Stiffness-strength-ductility design approach

As recently also recalled by Priestley in a keynote lecture given at the 1st ECEES, Geneve 2006, the emphasis on Performance-Based Seismic Design has forced a re-examination of the methodology employed in the seismic design of structures. Skipping all details, direct Displacement-Based Design of structures has opened up new possibilities for the structural engineer in terms of conceiving and dimensioning a structural system which offers optimized seismic performances. Focusing directly on the non-linear behavior of the lateral-resisting elements of a given building structure, the secant stiffness approach allows “logical choices regarding the force distribution” between different lateral-resisting elements. This paper aims at investigating how these logical choices concerning strength, stiffness and ductility of lateral-resisting elements can be made. In general, the lateral-resisting system of a given building structure can be seen as composed of a series of single lateral-resisting elements working together. The mechanical characterization of each component necessarily requires to capture both its elastic and inelastic behavior and can be assumed to be an elastic-perfectly plastic one, univocally identified by three independent parameters: stiffness (secant at yield point), strength and ductility. The mechanical characterization of the whole lateral-resisting system, as composed of the n lateral-resisting components working in parallel, can be directly obtained from the mechanical characterization of each single component and is therefore controlled by 3n parameters. Consequently, depending upon the type of lateral-resisting used, the structural designer is allowed to control up to 3n parameters upon which he can “act” to reach the desired seismic performance objectives.