Calculation of displacements for base isolation systems with bilinear behaviour

The concept of seismic isolation is, beside the ductile design, state-of-the-art in the field of the seismic design of structures. In the case of seismic isolation, the relative displacement due to earthquake is an important effect, which must be calculated with an adequate reliability.

Since in isolation systems devices with nonlinear deformation characteristics are often used, the isolation system is usually to be considered with its nonlinear characteristics. This paper deals with the computation of the relative displacements for bilinear elastic-plastic isolation systems by means of equivalent linear computations using different linearization methods, the method ‘Geometric Stiffness’, recommended in Eurocode8, and the method ‘Average Period and Damping’.

In extensive parameter studies the relative displacements determined with these procedures will be compared with the accurate results, which were determined with nonlinear time history calculations. Based on the statistical evaluation of the differences the quality of results will be shown and rated. The conditions for using this procedure will be taken into account.