Seismic Vulnerability Assessment: Masonry Structures

Abstract

The assessment of the vulnerability of the building stock of an urban center is an essential prerequisite to its seismic risk assessment. The other two ingredients are the expected hazard over given return periods and the distribution and values of the assets constituting the building stock. All three elements of the seismic risk assessment are affected by uncertainties of aleatory nature, related to the spatial variability of the parameters involved in the assessment, and epistemic nature, related to the limited capacity of the models used to capture all aspects of the seismic behavior of buildings and to describe them in simple terms, suitable for this type of analysis. Hence it should always be kept in mind that the computation of a risk level is highly probabilistic and that to accurately represent the risk, the expected values should always be accompanied by a measure of the associated dispersion. A very preliminary estimate of the seismic capacity of the local building stock can be obtained by consulting requirements included in seismic standards and code of practices in force at the time of construction of such buildings. This information together with a temporal and spatial record of the growth of the urban center can provide a definition of classes of buildings assumed to have different capacities. This information can be obtained by looking at past and present cadastral maps with ages of buildings and knowing the historical development and enforcement of codes at the site. In general however for a correct assessment of the seismic risk, a more detailed inventory and classification should be considered, the extent of which is a function of the economic and technical resources available and of the dimensions and building density of the area under investigation, as well as the diversity within the building stock. Seismic vulnerability analysis of masonry structures has traditionally been carried out using “empirical” (or statistical) methods. These can be divided in categorization methods, which classify buildings into typologies characterized by propensity to damage, or inspection and rating methods, wherein scores are attributed to each significant characteristic of the buildings thought to have a bearing on its overall vulnerability (UNDP/UNIDO 1985). Several of these methods have been developed in the past 30 years, and the more robust and commonly used are reviewed in section “Empirical Methods.” Recent dramatic increase in computational powers and consequent improvement and refinement in numerical modelling of relatively complex structures, using either static or dynamic approaches (e.g., Fajfar and Gaspersic 1996; Vamvatsikos and Cornell 2005), has facilitated greater exploitation of analytical approaches for vulnerability assessment purposes. Analytical methods need experimental validation of the various parameters used to define the vulnerability, and so far relatively little experimental work on the behavior of masonry structures has been undertaken by the international community, albeit a wide variety of existing unreinforced masonry typologies, both historic and modern, exists. The framework within which analytical methods can be applied and their limitation are presented in section “Analytical Methods.”