Seismic response of flexible-edge elements in code-designed torsionally unbalanced structures

Abstract

This study investigates the adequacy of static code torsional provisions in controlling the ductility and deformation demands in the flexible-edge element of structures with asymmetric stiffness distributions (termed torsionally unbalanced), excited into inelastic behaviour by intense earthquakes. The model configurations are designed strictly according to seismic building standards applied in the US, New Zealand, Canada, Australia and Europe. Significant additional ductility demand is found to arise in short-period systems designed according to the US and New Zealand provisions. The remaining codes, which amplify the static eccentricity to account for dynamic torsional effects, adequately control the additional ductility demand. However, the Australian and European codes may be considered over-conservative with respect to this response parameter. The deformation demand for the flexible-edge element increases rapidly with the magnitude of the static eccentricity, and for the US, New Zealand and Canadian codes may reach values as high as five or six times that of corresponding torsionally balanced systems. The Australian and European provisions control the flexible-edge element deformation to levels similar to that of the torsionally balanced system for moderate levels of eccentricity, but exhibit significant additional deformation demand for highly eccentric systems. For increasing values of the force reduction factor, code provisions give improved control of both the additional ductility and deformation demands, but both these response parameters are relatively insensitive to variations in the building's plan aspect ratio above the value 2 : 1.