| Abstract: | A safety format is proposed for the flexural design of reinforced concrete members for the combination of seismic and gravity loads, with load and resistance factors which depend on member type, on the value of the target theoretical probability of failure and on the ratio of the load effect due to gravity loads to that due to the nominal value of the seismic action, both obtained by elastic analysis. Safety factors are computed through an advanced Level II reliability procedure, using a limit state inequality between the member rotation ductility supply under monotonic loading and the peak rotation ductility and cyclic energy dissipation demands. Uncertainties considered are: for resistance, the uncertainty of failure under imposed cyclic deformations, and for action, the maximum peak ductility and energy dissipation demands in the structure's lifetime, as obtained through a series of non-linear dynamic analyses of multistorey buildings in 3D. using as input ensembles of bidirectional acceleration time-histories which describe probabilistically the extreme bidirectional seismic action in the structure's lifetime. Computed load and resistance factors are practically independent of the load-effects ratio. The load factor on the seismic action is found to be independent of member type and to increase with the theoretical probability of failure much faster than the elastic spectral value at the structure's fundamental period with probability of exceedance in the structure's lifetime. Simple rules for the dependence of the resistance modification factors on the theoretical failure probability are also derived. As for the computed values of the load factors the moment due to gravity loads is negligible in comparison to the factored seismic moment, a simplified safety checking inequality between the design flexural capacity and a reduced seismic moment is proposed, in which the ratio of the resistance to the load factor plays the role of a force reduction or effective behaviour factor for the member. |
