Effects of high-strength reinforcing bars and concrete on seismic behavior of RC beam-column joints

Abstract

This paper reports an experimental investigation of the seismic performance of exterior beam-column joints reinforced with Grade 600 longitudinal reinforcing bars in beams and columns made with high strength concrete of 70 MPa. For this purpose, six half-scale reinforced concrete (RC) exterior connections were designed in accordance with the ACI 318-14 Code requirements for a special moment frame. The variables yield strength of longitudinal bars, concrete compressive strength, flexural reinforcement ratio, ratio of the column to beam flexural capacity, and longitudinal bar size were investigated by subjecting all the specimens to quasi-static reversed cyclic loading at drift ratios of up to 5.3%. In specimens with identical flexural strengths, the amounts of longitudinal reinforcing bars were reduced in beam and column by approximately 27% due to enhancement in their yield strength. Up to a drift ratio of 4.5%, all the specimens reinforced with high-strength steel (HSS) reinforcing bars exhibited an overall seismic behavior comparable to that observed for joints containing Grade 420 MPa bars regarding load-carrying capacity, failure mode, energy dissipation capacity, pinching width ratio, and secant stiffness. It was while all the specimens reached their theoretical load-carrying capacities. In the case of specimens with identical reinforcing bar ratio of Grade 600, the experimental evidence revealed that utilizing high-strength concrete (HSC) was able to improve the cumulative energy dissipation and pinching width ratio by a maximum of 30% and 26%, respectively, at a drift ratio of 4.5%; while HSC did not considerably affect secant stiffness and average peak load. A parametric study was also performed through finite-element analysis (FEA) to investigate the influence of different design parameters such as the grade and spacing of joint hoops, the ratio of the column-to-beam flexural capacity, and column depth-to-bar diameter, on the strength, stiffness, energy dissipation capacity and equivalent damping of the specimens.