Towards a new base shear format in future NBCC seismic provisions

Abstract

Recent trends in the specifications of earthquake design forces are moving away from spectral amplification of ground motion towards direct evaluation of spectral values. Also, the 1988 Saguenay earthquake produced strong evidence for the need to modify the present provisions for low period structures in eastern North America. A new database consisting of 75 time histories was assembled and an extensive parametric study was undertaken to prepare new seismic response factors (S curves) for the National Building Code of Canada (NBCC). These time histories were divided into five categories (15 time histories each) on the basis of their frequency content as measured by their peak ground acceleration to peak ground velocity ratios, ranging from very high (=3) to very low (=0.5). Mean and mean + one standard deviation response spectra for one-degree-of-freedom and multi-degree-of-freedom systems were computed for the five categories using several normalizing (scaling) rules for the time histories. The results were compared and spectral amplification factors are presented. Spectra based on peak spectral velocity scaling were compared with spectra based on the present peak ground velocity scaling approach, and some differences are noted. Also, comparisons with the design spectra given in the 1991 edition of the National Earthquake Hazard Reduction Program provisions are presented. On the basis of this study, new S curves are proposed for consideration in the development of future editions of the NBCC.

Insights and challenges associated with determining seismic design forces in a loading code

“…It is therefore instructive to relate spectral shapes to A/V ratios. This relationship was studied by Heidebrecht et al [ 1994] for the previously mentioned ensembles of strong ground motion records [Naumoski et al 1993]. Based on a regression analysis, this study found that the ratio of peak spectral acceleration (SAm) to peak spectral velocity (SVm) equals 1.20 A/V.…”

Compatible ground-motion time histories for new national seismic hazard maps

“…Ground-motion relations can be given in terms of peak ground acceleration or velocity, or for response spectral ordinates for given periods of vibration. Within the last 10 years, practice has shifted to the use of response spectra instead of peak ground-motion parameters, due to their greater engineering relevance (EPRI 1986;EERI 1989;Atkinson 1991;Heidebrecht et al 1994). Thus the GSC maps are based on response spectral ordinates.…”