Characterization of CFS Framed Diaphragm Behavior

Knowledge of the in-plane response to loading of cold-formed steel (CFS) framed diaphragm structures is essential for seismic and wind design. The information available specific to diaphragm design in the North American CFS design standards (AISI S240 & S400) is based on limited experimental work and the extrapolation of design methods developed for the wood industry. To address this shortcoming a test-based research project was performed in the Jamieson Structures Laboratory at McGill University focusing on the characterization of the behaviour under in-plane loading of CFS framed - wood sheathed diaphragms. This research was a complementary study to a research project involving the dynamic testing of CFS framed buildings, known as the CFS -- NEES project. This Johns Hopkins University project involved full-scale two storey CFS framed buildings tested on a shake table under earthquake loading. Although the CFS -- NEES project provided for the first time an insight into the seismic response of CFS structures, more information is required regarding the response of isolated diaphragm systems. The research reported herein included diaphragm test specimens that were based on the floor and roof configurations used in the CFS - NEES Building; which were comprised of oriented strand board (OSB) sheathing screw connected to CFS C-Channel joists. The cantilever diaphragm test approach was followed and the specimen dimensions were chosen considering the AISI S907 Standard requirements. Four diaphragm configurations were selected, each of which was tested under in-plane monotonic and reversed cyclic loading, resulting in a total of eight test specimens. A self-reacting frame was initially designed and constructed as a test-setup in order to accommodate the 12’x 24’ (3.66 m x 7.31 m) diaphragm specimens. It was demonstrated that screw size constitutes a decisive factor in the resulting diaphragm response. In addition, the fully blocked diaphragm configurations, i.e. where the edges of all sheathing panels are supported by an underlying CFS frame, exhibited a considerable increase in shear strength. The bare steel frame contribution to shear strength and stiffness was negligible. An effort to predict the shear strength and deflection of the diaphragm specimens was made using the information available in the AISI S400 Standard. However, the limited information provided in the aforementioned Standard prevented a meaningful comparison to be realized between the predicted design values and the measurements from the tests