Refined force reduction factors for seismic design

Whereas seismic design based on deformations is a concept that is gaining ground, existing codes are fundamentally force-based, with a final check on deformations. A central feature of force-based seismic design is the response modification factor (R or q). Many studies have attempted to quantify the potential of structural systems to delimit the level of force imposed by virtue of their ductility and energy absorption capacity. This paper employs a well controlled and evenly distributed earthquake data-set (in magnitude, distance and site characterization spaces) to derive values for force reduction factors needed for the structure to reach, and not exceed, a pre-determined level of ductility. It is observed that the force modification factors are only slightly influenced by the shape of the hysteretic model used in their derivation and even less sensitive to strong motion characteristics. A linear representation is recommended for use in a benchmark for ‘demand’ considerations and given in an easy-to-use parametric form.