Does amplitude scaling of ground motion records result in biased nonlinear structural drift responses?

Abstract

Limitations of the existing earthquake ground motion database lead to scaling of records to obtain seismograms consistent with a ground motion target for structural design and evaluation. In the engineering seismology community, acceptable limits for 'legitimate' scaling vary from one (no scaling allowed) to 10 or more. The concerns expressed by detractors of scaling are mostly based on the knowledge of, for example, differences in ground motion characteristics for different earthquake magnitude-distance (M w -R close ) scenarios, and much less on their effects on structures. At the other end of the spectrum, proponents have demonstrated that scaling is not only legitimate but also useful for assessing structural response statistics for M w -R close scenarios. Their studies, however, have not investigated more recent purposes of scaling and have not always drawn conclusions for a wide spectrum of structural vibration periods and strengths. This article investigates whether scaling of records randomly selected from an M w -R close bin (or range) to a target fundamental-mode spectral acceleration (S a ) level introduces bias in the expected nonlinear structural drift response of both single-degree-of-freedom oscillators and one multi-degree-of-freedom building. The bias is quantified relative to unscaled records from the target M w -R close bin that are 'naturally' at the target S a level. We consider scaling of records from the target M w -R close bin and from other M w -R close bins. The results demonstrate that scaling can indeed introduce a bias that, for the most part, can be explained by differences between the elastic response spectra of the scaled versus unscaled records.(7) Regress (in log-log space) the resulting ratios of scaled over unscaled nonlinear structural responses on the corresponding scale factors. AMPLITUDE SCALING OF GROUND MOTION RECORDS 1825Depending on the vibration period (T ) and strength (R) of the SDOF structure, these results demonstrate that intra-bin scaling records up can result in nonlinear structural responses (here S I d ) that are biased high, whereas the converse is true for scaling down. The magnitude of the bias for a given scale factor is smaller for longer-period structures and for stronger (closer to elastic) structures; it also depends on the characteristics (e.g. M w and R close ) of the records that are scaled. These results are summarized further in the conclusions section. SDOF structures-inter-bin scalingAlternatively, one could either 'correct' for a scaling-induced bias by using results like those presented in this paper, or select records with spectra that are, once scaled, similar to that of the target S a , M w , and R close . Such a selection of the records to be scaled has been demonstrated to significantly reduce the potential for biased responses.

Prediction of spatially distributed seismic demands in specific structures: Ground motion and structural response

“…scaling amplitude without modifying frequency content or duration) ground motion records to a specific IM level has been scrutinized for introducing bias compared with some 'true' response that would be obtained using un-scaled ground motions. Luco and Bazzurro [11] have illustrated that when using S de as an IM, scale factors that are significantly different from 1.0 can introduce significant bias in seismic response analysis. Baker [37] has shown however that careful ground motion selection (similar to what is used here) can significantly reduce such bias.…”

“…With the increased interest in ground motion selection methods (e.g. [10]), scaling robustness seeks to determine if the distribution of EDP using scaled ground motions is biased compared with that obtained using un-scaled ground motions [11]. Optimal intensity measures for total floor accelerations have received less attention than that of peak interstorey drifts, with the exception of Taghavi and Miranda [12] who examined the efficiency of four different IMs at predicting peak floor accelerations using simple elastic structural models.…”

Yield frequency spectra and seismic design of code‐compatible RC structures: an illustrative example

“…This is the case here due to considerable cracking of the initially unstressed fiber sections. Still, the spectral acceleration at any single period may not be fully sufficient IM for any MDOF structure subjected to high levels of inelastic deformation [35]. Nevertheless, this is not the case here, as the DL checking governs.…”

Ground Motion Selection and Scaling in Practice

“…But recent work suggests that in some other situations record scaling may induce some bias in structural response [39,40]. This bias appears to result from the scaled ground motions having inappropriate values of spectral shape or the parameter ε, which is an indirect measure of spectral shape [9,41].…”