WIND RISK ASSESSMENT IN A MULTIHAZARD PERSPECTIVE: THE CASE OF STEEL AIRCRAFT HANGARS
CHIODI, REMO (2011) WIND RISK ASSESSMENT IN A MULTIHAZARD PERSPECTIVE: THE CASE OF STEEL AIRCRAFT HANGARS. [Tesi di dottorato] (Inedito)
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In the field of structural engineering, current design procedures use the envelope of individual hazard demands on a structure to ensure safety against multiple hazards. With regard to wind and seismic hazard, these actions can reasonably considered uncorrelated and, therefore, the design can be carried out separately; however, a difficulty in multi-hazard design for wind and earthquake is that the load and resistance factor method makes use of different design philosophies developed by different subdisciplines. Seismic design explicitly allows for inelastic behavior. In contrast, wind design assumes that the structures behaves in elastic range both for damage limit state (DLS) and ultimate limit state (ULS), although ULS typically refers to return periods shorter than those used for seismic design. In this context, a probabilistic multi-hazard approach can be employed to investigate the performance of a structure under critical events and to ensure its acceptable performance during its entire lifetime. Following the approach proposed by the Pacific Earthquake Engineering Research Center (PEER) for Performance-Based Earthquake Engineering, the purpose of the thesis is to define a reliable methodology for probabilistic estimation of the annual wind risk associated to the achievement of specific limit states. Such approach must be implemented in probabilistic terms due to the stochastic nature of both resistance and loading parameters. These uncertainties affect the wind field, the structural response and also the aerodynamic interaction between the environment and the structure. Therefore, a reliable evaluation of structural performances needs the statistical treatment of recorded data, that is the first step towards the investigation of the performance of wind-exposed structures, the characterization of the interaction by wind tunnel testing or computational fluid dynamics (CFD) techniques and also the knowledge of the structural behavior, i.e. the analysis of all possible failure mechanism induced by wind loads. Starting from this point, the main statistical methods for treatment of extreme wind speeds are presented in the Chapter 2, including the methods for correction of non-standard conditions in terms of roughness and orography. The theoretical background is then applied by performing the statistical analyses of observed data collected starting from 1951 by the Air Force meteorological service. Some of the results are presented in the Chapter 3 whereas the detailed results in terms of fitted parameters, 50-year return period wind speeds, directional and seasonal coefficients estimated by different methods are presented in the Appendixes. The effect of the dowsampling is also investigated and the underestimation of 50-year return period wind speeds is quantified for all the stations available. A further step of the research activity has consisted in the characterization of the structural vulnerability. In fact, the Chapter 4 provides a brief review of properties of materials, structural details and structural types adopted in the past for steel hangars, that are considered as representative wind-exposed structures. These structural types are characterized by large spans and, in some cases, have borrowed design solutions from other industrial buildings. The attention is focused on historical evolution of structural types and adopted design standards; some failure cases which occurred during the last years due to extreme wind events are illustrated. Hence the main elements of vulnerability are discussed. Furthermore, the aerodynamic interaction is investigated; the role of the location and size of the openings is outlined and, in particular, the main theories about the propagation of the internal pressure due to a dominant opening are examined. In fact, due to their particular use, the steel aircraft hangars have a dominant opening that allows increased internal pressures to occur. By assembling of all the previously mentioned tools, a methodology for assessing wind risk is proposed aiming at the evaluation of the annual probability of achievement a fixed limit state due to wind actions. In a multihazard framework, the resulting value can be compared with the same probability referred to seismic actions obtained by applying the IDA approach. Finally, in order to implement and explain the multihazard risk assessment, two case studies are presented and briefly discussed.
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