Developing collapse fragility curves for base‐isolated buildings

Abstract

Summary Seismic isolation technique is increasingly used both for the design of new buildings and for the seismic retrofit of existing buildings. Nevertheless, so far, little attention has been paid on the collapse capacity of these structures, mainly because it requires refined nonlinear models and careful consideration of different sources of uncertainties. To fill this gap, a set of collapse fragility functions for existing reinforced concrete‐frame buildings, designed for gravity loads only and then retrofitted with different isolation systems (including rubber‐based and friction‐based isolation systems), are derived in this study. For completeness, buildings with low and high seismic resistance are also considered. Collapse fragility functions are derived through incremental dynamic analysis, considering different collapse conditions both for isolation system and superstructure. For each case study building, mean and dispersion values are obtained considering both aleatory and epistemic uncertainties, due to record‐to record and model variability, respectively. Finally, some comments on the possible use of the results of this study for practical applications are made.

Evaluating Collapse Fragility Curves for Existing Buildings Retrofitted Using Seismic Isolation

“…Finally, record-to-record dispersion values ranging between 0.19 and 0.27 have been obtained. These results are aligned with those proposed by Cardone et al [6] for similar building typologies. On the other hand, a significant reduction of the number of superstructure failures is observed at each IM.…”

“…In both cases, slight variations of the record to record dispersion, β RTR , (lower than 10%) are obtained. It is worth noting that the derived dispersion values are in line with those proposed in literature [6].…”

“…In the last decade, several studies aimed at the evaluation of the CFFs for new and existing fixed-base reinforced-concrete buildings [3,4] have been proposed. Conversely, a few studies [5,6] investigated the collapse performances and capacity of existing buildings retrofitted with different isolation systems. The building located in L'Aquila (high seismicity) has been designed with reference to pre-96 seismic regulations (Seismic Load Design-SLD-building) while the one located in Naples (medium seismicity) has been designed for gravity loads only (Gravity Load Design-GLD-building).…”

“…The collapse margin ratio (median collapse capacity S a,C , namely the spectral acceleration associated to a probability of exceedance equal to 50%, divided by the design spectral acceleration at the collapse prevention limit state) has been evaluated for each examined case-study. Values ranging from 1.10 to 1.45 were found.Among those, Cardone et al [6] proposed CFFs for existing reinforced concrete (RC) framed buildings, located in medium or high-seismicity areas, designed for vertical loads only and equipped with different base-isolation systems.Recently, the RINTC (Implicit Risk of code-conforming Italian buildings) research project [7], funded within the ReLuis 2015-2018 program (promoted by the Italian Department of Civil Protection), developed a systematic methodology for the estimation of collapse probabilities of different building typologies, included existing buildings retrofitted using seismic isolation. Pragmatic choices regarding the definition of the collapse conditions and the numerical non-linear modeling of the isolation system have been adopted within [7].…”

“…Among those, Cardone et al [6] proposed CFFs for existing reinforced concrete (RC) framed buildings, located in medium or high-seismicity areas, designed for vertical loads only and equipped with different base-isolation systems.…”