Advancing the seismic performance of cold-formed steel framed building structures

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Ease of fabrication, high strength-to-weight ratio, dimensional accuracy, infinite recyclability & reusability, among other attributes, have led to the increase in construction of cold-formed steel (CFS) framed multi-storey residential and commercial buildings. CFS framed buildings are complex systems that under earthquake excitations experience the most severe demands of their life cycle. Improved design procedures will lead to a direct enhancement of life safety for building occupants & owners. New North American design standards to address the lateral loading demands on CFS structures are being developed. However, much of the existing research on which one can base the new lateral loading design standards is limited to tests of single-storey shear & braced walls under static or reversed cyclic displacement-based loading, approx. 20 dynamic tests of shear & braced walls, and a handful of CFS framed diaphragm tests. While informative, this existing research has not incorporated the influence of a building's overall response to ground motions, which is dependent on; the diaphragms, the interconnectivity of the roof & floor diaphragms with the vertical walls and the presence of non-structural components. To reach the next phase of innovative, efficient and resilient buildings engineers often find themselves modeling the full 3D structural system. Given the current state of knowledge of CFS buildings engineers are left in the situation where they must make gross simplifications in the building models for the floor & roof diaphragms, connections between the diaphragms & walls, along with other structural & non-structural components. Simplifications that lead to i) inefficiencies, because the influence of the diaphragm & diaphragm-to-wall interface is ignored; ii) inaccuracies because the forces transmitted by the diaphragm to the walls are approximated & simplified, and the forces that develop in the diaphragms themselves are not well understood; iii) incorrect period of vibration estimates because the ignored non-structural components (gypsum sheathed interior walls & clad exterior walls) often have equivalent stiffness to the structural CFS walls; and iv) over-design because the ignored non-structural components have similar shear resistance to the structural CFS walls. Taken together these lead to v) an incomplete understanding of the full 3D CFS seismic force resisting system. The long term objective of the research program is to develop proven seismic design methods for CFS framing systems The specific 5 year short term objectives of the proposed research are: i) provide characterization of typical floor diaphragm & wall force vs. deformation hysteretic response with & without non-structural components; ii) devise a numerical model, appropriate for building response evaluation under dynamic seismic loading, to investigate the influence of the main structural components in a building including the walls, the diaphragms (floors and roofs), the connections between walls, diaphragms & non-structural components; iii) carry out evaluations using this numerical model to identify & quantify building (diaphragm/wall) response to ground motions; iv) recommend guidelines as how to accurately address the influence of diaphragms & non-structural components in the practical engineering design of CFS framed structures; and v) include the recommended CFS seismic design procedures in the relevant design standards; AISI S400, CSA S136 & the National Building Code. Given that the design standards for CFS are intended for use throughout North America it is imperative that issues relevant to Canadian engineers be incorporated within the design documents; this requires the participation of Canadian universities in design standards development.