Atilla ANSAL, Civil Engineering Dep., Ozyegin University, Istanbul, Turkey

PERFORMANCE BASED SITE RESPONSE ANALYSIS AND MICROZONATION
One of the controlling issues in the performance based design involves the definition of performance objectives. In the case of site response analysis, the performance objectives may be considered as the probabilistic definition of the uniform acceleration hazard spectrum and acceleration time histories calculated on the ground surface. Thus, site-specific response analyses need to produce a probabilistic uniform hazard acceleration response spectrum on the ground surface. A performance based approach thus requires a probabilistic estimation of soil stratification and engineering properties of encountered soil layers in the soil profile. The major uncertainties in site-specific response analysis arise from the variabilities of (a) local seismic hazard assessment, (b) selection and scaling of the hazard compatible input earthquake time histories, (c) soil stratification and engineering properties of encountered soil and rock layers, and (d) method of site response analysis. One option may be conducting site response analyses for large number of soil profiles produced by Monte Carlo simulations for the investigated site to assess probabilistic performance based design acceleration spectra and acceleration time histories calculated on the ground surface.

The purpose of seismic microzonation is to estimate probable earthquake characteristics on the ground surface to mitigate earthquake damage in the foreseeable future for the new buildings as well as for the existing building stock. At this stage, the probability distribution of the related earthquake parameters on the ground surface may be determined considering all possible input acceleration time histories, site profiles, and dynamic soil properties.  A seismic microzonation methodology is proposed based on the probabilistic assessment of the factors involved in site response analysis. The second important issue in seismic microzonation procedure is the selection of microzonation parameters. The purpose being mitigation of structural damage, it is possible to adopt earthquake parameters like cumulative average velocity (CAV) or Housner intensity (HI) that was observed to have better correlation with building damage after earthquakes.