Seismic load tests on two concrete interior beam-column joints reinforced by plain round bars designed to pre-1970s codes

Abstract

Two identical reinforced concrete interior beam-column joint assemblies were tested under simulated seismic loading as part of an investigation of the behaviour of existing reinforced concrete structures designed to pre-1970s codes when subjected to severe earthquake forces. The test units were full-scale in size, and were replicas of part of the moment resisting frame of an existing building in Christchurch that was constructed in the 1950s. The longitudinal and transverse reinforcement were from plain round bars, the beams and columns had low quantities of transverse reinforcement and the joints had no shear reinforcement at all, as was typical of pre-1970s construction in New Zealand. One unit was tested with zero axial column load, and the other unit with a constant axial column load of 0.12 f’c Ag, where f’c = concrete cylinder compressive strength and Ag = the gross column section area. The units displayed very low structural stiffness, flexural strength less than conventionally calculated, and significant degradation of stiffness and strength during testing. The low structural stiffness and flexural strength could be attributed to the slip of the plain round longitudinal bars through the joint. However, the utilisation of plain round bars although leading to bond slip was found to improve the joint shear strength and to result in reduced shear distortion of the joint. Buckling of longitudinal column bars was found to initiate failure, due to inadequate ties.