Seismic Behaviour of Torsionally-Weak Buildings with and Without Base Isolators

Abstract

Seismic isolation is a powerful tool to reduce structural damage caused by large magnitude earthquakes. This paper investigates the extent to which seismic isolation may be used to reduce torsional response of very irregular, torsionally unbalanced buildings. Designed according to an old code (UBC-85), the three-story steel buildings analysed in this study were retrofitted using seismic isolators to meet the requirements of a modern code. The influence of seismic isolation on decreasing torsional effects is evaluated in the linear range by a parametric study. Then, nonlinear models are used to assess the effectiveness of seismic isolation. The analysis of the buildings is conducted using nonlinear bidirectional models that include strength loss. The main conclusions of this research are: (1) using similar isolators for all columns of a unsymmetric-plan building reduces, but not eliminates torsional behaviour of the structure leaving a residual torsion that gets amplified by higher mode effects and nonlinearities of the super-structure. (2) The flexural stiffness of the base beams (above the isolation system) has an important effect on the magnitude of story drifts of isolated buildings. (3) For unsymmetric-plan buildings, linear analyses can significantly underestimate story drifts when the super-structure goes into the nonlinear range.