Proposal of horizontal elastic response spectra for low and high seismicity regions towards the revision of Algerian seismic code

Abstract

The elastic response spectra are fundamental engineering tools provided by seismic codes for performing structural seismic analysis and seismic risk reduction. This work is performed in the framework of the ongoing revision of the current Algerian seismic code RPA99/2003, which provides elastic spectra whose reproduction performance of real seismic actions, is called into question by recent earthquakes. Algerian seismicity is characterized by frequent moderate earthquakes, as well as rare severe earthquakes such as those of Boumerdes (Mw = 6.8 on May 21, 2003) and El Asnam (Ms = 7.3 on October 10, 1981). The compilation of a large accelerometric database from Algeria and the surrounding areas has recently made it possible to develop new horizontal elastic acceleration response spectra and soil amplification factors for four different soil classes— very soft, soft, firm, and rock—as well as for two seismicity levels—weak to moderate seismicity (wms) for the central and southern regions and moderate to high seismicity (mhs) for the northern region. 153 earthquakes are used, with a total of 773 3-component records from events with magnitudes ranging from 3.0 to 7.4 and hypocentral distances less than 200 Km. A statistical approach is used which leads to the estimation of new constant spectral acceleration branch limits and soil factors for the proposed horizontal elastic response spectra, for two seismicity levels and four soil classes. Soil factors for the two seismicity levels are estimated using the intensity of the Housner spectrum. Significant variations in elastic spectra developed for the two seismicity levels, wms and mhs, are highlighted. The first difference involves the branch plateau of the constant spectral acceleration, which swings toward the high periods in mhs but is dominated by the low periods in wms. The second difference is the spectrum amplitude related to site factors, which is less important in the case of mhs due to the effects of nonlinearities in soils subjected to strong seismic accelerations. Furthermore, comparing the proposed elastic acceleration spectra with the spectra provided by Pitilakis et al. (2018), developed in the framework of the ongoing revision of Eurocode 8, reveals comparable trends.