Stiffness Strength Ductility Design Approaches for crescent shaped braces

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 optimised seismic performances. Focusing directly on the non-linear behaviour 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 (through a specific application) how these logical choices concerning strength, stiffness and ductility of lateral-resisting elements can be rationally made. The key idea lies in the design of the structure so as to achieve the separation between the lateral resisting system and the vertical resisting system. The lateral resisting system is calibrated, within a Performance-Based Seismic Design approach, to satisfy a multiplicity of performance objectives through the identification of a "objectives curve", in the Force-Displacements diagram, of the mechanical characteristics of the structure ("target push-over"). The calibration is obtained by methods borrowed from Displacement Based Design. In more detail, 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 characterisation of each component (e.g. shear wall, bracing system, …) necessarily requires to capture both its elastic and inelastic behaviour and, without loss of generality, 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 characterisation of each single component and is therefore controlled by 3n parameters. Consequently, depending upon the type of lateral-resisting components which are used, the structural designer is allowed to control up to 3n parameters (“degrees of freedom”), upon which he can “act” to reach the desired seismic performance objectives. Within the approach here proposed, the structural designer acquires an active role in shaping the linear and non-linear dynamic behaviour of the structural system (instead of passively designing member strength according to the usual Force-Based Design approach). From a practical point of view, in order to apply this innovative design approach, the lateral resisting system is thought as composed of special bracing elements capable of realizing a sort of properly-calibrated seismic isolation (“revised/tuned soft-storey") called crescent-shaped braces.