Seismic Behavior of Strengthened RC Columns under Combined Loadings

Twenty-five reinforced concrete (RC) columns of section size 250 × 250 mm were designed and tested to study the seismic response considering the effect of loading case, strengthening method, and the predamage level, containing 21 columns reinforced with high-performance ferrocement laminate (HPFL)-bonded steel plates (BSPs), i.e., the intact strengthened columns (ISCs), earthquake-damaged strengthened columns (EDSCs), corrosion-damaged strengthened columns (CSCs) and coupled-predamaged strengthened columns (CPSCs). The bearing capacity of the specimens under the four different types of loading methods is ranked as follows: uniaxial compression–bending–shear (CBS) members, biaxial CBS members, biaxial CBS-torsion (CBST) members, and uniaxial CBST members. Compared with nonstrengthened specimens, the cracks of the strengthened RC columns are more fully developed, and the failure modes have been changed after strengthening. The failure modes and load–deformation curves had little significant difference for the strengthened RC columns with different damage under combined loading levels. The bearing capacity of strengthened RC columns with the applied loading of 400 kN improved, which increased to 60.1%–114.7%, 29.9%–103%, 65.2%–127%, and 49.2%–104.5% for ISCs, EDSCs, CSCs, and CPSCs, respectively. Moreover, the bearing capacity of specimens decreased due to the existence of horizontal eccentricity. Finally, based on the degraded trilinear restoring force model, the strengthened influence coefficient for loading α and displacement β and the torsion influence coefficient for loading γ and displacement ξ were introduced. A modified restoring force model of RC columns was presented, reflecting the loading method, predamage level, and strengthening method. The theoretical calculation values align with the test load–deformation curves, and the mean absolute error is almost less than 15%.