Length of critical region for confinement steel in limited ductility high-strength reinforced concrete columns

The length of the critical region is one of the essential parameters in designing earthquake resistant structures where adequate transverse steel needs to be provided to confine the concrete core and subsequently to avert brittle failure under large inelastic deformation/curvature. In countries having high seismic risk such as China, Japan, New Zealand, USA etc.; guidelines have been provided in their reinforced concrete (RC) design codes for the design of fully ductile RC columns/piers. Nevertheless, in countries having low to moderate seismic risks, RC columns/piers can be designed for limited ductility, the design guidelines for which are very scarce. From the experimental tests carried out by the authors on limited ductility high-strength RC columns, it was found that this type of column/pier can be confined effectively with less confinement steel over a shorter extent of critical region compared with the existing code provisions for ductile columns/piers. In this paper, the length of the critical region in which the properly enhanced amount of confinement steel should be installed is investigated by two methods, which are denoted as direct methods in this paper. The evaluated critical region lengths have also been compared with those obtained by other researchers and the New Zealand Code.