A comparative study of design seismic hazard in major national codes

The codes, invariably, define the design seismic hazard in terms of an idealized acceleration response spectra, which is scaled to the desired hazard level either using PGA or spectral ordinates corresponding to different period (usually at 0.2 sec and 1 sec). Design hazard levels are usually characterised by a return period or exceedence rate, and are associated with desired performance levels. Some of the codes are very clear about the return periods considered for design, but all the codes are vague about the intended performance objectives. Another important issue associated with design hazard is the effect of local soil strata. Currently, the codes handle this issue by classifying sites into different classes and proposing site (amplification) factors to scale the design spectra. Lately, displacement-based seismic design is becoming popular, for earthquake resistant design of structures. Design displacement spectrum is the key input in displacement based design, and applicability of displacement spectrum derived from code design spectra needs to be evaluated. This paper presents a comparative study of the design hazard levels, site classifications, and corresponding design response spectra specified by four major codes, viz. ASCE7, Eurocode 8, NZS 1170.5 and IS 1893. The design displacement spectra based on code spectra also compared with the empirically obtained displacement spectra using Next Generation Attenuation relationship