Minimum Shear Reinforcement for Columns with High-Strength Reinforcement and Concrete

Large-scale reinforced concrete columns with high-strength reinforcement and concrete were tested. The nominal compressive strength of concrete was 70 MPa. The nominal yield strengths of longitudinal and shear reinforcement were 690 and 790 MPa, respectively. The columns were subjected to double-curvature lateral cyclic loading under constant axial compression. Test variables included the amount of shear reinforcement and level of axial compression. Test results showed that all the nine columns tested failed in shear and showed a successful redistribution of internal forces at diagonal cracking. The stress of shear reinforcement at the peak load increased with an increasing amount of shear reinforcement. Three of four columns with the highest amount of shear reinforcement (0.56%) showed yielding of shear reinforcement at the peak load. By comparing with the test results of 86 high-strength columns from this research and the literature, the minimum shear reinforcement equation of the ACI 318 code was found not able to prevent failure at diagonal cracking and failed to provide a clear trend between the reserve shear strength and the amount of shear reinforcement. A minimum shear reinforcement equation is thus proposed. The equation is based on the Vc equation of the ACI 318 code and can consider the effect of axial compression. Comparing 86 columns shows that the columns that failed at diagonal cracking do not satisfy the proposed equation. Moreover, the proposed equation can provide a clear trend between the reserve shear strength and the amount of shear reinforcement.