OPTIMUM ASEISMIC DESIGN OF MULTISTOREY STEEL FRAMES

The problem of optimum aseismic design of multistorey frames has been cast as a nonlinear programming problem. The multistorey frame has been idealized as a shear beam subjected to ground acceleration represented by white noise. The objective function is the weight of the frame which is to be minimized. The moments of inertia of the columns of different storeys are the design variables. Constraints have been imposed on the upper bound on the probability of failure, defined as the probability that the stresses due to earthquake acceleration exceed the yield stress of the material of the column. An upper bound solution has been obtained. The probabilistic optimum design solution has been compared with the deterministic optimum design solution using the response spectra approach which ignores the randomness of the ground motion. This comparison establishes the compelling need to formulate the problem of optimum design of multistorey frames subjected to strong earthquake motion in a reliability based framework.