Keywords: behavioural factor, total cost, fragility analysis, design codes.

Most of the current seismic design codes are based on a force based design procedure and on the principle that a structure will avoid collapse if it is designed to absorb energy through inelastic deformation and dissipate the kinetic energy that is imparted to it during the seismic excitation. The capacity of a structure to resist seismic actions in the nonlinear range generally permits lower seismic loads for its design than those corresponding to a linear elastic response. According to the design method the seismic excitation is defined by a single design earthquake that is used for assessing the structural performance against earthquake hazard. The lower seismic loads are calculated by dividing the base shear that develops in the structure if it were to remain in the elastic range under the single earthquake by a behavioural factor q. Seismic design codes recommend constant values of q for various types of structures with the aim of restricting inelastic deformation at a level adequate for the protection of human life in the case of a major seismic event. The numerical verification of the behaviour factor and the sufficiency of a designed structure, using the recommended values of q, to satisfy the demanded limit states is the subject of the present paper.

In this paper a number of design approaches for three-dimensional reinforced concrete (RC) buildings are formulated in the framework of structural optimization problems and they are assessed in terms of structural performance under earthquake loading. The basic objective of this study is the assessment of the european seismic design code with respect to the behavioural factor q on the basis of the final design achieved. In particular, this work consists of two parts. In the first part the total cost of an irregular in plane RC structure designed for seven different values of the factor q is calculated. The total cost consists of the initial cost of the concrete and the steel reinforcement as well as the life-cycle cost of the structure. In the second part, for each optimum design of a regular RC building, achieved for six different values of the behavioural factor, fragility curves are developed in four damage states. The optimum designs are compared based on limit-state probabilities of exceedance encountered for the design earthquake.

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