Influence of prior shaking on seismic drift demands

Following the 2011 downtown Christchurch earthquake, 60% of buildings in Christchurch were demolished and reconstructed. Such practices are costly in terms of money and carbon emissions. Structures need to be proportioned so that they can be reused after one or several ground motions. To do so, it is important to understand the effect of previous shaking on drift demands. Past studies conducted to investigate the effect of previous shaking were based almost exclusively on numerical analyses that used a limited set of parameters. Conclusions from past numerical analyses do not lead to a clear consensus, and results from past studies have not been systematically vetted against measurements from structures tested in the laboratory or evidence from the field. To address these issues, the plausible effects of previous shaking on drift demands were re-evaluated using (1) numerical analyses of Single and Multi-Degree-of-Freedom oscillators with an expanded set of parameters and (2) measurements from 220 experimental tests of structures subjected to repeated simulated base motions. The compiled evidence supports at least two conclusions: (1) Repeats of medium or high-intensity motions strong enough to cause yielding in their first occurrence are unlikely to produce large relative increases in peak drift demands in structures with stable force–displacement relationships not susceptible to brittle failures. This phenomenon was observed in repeated motions not preceded by more intense motions, as well as in repeated motions preceded by more intense motions. The latter observation had not been reported in the literature to date. (2) Increases in peak drift demands were observed to be likely to occur if the first motion (in a sequence of motions containing a pair of repeated motions) is mild enough not to cause cracking and/or yielding, but the second instance of the same motion is preceded by larger intensity motions causing cracking and/or yielding.