Definition of fragility curves through nonlinear static analyses: procedure and application to a mixed masonry-RC building stock

Abstract

Seismic risk analyses at large scale represents a fundamental support to effective mitigation policies. Evaluating fragility curves able to capture the huge variety of existing buildings is one key point of this analysis. Within this context, this paper proposes a procedure for the evaluation of the fragility curves that aims to limit the computational effort without losing the reliability of the achieved results. This is reached through the execution of a limited number of nonlinear static procedures based on the use of the sensitivity analysis carried out according to the simplified star design with central point approach. The main strength of the procedure is the ability to explicitly quantify the various contributions of uncertainty to the dispersion, associated to those on the structural capacity (taking into account both aleatory and epistemic sources) and on the seismic input. As known, the adoption of a nonlinear static approach for the seismic assessment implies various assumptions, such as the load pattern applied, the criteria adopted to compare the capacity and the demand, and the definition of the damage levels. All these issues potentially affect the reliability of the final fragility curves, which are defined through a proper combination of such various options or they can be selected based on the ones more representative of the expected behaviour of the class. To improve this aspect, the evidences from nonlinear dynamic analyses are used. The feasibility and effectiveness of the procedure is duly demonstrated in this paper through its application to a building stock typology, consisting of existing mixed masonry-reinforced concrete structures, representative of one of the largest portions of the existing residential buildings in Lisbon. The attention is focused only to the global in-plane behaviour by adopting as modelling approach the equivalent frame method, that has been proven particularly efficient and accurate enough in representing the nonlinear behaviour of the examined structures.