Harmonizing Seismic Performance via Risk Targeted Spectra: State of the art, dependencies, and implementation proposals

Abstract

Design spectra are commonly defined for a constant value of hazard associated to an “ultimate” limit state (LS) of reference, typically 10% in 50 years. Given a specific structure, this approach results in different limit‐state exceedance risk levels even for sites characterized by the same design peak ground acceleration (PGA), mainly because of differences in hazard curve shape/slope. Instead, Risk Targeted design maps (first applied in ASCE7‐10) suggest the application of suitable spectra adjustment factors, the so‐called risk coefficients, in order to ensure a uniform collapse risk across different sites and buildings. Making use of simplified single‐degree‐of‐freedom (SDoF) structures for several configurations of vibration period and ductility, we test the effectiveness of the adjustment factors computed under different assumptions in matching a specific target risk or, at least, in harmonizing the risk of multiple buildings at different sites with respect to different LSs. Although risk matching is shown to be only theoretically possible and unachievable in practice, we claim that harmonization remains a viable target, and we offer insights for possible future adoption of Risk Targeted Spectra in the European code provisions.

Some issues in the practical application of risk‐targeted ground motions

“…It is worth pointing out that, for this comparison, both the risk‐targeted and uniform hazard spectra have shapes directly derived from the calculations, courtesy of the ground motion model(s) therein, without any smoothing adjustments for cosmetic or practical purposes. This was a conscious choice, since other works have shown that the way such spectral shape modifications are implemented has far from negligible effects 13 which are not examined here. For all three sites, the assumptions made about the target reliability against failure, have produced higher risk‐targeted spectral ordinates than the corresponding 475 years return period uniform hazard spectra, across all vibration periods ( T ).…”

“…10 This entails choosing an appropriate target rate of failure (often taken as interchangeable with the annual failure probability) to meet said objective, 𝜆 * 𝑓 , and making some a-priori assumptions about the corresponding structural fragility, that is, the conditional probability of failure, given the level of shaking intensity. 11,12 Despite the apparent simplicity of linking the seismic design actions to a risk objective, in practice this approach of deriving, so-called, risk-targeted ground motion (RTGM), presents some issues 13 that this study explores by implementing the original proposal for their derivation by Luco and co-authors, 14 for seven Italian sites. Risk-targeted and uniformhazard elastic design spectra are calculated for these sites and the available results of a recent research project are used to simulate the design of low-and mid-rise reinforced concrete (RC) moment-resisting frames at each site.…”

“…Despite the apparent simplicity of linking the seismic design actions to a risk objective, in practice this approach of deriving, so‐called, risk‐targeted ground motion (RTGM), presents some issues 13 that this study explores by implementing the original proposal for their derivation by Luco and co‐authors, 14 for seven Italian sites. Risk‐targeted and uniform‐hazard elastic design spectra are calculated for these sites and the available results of a recent research project are used to simulate the design of low‐ and mid‐rise reinforced concrete (RC) moment‐resisting frames at each site.…”

Risk‐targeted seismic design: Prospects, applications, and open issues, for the next generation of building codes

“…20 The paper by Suzuki et al, 21 provides opinions on the forces opposing seismic design paradigm shifts in Japan, which is certainly interesting beyond Asia, and is likely applicable to all countries of advanced technical culture. Finally, the work of Spillatura et al 22 provides a comprehensive overview of risk-targeted ground motion (RTGM). RTGM was the subject of several submissions, showing that this is one of the prominent approaches towards implementing risk-targeted design since the seminal work of Luco et al 23 (Nevertheless, some limitations, which seem not to completely solve the issues with the current approach to seismic design, and some assumptions-mainly on the fragility of the designed structure-may still require developments.…”