Effect of soil type on seismic demand
This paper investigates the validity of the soil considerations used in the determination of seismic demand as part of NZS1170.5, which currently specifies seismic design spectra corresponding to 5 different soil types. According to the current provisions stipulated in NZS1170.5, for all natural periods, the building demand for soft soil is either equal to or greater than that for hard soil. It is noted that 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 1-D nonlinear site response analysis 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 column (represented by its shear wave velocity, Vs) are the main parameters affecting the surface response. 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 soil types 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 seismic structural demand is proposed and demonstrated.