Amplification of Seismic Loads on Cold-Formed Steel Mid-Rise Podium Structures

Increasingly, cold-formed steel framing (CFS) is appended to reinforced concrete or structural steel podiums, forming structures with a vertical combination of seismic force-resisting systems (SFRS). These systems are vertically irregular and so often must be analysed dynamically according to National Building Code of Canada (NBCC) provisions. Amplification may occur in the CFS structure such that static methods underestimate the upper structure's seismic base shear. However, while simplified methods such as the ASCE 7 two-stage analysis procedure may apply in certain cases, NBCC 2015 provides no simplified methods for irregular structures more than 20min height. This paper investigates the two-stage assumption as applied to these structures. The results are generalized using an equivalent 2DOF model and quantified in terms of amplification factora alpha(U). Two examples are developed satisfying the ASCE 7-16 two-stage procedure using NBCC 2015 design parameters. The applicability of the two-stage procedure is discussed, and it is proposed that the ratio of alpha(U) in an MDOF and a 2DOF model, alpha(U,MDOF)/alpha(U,2DOF), may be used to gauge the applicability of the two-stage procedure.