Collapse strength ratios for structures under mainshock-aftershock subduction seismic sequences

The effects of aftershocks on civil structures have called the attention of the earthquake engineering community mainly in the last two decades. However, neither seismic design codes nor seismic assessment guidelines for existing structures worldwide consider the possible detrimental effects of strong aftershocks nowadays. From this aim, this paper presents the results of a statistical study focused on computing the relative lateral strength to avoid collapse, named lateral strength ratio Rc, of degrading (i.e., having stiffness and in-cycle strength degra-dation) single-degree-of-freedom, SDOF, systems under mainshock-aftershock seismic sequences. For this pur-pose, the degrading SDOF systems have a trilinear force-displacement backbone mainly defined by their displacement ductility capacity, & mu;c, and the negative slope after peak strength, & alpha;c. Therefore, median Rc ratios were computed for a site-specific mainshock-aftershock scenario at rock sites considering as-recorded and arti-ficial seismic sequences. From the results of this investigation, it is shown that median Rc ratios that take into account strong aftershocks are smaller than those computed for only the mainshocks, which means that the yield strength coefficient, Cy, for structures subjected to mainshock-strong aftershock pairs should be greater than that for structures under only the mainshocks. The ordinates of median Rc ratios depend on the site class, period of vibration, T, as well as & mu;c and & alpha;c. A predictive equation for obtaining estimates of period-dependent Rc ordinates under mainshock and mainshock-aftershock sequences at rock sites is introduced whose fitted coefficients depend on & mu;c and & alpha;c. The predictive equation of Rc allows obtaining estimates of the yield strength coefficient required to avoid collapse, Cyc, under mainshock-aftershock sequences for the design of new structures.