5. Hybrid Force-Displacement Design

The hybrid force/displacement (HFD) seismic design method for plane and space, regular and irregular, unbraced and braced steel building frames is presented. The method combines the advantages of the force-based design (FBD) and displacement-based design (DBD) methods. The HFD design method starts by considering both non-structural and structural target deformations in the form of maximum interstorey drift ratios (IDR) and member rotational ductilities μ_θ, respectively. These target deformations are transformed to a target roof displacement. Thus, the behavior (strength reduction) factor q is determined as a function of the target roof displacement ductility. After that, the HFD method proceeds as a FBD approach for strength checking using the pseudo-acceleration design spectrum analysis for seismic force determination. The necessary explicit empirical expressions needed for the determination of q are obtained by regression analysis on response databanks created for the various types of frames considered here. These databanks are obtained with the aid of extensive parametric studies involving many frames under many seismic motions analyzed by nonlinear time history analyses. The HFD method is constructed to be a performance-based seismic design method. Numerical examples are presented to illustrate the application of the method and demonstrate its advantages over the FBD and DBD seismic design methods.