ADVANCED SEISMOLOGICAL AND ENGINEERING ANALYSIS FOR STRUCTURAL SEISMIC DESIGN

Nowadays, standard “Performance Based Seismic Design” (PBSD) procedures rely on a “Probabilistic Seismic Hazard Analysis” (PSHA) to define the seismic input. Many assumptions underlying the probabilistic method have been proven wrong. Many earthquakes, not least the Italian earthquake sequence of 2016 (still in progress), have shown the limits of a PBSD procedure based on PSHA. Therefore, a different method to define the seismic hazard should be defined and used in a PBSD framework. This thesis tackles this aspect. In the first chapter a review of the standard PBSD procedures is done, focusing on the link between the seismic input and the acceptable structural performance level for a building. It is highlighted how, at least when evaluating the Collapse Prevention Level (CP), the use of a probabilistic seismic input should be avoided. Instead, the concept of “Maximum Design Seismic Input” (MDSI) is introduced. This input should supply Maximum Credible Earthquake (MCE) level scenario ground motions, in other words an “upper bound” to possible future earthquake scenarios. In the second chapter an upgrade of the “Neo Deterministic Seismic Hazard Assessment” (NDSHA) is proposed to find MDSI, henceforth called NDSHA-MDSI. NDSHA is a physics-based approach where the ground motion parameters of interest (e.g. PGA, SA, SD etc.) are derived from the computation of thousands of physics-based synthetic seismograms calculated as the tensor product between the tensor representing in a formal way the earthquake source and the Green’s function of the medium. NDSHA accommodates the complexity of the source process, as well as site and topographical effects. The comparison between the NDSHA-MDSI response spectra, the Italian Building Code response spectra and the response spectra of the three strongest events of the 2016 central Italy seismic sequence is discussed. Exploiting the detailed site-specific mechanical conditions around the recording station available in literature, the methodology to define NDSHA-MDSI is applied to the town of Norcia (about five km from the strongest event). The results of the experiment confirm the inadequacy of the probabilistic approach that strongly underestimated the spectral accelerations for all three events. On the contrary, NDSHA-MDSI supplies spectral accelerations well comparable with those generated by the strongest event and confirms the reliability of the NDSHA methodology, as happened in previous earthquakes (e.g. Aquila 2009 and Emilia 2012). In the third chapter a review of the PBSD is done. It emphasizes the arbitrariness with which different choices, at present taken for granted all around the world, were taken. A new PBSD framework based on the use of MDSI is then proposed. This procedure is independent from the arbitrary choice of the reference life and the probability of exceedance. From an engineering point of view, seismograms provided by NDSHA simulations also allow to run time history analysis using site specific inputs even where no records are available. This aspect is evidenced in chapter four where a comparison between some Engineering Demand Parameters (EDP) on a steel moment resisting frame due to natural and synthetic accelerograms are compared. This thesis shows that, at least when assessing the CP level, the use of PSHA in a PBSD approach should be avoided. The new PBSD framework proposed in thesis and based on NDSHA-MDSI computation, if used, could help to prevent collapse of buildings and human losses hence to build seismic resilient systems and to overcome the limits of probabilistic approaches. Not least, the availability of site specific accelerograms could lead to wider use of Non-Linear Time History Analysis (NLTHA) hence to a better understanding of the seismic behaviour of structures.

Nowadays, standard “Performance Based Seismic Design” (PBSD) procedures rely on a “Probabilistic Seismic Hazard Analysis” (PSHA) to define the seismic input. Many assumptions underlying the probabilistic method have been proven wrong. Many earthquakes, not least the Italian earthquake sequence of 2016 (still in progress), have shown the limits of a PBSD procedure based on PSHA. Therefore, a different method to define the seismic hazard should be defined and used in a PBSD framework. This thesis tackles this aspect. In the first chapter a review of the standard PBSD procedures is done, focusing on the link between the seismic input and the acceptable structural performance level for a building. It is highlighted how, at least when evaluating the Collapse Prevention Level (CP), the use of a probabilistic seismic input should be avoided. Instead, the concept of “Maximum Credible Seismic Input” (MCSI) is introduced. This input should supply Maximum Credible Earthquake (MCE) level scenario ground motions, in other words an “upper bound” to possible future earthquake scenarios. In the second chapter an upgrade of the “Neo Deterministic Seismic Hazard Assessment” (NDSHA) is proposed to compute NDSHA-MCSI, henceforth shortly called MCSI. In other words, MCSI is fully bolted to NDSHA and aims to define a reliable and effective design seismic input. NDSHA is a physics-based approach where the ground motion parameters of interest (e.g. PGA, SA, SD etc.) are derived from the computation of thousands of physics-based synthetic seismograms calculated as the tensor product between the tensor representing in a formal way the earthquake source and the Green’s function of the medium. NDSHA accommodates the complexity of the source process, as well as site and topographical effects. The comparison between the MCSI response spectra, the Italian Building Code response spectra and the response spectra of the three strongest events of the 2016 central Italy seismic sequence is discussed. Exploiting the detailed site-specific mechanical conditions around the recording station available in literature, the methodology to define MCSI is applied to the town of Norcia (about five km from the strongest event). The results of the experiment confirm the inadequacy of the probabilistic approach that strongly underestimated the spectral accelerations for all three events. On the contrary, MCSI supplies spectral accelerations well comparable with those generated by the strongest event and confirms the reliability of the NDSHA methodology, as happened in previous earthquakes (e.g. Aquila 2009 and Emilia 2012). In the third chapter a review of the PBSD is done. It emphasizes the arbitrariness with which different choices, at present taken for granted all around the world, were taken. A new PBSD framework based on the use of MCSI is then proposed. This procedure is independent from the arbitrary choice of the reference life and the probability of exceedance. From an engineering point of view, seismograms provided by NDSHA simulations also allow to run time history analysis using site specific inputs even where no records are available. This aspect is evidenced in chapter four where a comparison between some Engineering Demand Parameters (EDP) on a steel moment resisting frame due to natural and synthetic accelerograms are compared. This thesis shows that, at least when assessing the CP level, the use of PSHA in a PBSD approach should be avoided. The new PBSD framework proposed in thesis and based on MCSI computation, if used, could help to prevent collapse of buildings and human losses, hence to build seismic resilient systems and to overcome the limits of probabilistic approaches. Not least, the availability of site specific accelerograms could lead to wider use of Non-Linear Time History Analysis (NLTHA), therefore to a better understanding of the seismic behaviour of structures.