Seismic behaviour and retrofit of pre-1970's as-built exterior beam-column joints reinforced by plain round bars

Abstract

Four exterior beam-column joints reinforced by plain round bars designed according to pre-1970's codes were subjected to simulated seismic loading. Each of the test units was full size in scale. The main test variables were the manner in which the longitudinal beam bars were hooked in the joint core and the level of the axial load applied to the columns. The amount of transverse reinforcement in the beams, columns and joint cores was very small, as was typical of the pre-1970's. The tests on the units demonstrated that the available stiffness and strength was low when the axial load was zero. The final failure occurred due to opening of the beam bar hooks in tension and column bar buckling, irrespective of the hook details of the beam bars. The presence of axial compressive load of 0.25fc′Ag on the columns delayed the failure initiated by the beam bar hooks, leading to much improved stiffness and strength of the units. In one unit with beam bar hooks bent away from the joint core, the column regions adjacent to the joint core were jacketed with fibre-glass and the unit when tested showed much improved stiffness and strength. When compared with the results of similar units reinforced by deformed bars, the units reinforced by plain round bars showed less joint shear distortion but more opening of beam bar hooks in tension and column bar buckling. As a result, premature concrete tension cracking failure along the outer layer of column bars adjacent to the beam bar hooks was enhanced, and the attained stiffness and the force strength were significantly lower, especially the stiffness.