IM-based and EDP-based decision models for the verification of the seismic collapse safety of buildings

Decision models for the verification of seismic collapse safety of buildings are introduced. The derivations are based on the concept of the acceptable (target) annual probability of collapse, whereas the decision making involves comparisons between seismic demand and capacity, which is familiar to engineering practitioners. Seismic demand, which corresponds to the design seismic action associated with a selected return period, can be expressed either in terms of an intensity measure (IM) or an engineering demand parameter (EDP). Seismic capacity, on the other hand, is defined by dividing the near-collapse limit-state IM or EDP by an appropriate risk-targeted safety factor (gamma(im) or gamma(edp)), which is the only safety factor used in the proposed decision model. Consequently, the seismic performance assessment of a building should be based on the best possible estimate. For a case study, it is shown that if the target collapse risk is set to 10(-4) (0.5% over a period of 50years), and if the seismic demand corresponds to a return period of 475years (10% over a period of 50years), then it can be demonstrated that gamma(im) is approximately equal to 2.5 for very stiff buildings, whereas for buildings with long periods the value of gamma(im) can increase up to a value of approximately 5. The model using gamma(edp) is equal to that using gamma(im) only if it can be assumed that displacements, with consideration of nonlinear behavior, are equal to displacements from linear elastic analysis.