A hybrid force/displacement seismic design method for steel building frames

This paper proposes a preliminary performance-based seismic design method for steel building frames which combines the advantages of the force-based and displacement-based seismic design methods in a hybrid force/displacement design scheme. The proposed hybrid method starts by transforming target values of the peak storey drift and local ductility to a target roof displacement and then calculates the strength reduction factor associated with the roof displacement ductility. The proposed method (1) uses both drift and ductility demands as input variables for the initiation of the design process; (2) does not use a substitute single degree of freedom system; (3) uses conventional elastic response spectrum analysis and design; (4) includes the influence of the number of stories; (5) considers the influence of the type of the lateral load resisting system (moment resisting frame or concentrically braced frame); and (6) recognizes the influence of geometrical (setbacks) or mass irregularities. Realistic design examples demonstrate the advantages of the proposed method over the force-based seismic design procedure of EC8.