Drift limits of high-strength concrete columns subjected to load reversals

A series of experiments was carried out to investigate the behavior of columns with high-strength concrete subjected to shear reversals into the nonlinear range of response. The main experimental variables were loading history, axial load, and concrete strength. Columns measured 0.20 x 0.20 m (8 x 8 in.) in cross section and had shear span to depth ratios of 3.4. The transverse reinforcement ratio was 1%. Column behavior was closely related to the maximum strain that was measured in the stirrups during each loading cycle, which was affected by the displacement history and axial load applied to the specimens. The level of axial load had an increasingly adverse effect on the limiting drift of specimens subjected to axial loads larger than one half the balanced load, while the opposite effect was observed in specimens with lower values of axial load. Despite its apparent brittleness under axial compression, high-strength concrete was found to improve the toughness of reinforced concrete columns and extended the drift range within a column would not require repairs after an earthquake. In specimens made with high-strength concrete, the amount of reinforcement dictated by ACI 318-99 in 1999 provided sufficient confinement to obtain limiting drift ratios exceeding 4%.