Influence of non-structural components on the seismic response of cold-formed steel structures

Shortcomings exist with respect to accounting for the influence of non-structural components, e.g. gypsum panels installed on the ceilings (floors and roof diaphragms) and walls, on the response of cold-formed steel (CFS) framed structures subjected to seismic excitation. The current North American building codes and CFS related design standards (AISI S400 North American Standard for Seismic Design of Cold-Formed Steel Structural Systems & AISI S100 / CSA S136 North American Specification for the Design of Cold-Formed Steel Structural Members) do not take into account the diaphragms’ contribution to the lateral resistance or stiffness of the structure, nor do they consider the contribution of the non-structural components found throughout the building. To improve on the seismic design of CFS structures, the CFS-NEES building, as tested by researchers at Johns Hopkins University, was chosen as a case study structure and modelled in the OpenSees platform, including interior non-structural gypsum sheathing and gravity walls. A 2D non-linear diaphragm model was initially created and, subsequently, incorporated in the 3D building model. The diaphragm model was calibrated based on a recently completed laboratory test program conducted at McGill University involving 9 cold-formed steel framed diaphragm configurations (16 diaphragm specimens) subjected to in-plane monotonic and reversed cyclic loading. Response history analyses revealed that, although interior gypsum panels increased the stiffness of the individual shear walls, the addition of gravity walls resulted in a reduction of the shear wall forces. The gravity walls contributed to the resistance of the lateral load. As the stiffness of the structure increased, the fundamental period and wall line interstorey drift ratios decreased, and the total base shear increased in both directions, with the gravity walls being the dominant lateral force resisting elements.