The peak over the design threshold in strong earthquakes

2020

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SUMMARY In performance‐based seismic design, as adopted by several building codes worldwide, the structural performance is verified against ground motions that have predetermined exceedance return periods at the site of interest. Such a return period is evaluated by means of probabilistic seismic hazard analysis (PSHA), and the corresponding ground motion is often represented by the uniform hazard spectrum (UHS). The structural performance for ground motions larger than those considered in this design approach is, typically, not explicitly controlled under the assumption that they are sufficiently rare. On one hand, this does not achieve uniform safety at sites characterized by different design ground motions corresponding to the same return period; on the other hand, exceedances of the design spectra are systematically observed over large areas, for example in Italy. The latter issue is because of the nature of UHS, the exceedance of which is likely‐to‐almost‐certain when the construction site is in the epicentral area of moderate‐to‐high magnitude earthquakes (ie, the design spectrum may be not conservative at these locations), especially if PSHA is based on seismic source zones. The former is partially because of the systematic difference of ground motions for return periods larger than the design one at the different sites. Quantification of the expected ground motion given the exceedance of the design ground motions (ie, the recently introduced as the expected peak‐over‐threshold or POT) can be of help in quantitatively assessing these issues. In the study, a procedure to compute the POT distribution is derived first; second, POT spectra are introduced and used to help understanding why and how seismic structural reliability of code‐conforming structures decreases as the seismic hazard of the site increases; third, expected and 95th percentile POT maps are shown for Italy to discuss how much high hazard sites are exposed to much larger peak‐over‐threshold with respect to mid‐hazard and low‐hazard sites; finally the POT is discussed with respect to the slope of the hazard curve (in log‐log scale) at the threshold, a known proxy for ground motion beyond design. All data presented in the maps are made available for the interested reader as a supplemental archive.

Section: Pot Distributions and Site's Hazardmentioning

confidence: 90%

Section: Pot Distributions and Site's Hazardmentioning

confidence: 96%

{sa}_{T_{r}}

. Following the common notation in probability, the expected POT can be indicated as

E ([]|, italicSa ((), T), italicSa ((), T) > {sa}_{T_{r}})

.…”

Section: Peak‐over‐threshold Distributionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Peak‐over‐threshold: Quantifying ground motion beyond design

Cito

2020

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Seismic damage accumulation in multiple mainshock–aftershock sequences

Chioccarelli

Suzuki

2020

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Earthquakes are generally clustered, both in time and space. Conventionally, each cluster is made of: foreshocks, the mainshock, and aftershocks. Seismic damage can possibly accumulate because of the effects of multiple earthquakes in one cluster and/or because the structure is unrepaired between different clusters. Typically, the performance-based earthquake engineering (PBEE) framework neglects seismic damage accumulation. This is because: (a) probabilistic seismic hazard analysis (PSHA) only refers to mainshocks and (b) classical fragility curves represent the failure probability in one event, of given intensity, only. However, for life-cycle assessment, it can be necessary to account for the building-up of seismic losses because of damage in multiple events. It has been already demonstrated that a Markovian model (i.e., a Markov chain), accounting for damage accumulation in multiple mainshocks, can be calibrated maintaining PSHA from the classical PBEE framework, and replacing structural fragility with a set of statedependent fragility curves. In fact, the Markov chain also works when damage accumulates in multiple aftershocks from a single mainshock of known magnitude and location, if aftershock-PSHA replaces classical PSHA. Herein this model is extended further developing a Markovian model that accounts, at the same time, for damage accumulation: (i) within any mainshock-aftershock seismic sequence and (ii) among multiple sequences. The model is illustrated through applications to a series of six-story reinforced-concrete moment-resisting-frame buildings designed for three sites with different seismic hazard levels in Italy. The time-variant reliability assessment results are compared to the classical PBEE approach and the accumulation model that only considers mainshocks, so as to address the relevance of aftershocks for life-cycle assessment.

Fatality rates implied by the Italian building code

Pacifico

2021

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The project Rischio Implicito – Norme Tecniche per le Costruzioni (RINTC) assessed the seismic structural reliability, in terms of the annual rate of earthquakes causing failure, of a large set of code‐conforming buildings, designed to be located in three different sites, representative of low, mid, and high seismic hazard in Italy. It was found that seismic reliability tends to decrease significantly as the site's hazard increases, despite the design actions having the same return period at all sites. Because this is a consequence of the code's approach, the simple study presented in this paper aims to contribute to the discussion on whether the code‐implied safety is yet acceptable. To this end, the annual fatality rates due to the seismic failure of the buildings from the mentioned project are computed, in a simplified manner, and compared with the annual risk from other common causes of death in Italy; the latter obtained based on data from the Italian Statistical Institute. The results, although subjected to the conventionality of the working assumptions, seem to indicate that seismic fatality risk is generally lower than that of other causes of death, by one or more orders of magnitude at the lower hazard sites. This can contribute to the discussion on seismic structural safety due to the characteristics of the Italian code that are common to state‐of‐the‐art codes internationally.