Petruzzelli, Fabio (2013) Scale-dependent procedures for seismic risk assessment and management of industrial building portfolios. [Tesi di dottorato]

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Abstract

In the last years the assessment and management of seismic risk in the manufacturing industry has received a continuously growing interest, due to the significant consequences, both direct and indirect, that can be triggered by earthquakes (e.g. (Tohoku, 2011; Emilia, 2012). The present thesis is focused on the development and the application to real case-studies of different procedures for seismic risk assessment and management in the industrial and insurance fields. The research addresses to the problem at different scales: (i) a “large scale”, typically the one of interest of the insurer, at which the number of buildings to deal with is of hundreds or thousands; a “meso scale”, typically the one of large industrial groups, at which the number of building is of tens to hundreds; (iii) a “site-specific” scale, that is the one of interest for a single plant. At each one of the previously listed scales, a different procedure has been proposed. At the “large scale”, a detailed assessment of vulnerability is, in general, unfeasible. Therefore, a prioritization scheme has been developed with the purpose of analysing a portfolio of structures and ranking their seismic risk in a coherent manner, in order to define a priority scale for further, more detailed, investigations. This approach is based on the evaluation of a conventional (or “nominal”) seismic risk index, on the basis of extremely poor and easy to retrieve data, at least the year of construction and geographic location, for the production of which it is not required any visual inspection of the structures. This index is defined as the gap between the current seismic demand and the seismic capacity, the latter evaluated on the basis of seismic demand at the time of the design, assuming a perfect code compliance. In the case of structures not designed in a seismic zone, the horizontal capacity may be obtained from the design for other horizontal actions, such as that from wind. In order to implement the proposed prioritization scheme and compare it to other similar ones, available in literature, a tool named “NODE – NOminalDEficit – v1.1. beta” has been developed. This tool enables to compute location-specific code-based horizontal performance demands, according to Italian code and seismic classification evolution since 1909. Moreover it contains the evolution of wind design and a map of soil categories according to current seismic regulations for the whole Italian territory. Thanks to the collaboration with AXA Matrix Risk Consultants, this approach has been applied to a real case-study of 19 Italian industrial plants, visually surveyed by means of knowledge forms realized ad-hoc. At the “meso scale”, the number of buildings to deal with is of hundreds or thousands. It is believed that at this scale it is possible, according to the resources of the stakeholder, to achieve a level of knowledge about the structures sufficient to allocate a fragility curve to each structure of the portfolio or to classes of structures individuated in the portfolio. The proposed approach, therefore, consists in the rapid computation of expected loss due to earthquakes by the integration of hazard, fragility and exposure. Although the use of fragility curves can be considered a well-established methodology for computing seismic risk, significant differences exist among fragility functions computed in different geographical contexts, reflecting the differences in structural typologies, construction practice and materials. Therefore their practical use requires instruments for their managing, conversion and use along with hazard and exposure. In order to overcome this shortcomings, the software suite named “FRAME - Fragility-based rapid seismic Risk AssessMEnt - v.1.0 beta” has been developed with the aim of providing for the management, the manipulation and the homogenization of an inventory of fragility curves. The inclusion in the software of seismic hazard at the global scale and the possibility of including exposure allow the computation of the expected losses worldwide. At the “site-specific scale”, the analysis of the seismic performance of an existing steel building has been performed in order to provide for the lack in fragility curves available in literature for this specific structural typology and to compute failure probabilities. The latter represent the most rational basis for assessing loss estimates and computing insurance premiums. Such an approach to seismic risk assessment, extremely demanding in terms of time and computational burden, is proposed to be applied at the scale of the single building, of a plant or, of a limited number of plants. The analysed structure is the main workshop building of one of the most risk-prone plants of the case-study portfolio, as resulting from the prioritization analysis. The study of such a structure, designed according to obsolete structural codes about both the definition of the seismic design action and the design of steel structures, allows to enlighten and critically discuss some peculiar modelling aspects. Moreover, some findings deriving from non-linear response history analysis, related to the onset of different failure modes and the inclusion of residual drift in the assessment, have been discussed. Although on principle independent each other, the three procedures outlined in the thesis can be structured in order to provide a unified framework for management and assessment of seismic risk of large dimensions structural portfolios.

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