OPTIMAL DESIGN OF FARTQUAKE-RESISTANT STEEL FRAMED STRUCTURES

Earthquake resistant steel structures must satisfy two basic criteria in order for the design to be economically feasible; (i) The structure must be serviceable under normal loads, and (ii) the structure must be sufficiently ductile to withstand rare, intense overloads. The traditional approach to the limit design of structures consists of assuring that the structure has strength to sustain a prescribed set of ultimate, or factored loads. Many algorithms have been proposed to optimize the resisting strength of framed structures. However, in most applications geometrically linear analysis of the structure is used, with the consequence that the capacity of the structure is overestimated. Overall stability and ductility of a structure have traditionally played a secondary role in the design process. However, in an earthquake environment, they are of fundamental importance to the robustness of structural performance. This paper presents a method improving the strength and ductility characteristics of framed structures which are governed by limit load behavior. The method is based on a simple model of the nonlinear behavior of framed structures. An iterative optimality criterion method is used to effect the improvements in the structural configuration. While other constraints might be used, the implementation presented here improves an initial design while holding the volume of the structure fixed.