Behaviour factor and seismic safety of reinforced concrete structures designed according to Eurocodes

Abstract

Eurocode 8 allows force-based seismic design and safety assessment of buildings as if they were indefinitely linearly elastic. However, to implicitly account for their inelastic deformation capacity, Eurocode 8 allows reduction of the design values of seismic forces through a structure-dependant behaviour factor. Recent studies showed that this design approach yields structures with different probabilities of failure in different sites. However, for optimal design and investment of economic resources, a structural code should ensure similar risk level (lower than an acceptable risk level) for all structures that are designed according to it. To achieve this goal, the use of risk-targeted behaviour factors may be considered; therefore, the behaviour factors adopted for the design of the same structure in zones with different seismicity levels should be different.In this study, reinforced concrete moment-resisting frame buildings designed according to Eurocodes with different design values of seismic acceleration and number of storeys were considered, and their “available” behaviour factors and probabilities of failure were evaluated through nonlinear static analyses. The potential dependency of the available behaviour factor and probability of failure on different design parameters, as well as their dependency on the model adopted to reproduce the nonlinear response of reinforced concrete members, were investigated, and are discussed in this paper. The calculation of the probabilities of failure of the case-study structures showed that their probability of failure in low-seismicity zones was significantly lower than the acceptable risk level, which was not optimal. Conversely, the structures designed for high-seismicity zones had probability of failure similar to or higher than the acceptable risk level, which was optimal or not admissible, respectively. Thus, based on the assessed values of the available behaviour factor and probability of failure, a modification to the current equations for the calculation of the behaviour factor is presented. This modification implies the use of hazard-dependent values for the behaviour factor to have structures with similar and hazard-independent probability of failure. Notably, because the probabilities of failure for structures designed for low-seismicity zones were significantly lower than the acceptable risk level, the proposed hazard-dependent behaviour factor was higher than that currently adopted according to the Eurocode.