Exposure modelling and loss estimation for seismic risk assessment of residential buildings: innovative methods and applications

Defining the seismic hazard, assessing the vulnerability of the main components of the built environment and, consequently, estimating the expected losses are key steps for setting up effective post-event emergency plans as well as medium-long term mitigation strategies. Despite the significant knowledge advancements achieved in the last years, several points need to be further developed. Among them the collection of reliable building inventories, the selection of appropriate measures of seismic intensity and the definition of accurate loss estimation models still propose some challenges for the scientific community. The present PhD thesis aims at providing a contribution in this direction. After a comprehensive state of the art on seismic risk components along with a literature review focused on the main models to estimate the expected seismic losses, some new procedures related to hazard, exposure and loss estimation, have been proposed and applied. Firstly, a model aimed at estimating the direct economic losses (i.e., building repair costs) has been developed by improving the models currently available in the literature. These models generally account for only the severity of damage (i.e., the maximum damage level), while damage extension and distribution, especially along the building height, are implicitly considered in the repair cost values. If on the one side, the assessment of safety condition depends essentially on damage severity, on the other side, damage extension strongly affects the estimation of economic impact. In this regard, the proposed model allows to explicitly consider both damage severity and distribution along the building height. The model is applicable to both Reinforced Concrete (RC) and masonry building types. It requires the determination of the more frequent damage distributions throughout the building height. At the current state, the procedure has been specifically implemented for existing Reinforced Concrete (RC) building types by performing Non-Linear Dynamic Analyses (NLDAs). As for seismic hazard, correlations between macroseismic intensities and ground motion parameters have been derived processing data related to Italian earthquakes occurred in the last 40 years. Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV) and Housner Intensity (IH) as instrumental measures, and European Macroseismic Scale (EMS-98) and Mercalli-Cancani-Sieberg (MCS) as macroseismic measures, have been considered. The correlations can be used both to adopt empirical damage estimation methods (e.g., Damage Probability Matrices) and to convert the macroseismic data of historical earthquakes into instrumental intensity values, more suitable to risk analyses and design practice. Concerning exposure, an innovative methodology has been developed to convert the information on the typological characteristics collected through the AeDES form (currently used in Italy in post-earthquake usability surveys) to recognized international standards such as the taxonomy proposed by the Global Earthquake Model (GEM) and the EMS-98 building types. The methodology allows to fully exploit the exposure and vulnerability data of post-earthquake surveys related to the Italian built environment and to define an exposure model in terms of risk-oriented classes more suitable for large-scale risk assessments. Furthermore, an approach based on the integration of data collected with the CARTIS procedure (i.e., a protocol used in Italy for the typological-structural characterization of buildings at regional scale) and using the RRVS web-based platform (i.e., for a remote visual screening based on satellite images) has been proposed and specifically applied to the village of Calvello (Basilicata region, Southern Italy). This approach represents a useful tool for compiling residential building inventories in a quick and inexpensive way thus being very suitable in data-poor and economically developing countries. To better illustrate the proposed methodological developments, some applications are provided in the last part of the thesis. The first one proposes a comparison among the results obtained applying some casualty estimation models available in the literature using the vulnerability and damage data collected in the L’Aquila urban area after the 2009 earthquake (data available on the Observed Damage Database Da.D.O. platform). After, by using the same data source, an exposure model in terms of EMS-98 types based on the 2009 post-earthquake data has been implemented for the residential buildings of L'Aquila town and the surrounding municipalities involved in the usability assessment surveys. The third - expansive - application deals with the seismic risk assessment of the Val d’Agri area (Basilicata region, Southern Italy). This area has a strategic role for Italy due to the large quantities of oil extracted from local deposits, making available large resources deriving from royalties. Specifically, earthquake damage scenarios for the residential building stock of 19 villages have been prepared. Considering a seismic vulnerability distribution obtained from the integration of a building-by-building inventory and information collected with the CARTIS and RRVS approaches, the expected losses deriving from a seismic event with an exceedance probability of 10% in 50 years (475 years return period) have been determined. Finally, an action plan for the seismic risk mitigation, essentially based on the reduction of vulnerability of the building stock through a structural strengthening program, has been proposed and specifically applied to one of the villages in the area under study

Methodology for seismic vulnerability assessment of existing masonry buildings in urban centres. Application to the Eixample district in Barcelona

(English) Unreinforced masonry buildings (URM), which prevail in many historic and urban centres, can be considered to be significantly vulnerable to seismic actions due to their peculiar constructive and structural features that could influence their seismic performance, even in low to moderate seismic hazard areas. These existing structures were usually designed considering only gravity loads without any seismic design requirements. Hence, they may endure severe consequences in the event of an earthquake due to the presence of many specific sources of structural vulnerability, such as the material's limited resistant capacity and ductility, the buildings’ height, very slender load-bearing walls, semi-flexible horizontal diaphragms, irregular plan configurations, presence of vertical extensions, large façade openings, among other structural features.The scientific literature currently offers a variety of methods for assessing seismic vulnerability of existing buildings on a large scale, as it is considered a challenging task. The selection of the appropriate approach is determined by several factors, including the purpose and nature of the study, the amount of data and resources available, the investigated building typologies, the level of analysis effort, and the cost required for the studies.The aim of this research is to contribute to the seismic vulnerability assessment of existing masonry buildings of the Eixample district of Barcelona, though the derivation of a general methodology, intensively based on numerical simulation due to lack of seismic damage observations from past earthquakes. This approach could be applied to similar problems involving the vulnerability assessment of historic urban centres in low to moderate seismic regions, by applying the necessary modifications. The first step of the proposed methodology is elaborating a detailed building taxonomy of the masonry buildings of the Eixample district according to their structural, material and geometrical characteristics, relevant to their seismic behaviour and possible sources of vulnerability. The most representative building typologies are selected based on the aforementioned building taxonomy, by using available statistical data of structural features of the analysed building stock. The next step is developing sophisticated numerical models of the previously identified representative masonry buildings, by using an efficient and realistic simulation of their seismic response based on the Finite Element Method (FEM). Non-linear static (pushover) analysis are performed for both main directions (parallel and perpendicular to the façade) in order to better understand their global seismic behaviour in terms of capacity and failure mechanisms. Moreover, parametric analyses are carried out to investigate the influence of different structural parameters on the building’s seismic performance. The N2 method is applied for the evaluation of the buildings’ seismic performance for the seismic hazard scenarios in Barcelona. The final step is the proposal of new forms of the Vulnerability Index Method (VIM) for both main directions, by defining the classes and calibrating the weights of the specific vulnerability parameters. The methodology is applied eventually to the Eixample district of Barcelona’s urban centre, by including two cases: a large number of existing masonry buildings and a typical urban block.(Español) Los edificios de obra de fábrica no armada (URM), abundantes en muchos centros urbanos históricos, resultan frecuentemente vulnerables ante las acciones sísmicas, incluso en zonas de baja o moderada peligrosidad sísmica, debido a sus características constructivas y estructurales. Por lo general, estas estructuras fueron diseñadas teniendo en cuenta únicamente las cargas gravitatorias y sin verificar su comportamiento sísmico. Ante un terremoto, dichas estructuras pueden sufrir graves consecuencias debido a varias fuentes específicas de vulnerabilidad estructural, como son la limitada capacidad resistente y limitada ductilidad del material, la altura de los edificios, los presencia de muros de carga muy esbeltos, forjados flexibles o semiflexible, las configuraciones de planta, frecuentemente irregulares, la presencia de extensiones verticales (remontas) y la frecuente presencia de grandes aberturas de fachada, entre otras.Como consecuencia del desafío que supone la evaluación de la vulnerabilidad sísmica de los edificios existentes a gran escala, en la literatura científica actual se han propuesto una gran variedad de métodos orientados hacia su caracterización. La selección del método más adecuado viene determinada por varios factores, entre los cuales se hallan el propósito y la naturaleza del estudio, la cantidad de datos y de recursos disponibles, las tipologías de los edificios investigados, el esfuerzo computacional requerido para el análisis y el coste de la investigación.El objetivo del presente trabajo reside en contribuir a la evaluación de la vulnerabilidad sísmica de los edificios de obra de fábrica existentes en el distrito del Eixample de Barcelona. Ello se lleva a cabo mediante la elaboración de una metodología general basada principalmente en la simulación numérica debido a la ausencia de observaciones, en el caso investigado, relativas a daños sísmicos producidos terremotos ocurridos en el pasado. Esta metodología podría aplicarse, con las modificaciones necesarias, a casos similares relativos a la evaluación de la vulnerabilidad de centros urbanos históricos en regiones de sismicidad baja a moderada. El primer paso de la metodología propuesta consiste en elaborar una taxonomía detallada de los edificios de obra de fábrica del distrito del Eixample en función de las características estructurales, materiales y geométricas que resultan relevantes para la caracterización de comportamiento sísmico. Los tipos de edificios más representativos se han seleccionado en base a dicha taxonomía, utilizando para ello datos estadísticos disponibles sobre sus características estructurales. El siguiente paso ha consistido en desarrollar modelos numéricos avanzados de los edificios seleccionados. Para este fin se ha utilizado el Método de los Elementos Finitos (MEF) por su simulación eficiente y realista de la respuesta sísmica. Se han realizado análisis estáticos no lineales (pushover) para las dos direcciones principales (paralela y perpendicular a la fachada) de los edificios, con el fin caracterizar su comportamiento sísmico global en términos de capacidad y mecanismos de fallo. Además, se han realizado análisis paramétricos con la finalidad de investigar la influencia de diferentes parámetros estructurales en el comportamiento sísmico. Se ha aplica el método N2 para la evaluación del comportamiento sísmico de los edificios para distintos escenarios de peligrosidad sísmica en Barcelona. Finalmente, se han propuesto unos formularios modificados para la aplicación del Método del Índice de Vulnerabilidad (VIM) según las dos direcciones principales, de los edificios. Ello ha comportado una definición de las clases de vulnerabilidad y la calibración de los pesos de los parámetros específicos de vulnerabilidad consistentes en (1) un conjunto amplio de edificios de obra de fábrica existentes y (2) el caso específico de un bloque de edificios típico del entorno urbano investigado.Finite Element Model, Pushover Analysis, N2 method, Vulnerability Index Method.Postprint (published version