Minimum flexural ductility design of high-strength concrete columns

Abstract

High-strength concrete (HSC) has been widely used in the column construction of tall buildings because of its larger stiffness and strength-to-weight ratio. The current design of HSC columns emphasises the strength design but pays little attention to the ductility design. From structural safety point of view, however, it is important to provide a minimum level of ductility to all structures even they are not subjected to earthquake attack. Currently, this minimum level of ductility in concrete columns is provided by imposing empirical deemed-to-satisfy rules, which limit the minimum size and maximum spacing of the confinement. However, these rules, which are concrete strength independent, have the shortcoming that the ductility level provided Is not consistent - generally lower at higher concrete strength or higher axial load level. To overcome such shortcoming, an extensive parametric study based on nonlinear moment-curvature analysis that investigates the combined effects of concrete strength, axial load level, confining pressure and longitudinal steel ratio on the ductility of concrete columns is conducted herein. Based on the results, a new design inequality and chart are developed, which ensures that a consistent level of ductility could be provided to all concrete columns by limiting the maximum axial load level and minimum confinement.