Improved ground motion intensity measures for reliability-based demand analysis of structures

In Performance-Based Earthquake Engineering (PBEE), evaluating the seismic performance (or seismic risk) of a structure at a designed site has gained major attention, especially in the past decade. One of the objectives in PBEE is to quantify the seismic reliability of a structure (due to the future random earthquakes) at a site. For that purpose, Probabilistic Seismic Demand Analysis (PSDA) is utilized as a tool to estimate the Mean Annual Frequency (MAF) of exceeding a specified value of a structural Engineering Demand Parameter (EDP). This dissertation focuses mainly on applying an average of a certain number of spectral acceleration ordinates in a certain interval of periods, Sa,avg (T1,…,Tn), as scalar ground motion Intensity Measure (IM) when assessing the seismic performance of inelastic structures. Since the interval of periods where computing Sa,avg is related to the more or less influence of higher vibration modes on the inelastic response, it is appropriate to speak about improved IMs. The results using these improved IMs are compared with a conventional elastic-based scalar IMs (e.g., pseudo spectral acceleration, Sa ( T(¹)), or peak ground acceleration, PGA) and the advanced inelastic-based scalar IM (i.e., inelastic spectral displacement, Sdi). The advantages of applying improved IMs are: (i ) "computability" of the seismic hazard according to traditional Probabilistic Seismic Hazard Analysis (PSHA), because ground motion prediction models are already available for Sa (Ti), and hence it is possibile to employ existing models to assess hazard in terms of Sa,avg, and (ii ) "efficiency" or smaller variability of structural response, which was minimized to assess the optimal range to compute Sa,avg. More work is needed to assess also "sufficiency" and "scaling robustness" desirable properties, which are disregarded in this dissertation. However, for ordinary records (i.e., with no pulse like effects), using the improved IMs is found to be more accurate than using the elastic- and inelastic-based IMs. For structural demands that are dominated by the first mode of vibration, using Sa,avg can be negligible relative to the conventionally-used Sa (T(¹)) and the advanced Sdi. For structural demands with sign.cant higher-mode contribution, an improved scalar IM that incorporates higher modes needs to be utilized. In order to fully understand the influence of the IM on the seismis risk, a simplified closed-form expression for the probability of exceeding a limit state capacity was chosen as a reliability measure under seismic excitations and implemented for Reinforced Concrete (RC) frame structures. This closed-form expression is partuclarly useful for seismic assessment and design of structures, taking into account the uncertainty in the generic variables, structural "demand" and "capacity" as well as the uncertainty in seismic excitations. The assumed framework employs nonlinear Incremental Dynamic Analysis (IDA) procedures in order to estimate variability in the response of the structure (demand) to seismic excitations, conditioned to IM. The estimation of the seismic risk using the simplified closed-form expression is affected by IM, because the final seismic risk is not constant, but with the same order of magnitude. Possible reasons concern the non-linear model assumed, or the insufficiency of the selected IM. Since it is impossibile to state what is the "real" probability of exceeding a limit state looking the total risk, the only way is represented by the optimization of the desirable properties of an IM.

Essays on Ethnic Diversity and Development

This dissertation consists of three papers. The first paper "Ethnicity, Migration and Conflict: Evidence from Contemporary South Africa” exploits some of the institutional changes intervened in South Africa during the end of apartheid to investigate the relationship between ethnic diversity and conflict. I find within-ethnic polarization to be significantly related to the intensity of armed confrontations among black-dominated groups. My investigation thus gives strong and robust empirical support to the theoretical arguments which identify ethnic diversity as one of the determinants of civil conflict. The second chapter, "Pre-Colonial Centralization, Colonial Activities and Development in Latin America", investigates the hypothesis that pre-colonial ethnic institutions shaped contemporary regional development in Latin America. I document a strong and positive relationship between pre-colonial centralization and regional development. Results are in line with the view that highly centralized pre-colonial societies acted as a persistent force of agglomeration of economic activities and a strong predictor of colonial state capacity. The results provide a first evidence of the existence of a link between pre-colonial centralization, colonial institutional arrangements and contemporary economic development. The third paper "Bite and Divide: Malaria and Ethnic Diversity” investigates the role of malaria as a fundamental determinant of modern ethnic diversity. This paper explores the hypothesis, that a large exposure to malaria has fostered differential interactions that reduced contacts between groups and increased interactions within them Results document that malaria increases the number of ethnic groups at all levels of spatial disaggregation and time periods (exploiting historical and current ethnic diversity). Regressions' results show that endogamous marriages are more frequent in areas with higher geographic suitability to malaria. The results are in line with the view that malaria increases intra-ethnic interactions while decreasing inter-ethnic ones.