Seismic collapse assessment of a hybrid cold-formed hot-rolled steel building

This paper investigates seismic collapse potential of a hybrid cold-formed hot-rolled system in order to quantify the response modification factor (R-factor) through a procedural method proposed in FEMA-P695. A series of hot-rolled steel (HRS) knee-braced frames in conjunction with cold-formed steel (CFS) stud walls are proposed to resist lateral and gravity loads. ASCE7-16 does not provide seismic performance factors for this hybrid HRS/CFS structural topology in lightweight steel construction and as a result, more sophisticated assessment is needed to measure reasonable seismic performance. A nonlinear numerical model that simulates post-peak response of HRS knee-braced frames is calibrated with experimental data. Post-buckling behaviour of CFS studs are measured according to various techniques in terms of finite strip method (FSM), finite element method (FEM) and AISI-S136-16 analytical formulations. The modelling approach is implemented into nonlinear analytical models of a six-storey steel building which is designed in accordance with ASCE7-16, ANSI/AISC360-16, and AISI-S316-16. A suite of twenty-two bidirectional far-field ground motions are chosen from PEER/NGA database subset and scaled to conditional mean spectrum (CMS) relevant to Urban California region. A set of nonlinear static analysis as well as incremental dynamic analysis (IDA) is conducted to measure collapse fragility and seismic performance of the building. It is concluded that initially assumed R-factor for the proposed structural system maintains the collapse prevention criterion as recommended by FEMA-P695 and is appropriate to be considered for design purposes.