Cold-Formed Steel Structures: Special Issue

Behavior and design of cold-formed steel members remains an active area of research. The thin-walled nature of cold-formed steel members requires designers and researchers to explore cross-section stability in great detail. The issue begins with five papers covering experimental and numerical examinations of cold-formed steel members, with a particular emphasis on the distortional buckling limit state. Distortional buckling, where the compression flange buckles as a group of plates instead of as individual plates, has only recently begun to work its way into governing design specifications around the world.

Little research and no design guidelines exist for distortional buckling of cold-formed stainless steel columns. In a set of companion papers “Distortional buckling of cold-formed stainless steel sections: Experimental investigation” and “Distortional buckling of cold-formed stainless steel sections: Finite-element modeling and design” Lecce and Rasmussen provide detailed experiments and finite-element models exploring distortional buckling. Their work shows that success depends on a detailed understanding of the mechanical properties of a stainless steel section. Extensive cold work in the corner regions coupled with a low proportional stress leads to experimental and numerical results that differ from conventional cold-formed steel sections. The test results coupled with further finite-element studies demonstrate that current specifications are unconservative for austenitic stainless steels in distortional buckling limit states. The authors also propose a new set of Direct Strength Method expressions for design application to austenitic and ferritic stainless steels in distortional buckling.

In the work of Yu and Schafer “Distortional buckling tests on cold-formed steel beams,” experiments on lipped channel and lipped zee sections commonly used in the United States are performed to explicitly demonstrate the loss of capacity that occurs in the distortional buckling limit state. Current North American specifications are shown to be inadequate when this limit state occurs. The work is a companion to an earlier paper by the authors focusing on similar sections in the local buckling limit state. Taken together, the papers provide independent experimental verification for the newly proposed Direct Strength Method of cold-formed steel design.

Post-buckling behavior is a key aspect in the response of cold-formed steel members. Motivated from a series of experiments conducted by Yang and Hancock at the University of Sydney, Silvestre and Camotim employ an improved version of Generalized Beam Theory in “Local-plate and distortional post-buckling behavior of cold-formed steel lipped channel columns with intermediate stiffeners” to investigate postbuckling of cold-formed steel columns. The work demonstrates the importance of shear in the post-buckling regime (as opposed to membrane deformations) and explains the asymmetric post-buckling response in distortional buckling, that is, why post-buckling strength differs whether or not the flange buckles inward or outward. Further, it is shown how and why the introduction of a small longitudinal stiffener in the web can have a marked change in the distortional postbuckling response of the member.

With proper care, nonlinear finite-element analysis is shown to be a good predictor of tested behavior and strength for cold-formed steel members comprised of high-strength steel with a nominal yield stress of $550\mathrm{MPa}$ $\left(80\mathrm{ksi}\right)$ , as shown in “Numerical simulation of high strength steel box-shaped columns failing in the local and overall buckling modes” by Yang and Hancock. The paper builds upon an extensive set of tests previously conducted by the authors and provides specific guidance for using ABAQUS to accurately predict high-strength cold-formed steel members.

The second group of six papers presented in this issue of the Journal covers cold-formed steel walls, shear walls, and frames. Conventional cold-formed steel wall systems consist of lipped channels (studs) with unlipped channels (track) capping the top and bottom. The studs generally rely on midheight bridging to brace against weak-axis flexure. Green, Sputo, and Urala in “Strength and stiffness of conventional bridging systems for cold-formed cee studs” provide, for the first time, experimental results on the stiffness and strength of bridging details used on studs in current practice. They conclude that that bridging used in conventional North American practice has adequate stiffness and strength to brace axially loaded and curtain wall steel studs.

Cold-formed steel shear walls have been the subject of considerable recent research, particularly with regard to seismic performance of these systems. Two companion papers from researchers at the University of Naples “Seismic behavior of sheathed cold-formed structures: Physical tests” and “Seismic behavior of sheathed cold-formed structures: Numerical study” by Corte, Fiorino, and Landolfo provide an overview of cold-formed steel shear wall research and add their own testing and analysis results to the body of work. The testing and analysis are used to provide recommended $R$ factors for this building system. The authors demonstrate that adequate ductility in cold-formed steel shear walls can be achieved experimentally, and ensured in design, if seismic capacity-based design principles are correctly employed.

Success of cold-formed steel shear walls relies a great deal on adequate performance of the connections. Indeed, much of the observed nonlinearity from tests on shear walls derives from the connection behavior. In “Design criteria for seam and sheeting-to-framing connections of cold-formed steel shear panels” Fülöp and Dubina provide results from small-scale connection tests and demonstrate how these results may be combined with detailed finite-element models to replicate full-scale shear wall testing. The work focuses primarily on shear walls employing corrugated steel panels but also includes OSB sheathed walls.

In “Cold-formed steel frame shear walls utilizing structural adhesives” Serrette, Lam, Qi, Hernandez, and Toback examine a wall system where OSB sheathing is attached to the steel studs by adhesive and pneumatically driven pins, instead of screws. Much of the initial static and cyclic research work conducted on conventionally attached, sheathed, cold-formed steel shear walls in the United States was completed by Serrette. Here, Serrette and his colleagues demonstrate that adhesives and pneumatic pins may provide a shear wall with reasonable strength and ductility and are a particularly promising new solution for narrow shear walls.

Cold-formed steel framing for load bearing low-rise construction generally employs in-line framing and platform construction methods. Walls, floors, and rafters, etc. are constructed out of commodity plain channel and lipped channel cross-sections. Lateral resistance of the walls comes from sheathing or adding steel straps to the walls, as discussed in the previous papers. However, other ideas also persist, including the use of specialized cold-formed steel cross sections and fully connected frames. In “Experimental study of connections for cold-formed steel portal frames” Kwon, Chung, and Kim present the details of a portal frame system utilizing high strength, $570\mathrm{MPa}$ $\left(83\mathrm{ksi}\right)$ yield, specially fabricated box sections, with unique connections details, tested in Korea. The authors provide details of finite strip analysis, connection tests, and portal frame tests that demonstrate the behavior of this system.

The third group of three papers presented in this issue of the Journal covers research on cold-formed steel trusses. Cold-formed steel trusses may be constructed from conventional plain channel and lipped channel sections or may use proprietary shapes designed specially for providing strength against local buckling and for providing convenient locations for connections as diagonals frame into chords.

Heel plate configurations for conventional cold-formed steel roof trusses are studied in detail by Dawe and Wood in “Investigation into the behavior of heel connections for use in cold-formed steel trusses.” Through a combination of small-scale experiments and finite-element analysis, the authors demonstrate the sensitivity of the truss strength to detail changes made in the heel connection. The same two authors also examined “Cold-formed steel roof trusses subjected to concentrated panel point loading.” In this second paper, full-scale testing of conventional cold-formed steel trusses was employed to examine the ramification of different limit states, including those from concentrated loads. The 2001 Canadian code, essentially the same as the current North American Specification for the Design of Cold-Formed Steel Structural Members, is found to be conservative in its strength prediction of the locally loaded truss.

The final paper in this group on trusses examines a variation of the hat-shaped chords that are often used in proprietary truss configurations. This paper could have easily been grouped with the first set of papers on member behavior and distortional buckling because Nuttayasakul and Easterling in “Behavior of complex hat shapes used as truss chord members” focus primarily on distortional buckling of these special truss members. The authors provide design guidance verified by testing and modeling for distortional buckling of these unique shapes.

The special issue concludes with two Technical Notes. The first, by Pham, Mills, and Zhuge from Australia, provides testing related to “Experimental Capacity Assessment of Cold-Formed Boxed Stud and C Stud Wall Systems used in Australian Residential Construction.” The final Note, by Chodraui, Neto, Gonçalves, and Malite from Brazil, examines “Distortional buckling of cold-formed steel members” and provides an examination of a simplified design method employed in the Brazilian code for distortional buckling.