عنوان مقاله [English]
Considering the nonlinear behavior of conventional structures under strong ground motion, application of earthquake forces, as an equivalent static force in the elastic region, will not lead to an optimum application of materials in the structure.Using an energy approach, the performance-based optimum design of concrete moment resisting frames is evaluated for the, so-called, operational, immediate occupancy, life safety and collapse prevention performance levels. Three objective criteria are identified for the performance-based seismic design, which include the least structural weight, uniform ductility demand and, also, uniform earthquake energy in all stories of the structure.Steel reinforcement affects the ductility of RC frames under inelastic seismic loads, so, it is considered a design variable in the optimization process. In order to control the performance of the structure, constraints are added in the process of optimization.To show the capability of the proposed analysis procedure, a three story concrete moment frame is utilized as an illustrative example. For comparison with real behavior, nonlinear dynamic analyses for the optimized frames, with elements of distributed inelasticity, are performed for fifty year period earthquakes, with 2\% intensity (Erzincan, Turkey record). Since there are some constraints on steel reinforcement areas and story drifts, in accordance with current codes, the differences between the results for various performance levels are not so much.As shown, the optimization process, on the basis of the energy function, f3, leads to a decrease of pulse intensity and the last drift, and, therefore, moderates the frame response. This case verifies that minor damage occurs in these frames compared with the optimized frames in cases i and ii.