Seismic design for torsional response of ductile buildings

Abstract

A simple approach to the consideration of torsional effects in the seismic design of ductile building structures is postulated. Instead of increasing torsional strength, the control of twist, which may amplify local inelastic translational deformations, is emphasised. This may be achieved by assuring that some residual torsional stiffness of the system is available. A number of examples illustrate the simple principles involved. It is demonstrated that traditional codified techniques, based on the evaluation of torsional effects in elastic systems, are largely irrelevant to ductile structural response. It is recommended that, in accord with the philosophy of capacity design, torsional response should be controlled by elastic elements, while hysteretic damping is derived from inelastic translational mechanisms. As a general rule critical conditions for torsional response of well proportioned structures are likely to arise only when inelastic displacements are imposed in a narrow band of directions diagonal to the principal reference axes of orthogonal systems. It is suggested that in the presence of significant eccentricities the displacement ductility demand on the system should be reduced. Thereby elements with defined ductility capacity can be protected against excessive imposed inelastic deformations. The presentation addresses primarily concepts of torsional behaviour rather than advancement in analytical techniques.