Use of crescent shaped braces for controlled seismic design of ductile structures

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 behaviour 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. In this paper, this concepts will be applied on a specific application, characterized by 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. 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.