confined concrete-filled steel tube (SCCFST) columns with HSC under high axial load ratio, four CFST columns and four SCCFST columns with concrete strengths not less than 110 MPa were tested under combined axial and lateral loading. The test variables included the concrete compressive strength, the axial load ratio, and the lateral-loading protocol. The test results demonstrated that adding spiral reinforcement can offset the reduction of ductility caused by the increase of the axial load ratio. The ultimate drift ratios of the columns under an axial load ratio of 0.5 increased from approximately 2.0% to 3.5% after adding spiral reinforcement with a volumetric ratio of 3.13%. Increasing the concrete strength from 110 to 140 MPa essentially did not affect the deformation capacity of the columns. Due to the cyclic degradation effect, the deformation capacity of the specimen under cyclic loading was much smaller than that of the corresponding specimen under monotonic loading. The column axial shortening accumulated during cyclic lateral loading and increased exponentially when significant strength degradation started. The rotation at the column base contributed a significant portion to the total lateral displacement, indicating the necessity of considering the flexibility of the column base in structural analysis.