Application of the hybrid force/displacement (HFD) seismic design method to composite steel/concrete plane frames

This paper proposes a performance-based seismic design method for composite steel/concrete moment-resisting frames (MRFs) consisting of I steel beams and square concrete filled steel tube (CFT) columns. The design method has to do with the hybrid force/displacement (HFD) method, which combines the advantages of both the force-based and displacement-based seismic design procedures. This hybrid method incorporates predefined values of the maximum story drift and local ductility to a target roof displacement and then determines the appropriate behavior (strength reduction) factor for limiting the roof displacement ductility. The HFD method uses conventional elastic response spectrum analysis and takes into account the influence of structural parameters, such as the number of stories, beam-to-column stiffness and strength ratio as well as the material strength. Comparisons of the proposed design method with those adopted by current seismic design codes demonstrate that the proposed procedure appears to be more rational and efficient indicating the tendency of the current seismic design codes to overestimate the maximum roof displacement and underestimate the maximum inter-story drift ratio along the height of the frames. Furthermore, comparisons between CFT-MRFs and pure steel ones reveal that the first type seems to be more cost-effective structures than the latter since they are associated with a higher behavior factor implying a better seismic behavior of the former.