Experimental Study on Minimum Depth of Interior Joints for Special Moment Frames with High-Strength Reinforcement and Concrete

Abstract

ACI 318-19 permits the use of Grade 690 bars for primary reinforcement of special structural walls, but not for special moment frames because of insufficient experimental evidence of frame joints. Where Grade 690 bars are used for longitudinal reinforcement, the bond and anchorage at beam-column joints become crucial in the design of special moment frames. Due to paucity of experimental evidence, ACI 318 set a minimum joint depth that is proportional to bar diameter and grade without accounting for effects of high-strength concrete and other parameters. In practice, higher-grade reinforcement may be used together with high-strength concrete, particularly for columns with limited architectural dimensions and high axial load at the lower levels of high-rise buildings. Therefore, the authors designed and conducted an experimental program of four interior beam-column joints reinforced with Grade 420 or 690 bars to investigate the beneficial effect of concrete strength on the bond of beam longitudinal bars passing through an interior joint. Cyclic test results show that the minimum joint depth could be reduced with the use of high-strength concrete for Grade 690 bars.