Formulation of risk-targeted seismic action for the force-based seismic design of structures

A risk targeted design spectral acceleration and the corresponding seismic design action for the force based design of structures is introduced by means of two formulations. The first one called direct formulation utilizes the seismic hazard function at the site of the structure. Because the seismic action defined in the codes is often associated with a designated return period, an indirect formulation is also introduced. It incorporates a risk targeted safety factor that can be used to define a risk targeted reduction factor. It is shown that the proposed formulations give analogical results and provide an insight into the concept of the reduction of seismic forces for the force based seismic design of structures if the objective is defined by a target collapse risk. The introduced closed form solution for the risk targeted reduction factor can be used to investigate how the target collapse risk, the seismic hazard parameters, the randomness of the seismic action, and the conventional parameters (ie, the overstrength factor and the deformation and energy dissipation capacity) affect the seismic design forces in the case of force based design. However, collaborative research is needed in order to develop appropriate models of these parameters. In the second part of the paper, the proposed formulations are demonstrated by estimating the risk targeted seismic design action for a six storey reinforced concrete building. By verifying the collapse risk of the designed structure, it is demonstrated that the risk targeted seismic action, in conjunction with a conventional force based design, provided structure with acceptable performance when measured in terms of collapse risk