DESIGN EARTHQUAKES AND SEISMIC DEMAND FOR PBEE IN FAR-FIELD AND NEAR-SOURCE CONDITIONS
Chioccarelli, Eugenio (2010) DESIGN EARTHQUAKES AND SEISMIC DEMAND FOR PBEE IN FAR-FIELD AND NEAR-SOURCE CONDITIONS. [Tesi di dottorato] (Inedito)
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In this thesis the problem of identification of design earthquakes and seismic demand for performance based earthquake engineering (PBEE) is studied referring to far-field and near-source conditions. Ordinary probability seismic hazard analyses (PSHA), usually referred to far-field conditions, are the base of hazard evaluation of the most advanced seismic codes (e.g. Eurocode 8, 2006, CS.LL.PP. 2008, etc.). PSHA allows to identify for each considered site the probability of exceedance of different ground motion intensity measure (IM) levels in a time interval of interest: choosing a return period, and assuming as IM the elastic spectral acceleration at different structural periods, it is possible to build the uniform hazard spectrum (UHS); i.e., the response spectrum with a constant exceedance probability for all ordinates (Reiter, 1990); e.g., 10% in 50 years in the case of design for life-safety structural performance. UHS is not the only possible PSHA-based design spectrum (e.g., Baker 2011), but it is, to date, the most used basis for the definition of design seismic actions on structures. Recent studies are focused on the possibility of reducing uncertainties of PSHA analyses because it will produce a significant reduction of hazard values. Despite that, ordinary PSHA and UHS can be considered as a consolidated procedure which is not discussed here. If the seismic assessment of structures is carried out via non-linear dynamic analysis, knowledge of design response spectra is not enough and selecting the seismic input is seen to be one of the most critical issue which is sometimes considered more important even than structural modeling. In general, the signals that can be used for the seismic structural analysis are of three types: (1) artificial waveforms; (2) simulated accelerograms; and (3) natural records (Bommer and Acevedo, 2004). Spectrum-compatible signals of type (1) are obtained, for example, generating a power spectral density function from a code-specified response spectrum, and deriving signals compatible to that. However, this approach may lead to accelerograms not reflecting the real phasing of seismic waves and cycles of motion, and therefore energy. Simulation records (2) are obtained via modeling of the seismological source and may account for path and site effects but, they often require setting of some rupture parameters, such as the rise-time, which are hard to determine. Finally, of type (3) are ground-motion records from real events. The availability of on-line, user-friendly, databases of strong-motion recordings, and the rapid development of digital seismic networks worldwide, have increased the accessibility to recorded accelerograms, which, therefore, have become the most promising candidates for the seismic assessment of structures (Iervolino and Manfredi, 2008). In the code approach, selection of natural records has to identify a set compatible with the code-specified spectrum which should include implicitly information about the features of the seismogenic sources determining the seismic hazard at the construction site. Moreover, prudently, the practitioner is often required to also account explicitly for them: for example, Eurocode 8 states that accelerograms should be adequately qualified with regard to the seismogenetic features of the sources […]. In practical engineering application, accounting for seismological features of the sources is usually not compatible with information and/or ability of practitioners but, in accordance with probabilistic approach, for a given UHS, disaggregation of seismic hazard (Bazzurro and Cornell, 1999), for the exceedance return period and for the spectral ordinate of interest, allows to identify the contribution to the hazard of each seismological features considered in PSHA. Thus earthquakes dominating the hazard at the site (or design earthquakes) may be considered as the events characterized by the seismological features with the maximum contributions to the hazard (McGuire, 1995). In the first part of this work issues and findings related to identification of design earthquakes are taken to the Italian national level extending preliminary investigations of Convertito et al., 2009 which were referred to a case study region in the Southern Italy. In a recent work of Barani et al. (2009). disaggregation analyses of Italy were presented but different criteria for identification of design earthquakes were chosen. A large part of results obtained here can be considered independent from the specific sites considered and correlated to the typology of analyses: for example it is discussed how and why design earthquakes change with the spectral period (i.e., the dynamic characteristics of the considered structure), or why return period of seismic action may increase the hazard contribution of moderate events respect to the strong distant earthquakes. Moreover general trends of results for Italian sites are identified and a methodology for extracting one or more design earthquakes given disaggregation results is proposed and discussed. Finally, it is illustrated how these concepts may be easily included in engineering practice complementing design hazard maps and effectively enriching definition of seismic action with relatively small effort. The attention to the practical applicability of presented analyses is summarized in their implementation in an already existing and freely distributed software addressed to the code-based record selection (Iervolino et al., 2010 and Galasso et al. 2010). In the second part of the work, near-source conditions are considered analyzing the problem of rupture directivity effects. Generally speaking sites that are in a particular geometrical configuration with respect to the rupture may be reached contemporarily by seismic waves generated in different instant of time and velocity fault-normal signals may show a large pulse which occurs at the beginning of the record and contains the most of energy (Somerville et al., 1997). The results are waveforms different from ordinary ground motions recorded in the far field or in geometrical conditions not favorable with respect to directivity (Singh, 1985 and Reiter, 1990). Current attenuations laws are not able to capture such effect well but, unfortunately, it is believed that structures with dynamic behavior in a range of periods related to the pulse period may be subjected to underestimated seismic demand (Tothong and Cornell, 2006). So although directivity effects are known since many years to both seismologists and earthquake engineers, many aspects are still to be deepening. A systematic procedure for (i) analyzing resultant signals, (ii) studying structural effects and (iii) including these issues in hazard assessment is still far to be consolidated and as consequence, main European seismic codes do not account for the problem. In this work an attempt of improving each one of the previous points is presented. Regarding quantification of structural effects, a strong-motions’ American database, already classified in pulse-like and non pulse-like signals, is analyzed and three main characteristics of pulse-like records are outlined: (1) the elastic demand is generally larger than that of ordinary recordings, particularly concerning the fault-normal direction; (2) the spectral shape is non-standard with an increment of spectral ordinates in the range around the pulse period; (3) because the pulse period is generally a low frequency one (i.e., in the same order of magnitude of that of the most of common structures) the inelastic demand can be particularly high (Tothong and Luco, 2007) and developed in a comparatively short time which may facilitate fragile collapse mechanisms in structures not properly designed. Using previous results as benchmark, signals of the recent L’Aquila earthquake mainshock are analyzed investigating if directivity effects occurred. In particular, near-source records from the mainshock, rotated in fault-normal (FN) and fault-parallel (FP) directions, are analyzed extracting pulses with the same procedure used for the NGA dataset. Those found as likely containing velocity pulses were compared to: (1) those not identified as pulse-like; (2) the un-rotated components, and (3) to predictive models for the occurrence of directivity pulses and for the pulse period. The features of L’Aquila records were also compared to those of the NGA database to check whether they are in agreement to what expected for impulsive and non impulsive near-source records. The analyses include also rupture-rotated and vertical components of motion. Finally all the recent models regarding pulse-like effects (applied also in the analyses mentioned above) are used for a PSHA which is modified in order to account for near-source conditions (Iervolino and Cornell, 2008). Also disaggregation analysis is adapted to near source conditions as suggested by literatures (Thotong et al., 2007). Illustrative cases are analyzed: they can be considered as some of the first numerical applications of the most recent procedures and they are a way for identifying advantages and limits and for contributing to future improvements.
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