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

This paper proposes a performance-based seismic design methodology for steel building frames which combines the advantages of the well-known force-based and displacement-based seismic design methods in a hybrid force/displacement design scheme. The proposed method controls structural performance by first transforming user-specified values of the interstorey drift ratio (non-structural damage) and local ductility (structural damage) to a target roof displacement and then, calculating the appropriate strength reduction factor for limiting ductility demands associated with the target roof displacement. The main characteristics of the proposed method are: (1) treats 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) makes use of current seismic code approaches as much as possible (e.g., conventional elastic response spectrum analysis and design); (4) includes the influence of the number of stories; (5) recognises the influence of the type of the lateral load resisting system (moment resisting frame or concentrically braced frame); (6) recognises the influence of geometrical (setbacks) or mass irregularities. A realistic design example serves to demonstrate the advantages of the proposed method over the currently used force-based design procedure.