Comparative Seismic Performance of Structural Steel and Mixed RC–Steel Building Systems 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. The support of the steel part on the RC one is considered through the two boundary conditions: (a) a fixed connection of the steel columns on the lower RC ones and (b) a pinned-fixed connection of the vertical steel elements to the corresponding RC ones.

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. Dimensionless comparison indices are employed to provide a more objective evaluation of the nonlinear response of the structures. The conclusions focus on the overall performance and potential of mixed and steel systems in seismic-prone regions. The findings of this study aim to contribute to a better understanding of steel-based mixed structures and may support future improvements in seismic design codes that explicitly include mixed RC–steel building typologies.