Effect of high axial load on seismic behavior of reinforced concrete beam-column joints with and without strengthening

An experimental program is described to investigate the effect of high axial load on seismic performance of non-seismically designed reinforced concrete beam-column joints with or without strengthening. Four two-thirds-scale interior joints, including two control specimens and two specimens strengthened by ferrocement jackets with embedded diagonal reinforcements, are tested. The specimens are subjected to two levels of axial load representing low and high axial load ratios at 0.2 and 0.6, respectively. Test results indicate that increasing axial load to 0.6fc'Ag is detrimental for the joint with and without strengthening. Joints under low axial load exhibit higher peak strength, higher drift ratio, and better energy dissipation compared to those under high axial load. Despite joints under high axial load having relatively higher initial stiffness, joint stiffness degrades rapidly. The proposed strengthening method is more effective for the joint under high axial load. Furthermore, a modification on softened strut-and-tie model (SSTM) is proposed to account for the influence of high axial load on joint shear strength. The comparison of predicted strength to tested strength indicates that the proposed modification increases the accuracy of SSTM for joints under high axial load.