An evaluation of the New Zealand code torsional provision

Abstract

This study evaluates the New Zealand torsional provision in the 1984 and 1992 editions of the New Zealand loading code (NZS 4203) based on the inelastic responses of a single mass model having lateral load resisting elements in two orthogonal directions and subjected to bidirectional base excitations. It is shown that for systems having torsional stiffnesses that exceed a minimum value, the provision in NZS 4203:1984 will restrict the ductility demands on the resisting elements no more than those of a similar but torsionally balanced system. This minimum torsional stiffness depends on the structural eccentricity of the system. For systems with torsional stiffness less than the minimum, the stiff edge elements can experience additional ductility demand because the 1984 edition of the Code permits excessive strength reduction on the stiff edge elements. In the 1992 edition, the Code imposes a minimum torsional stiffness of a structure in the farm of edge displacement ratios. With this new requirement, the danger of additional ductility demand on the stiff edge element is eliminated. Therefore, the torsional provision in the current edition will ensure no additional ductility demands on all lateral force elements caused by torsion.