Structural Steel Systems versus Mixed RC–Steel Buildings: A Seismic Performance Study under Strong Ground Motions

Typical structural steel buildings are widely used in everyday construction, and current codes provide comprehensive guidance for their analysis and detailing under various loading conditions, including strong seismic excitation. However, in practice, it is quite common to encounter mixed configurations in which a reinforced concrete (RC) lower structure supports an upper structural steel (SS) superstructure of similar geometry, forming a mixed RC–structural steel building system. Despite their frequent application, such mixed systems are not explicitly addressed in existing design codes, leaving an important gap for further investigation.
In this study, selected mixed RC-SS frames are analyzed using nonlinear dynamic simulations under strong earthquake excitations. For comparison purposes, geometrically similar all-steel frames are also examined under identical three-dimensional conditions and ground motion records. Nonlinear material behavior is incorporated into the analysis models to represent realistic structural response, with particular attention to the contribution of steel components.
Due to space limitations, selected response quantities and results are presented to highlight the differences, as well as the advantages and limitations, of mixed RC–steel and pure steel systems. The nonlinear structural response is objectively evaluated using dimensionless comparison indices, including the interstory drift ratio (IDR), the peak floor acceleration-to-peak ground acceleration (PFA/PGA), and normalized base shear and overturning moment ratios. The results indicate that the heightwise distribution of IDR differs significantly between steel and mixed buildings, reflecting the influence of the RC lower structure on global stiffness and deformation patterns. In addition, the generally higher PFA/PGA values observed in steel buildings indicate greater floor accelerations and consequently a higher vulnerability of non-structural components under strong seismic excitation. The comparison indices proved effective in identifying differences in stiffness, deformation demand, and seismic force transfer between the two structural typologies. The results highlight important behavioral differences between mixed RC–steel and all-steel systems and provide useful insights for the future development of seismic design recommendations and code provisions that explicitly address mixed structural configurations.