Seismic design spectra for different soil classes

Abstract

This paper investigates the validity of the soil class dependent spectral shape factors used to calculate seismic design actions in the New Zealand seismic design standard NZS1170.5, which currently specifies seismic design spectra corresponding to five different soil classes. According to the current provisions stipulated in NZS1170.5, for all natural periods, the seismic demand for structures on soft soil is either equal to or greater than that for structures on hard soil. This is opposite to the basic structural dynamics theory which suggests that an increase in stiffness of a system results in an increase in the acceleration response. In this pretext, a numerical parametric study is undertaken using a nonlinear site response analysis tool in order to capture the effect of soil characteristics on structural seismic demand and to scrutinize the validity of the current site specific seismic design spectra. It is identified that the level of input ground motion intensity and shear stiffness of the soil deposit (represented by its shear wave velocity Vs) greatly affect the maximum acceleration and frequency content of the surface motion. The study found some shortfalls in the way the current code defines seismic design demand, in particular the hierarchy of soil stiffness at low structural periods. It was found that stiff soils generally tend to have a higher spectral acceleration response in comparison to soft soils although this trend is less prominent for high intensity bed rock motions. It was also found that for medium to hard soils the spectral acceleration response at short period is grossly underestimated by the current NZS1170.5 provisions. Based on the outcomes of the parametric numerical analyses, a revised strategy to determine structural seismic demand for different soil classes is proposed and its application is demonstrated through an example.