Special Issue on Cold-Formed Steel Structures

Because of the recent increase and diversity in commercial activity dealing with the usage of cold-formed steel structures in the construction industry, the research community working with this type of structure has been faced with new challenges. This has fostered a fairly large number of considerable advances in structural engineering topics related to cold-formed steel applications. The aim of this special issue of the Journal of Structural Engineering is to put together, in a single comprehensive issue, a representative sample of the newest and most relevant analytical, numerical, and experimental original research devoted to investigating the behavior and design of cold-formed steel structures around the world. Indeed, this issue includes contributions from researchers working in 13 countries, located on four continents.

This special issue contains 18 technical papers and one technical note covering a wealth of topics concerning cold-formed steel structural systems. Without claiming to be exhaustive, it is possible to find papers dealing with members (columns and beams), panels, frames, walls, shear walls, and connections. Overall, the papers describe specific analytical, numerical, or experimental investigations that (1) address first-order buckling, postbuckling, or ductility problems; (2) involve static or dynamic (including seismic) analyses; (3) cover both carbon and stainless cold-formed steels, and (4) are concerned mainly with behavioral aspects or focus on design issues; namely, the development of rules for codes and specifications. For organizational purposes, the papers are divided into four groups, each one corresponding, as much as possible, to a given structural system type. The first group is comprised of eight papers on the analysis and design of isolated cold-formed steel structural elements, including seven papers on members and one on panels. The second group includes three papers that are concerned with the behavior and design of cold-formed steel connections, one aiming specifically at studying effects resulting from elevated temperatures. The technical note that closes the special issue also addresses a connection strength problem. Next, the third group consists of four papers that report investigations on the static behavior and design of cold-formed steel walls (one paper) and the cyclic/dynamic behavior and performance of shear walls (three papers). Finally, the last group contains the remaining three papers, all concerning cold-formed steel frames; two papers on buildings and one on storage racks. The content of each of the papers included in the special issue is succinctly outlined in the subsequent sections.

Keerthan and Mahendran (University of Queensland, Brisbane, Australia) present a numerical investigation of the shear behavior and strength of LiteSteel beams (LSBs) with web openings. The LSB is a new cold-formed steel hollow flange channel section, which is produced by a combination of cold-forming and welding and used as floor joists and transfer beams in residential, industrial, and commercial buildings. After showing that the existing design equations do not provide acceptable estimates of the beam shear capacity, an alternative shear design method is proposed, which compares well with the presented numerical values and experimental results. The next paper, coauthored by Basaglia and Camotim (Technical University of Lisbon, Lisbon, Portugal), deals with the numerical assessment of the buckling, postbuckling, strength, and collapse of lipped channel continuous beams subjected to nonuniform bending. Using results obtained from buckling analyses based on generalized beam theory and shell finite-element first-order plastic analyses, it is shown that the current direct strength method (DSM) design equations, duly modified, may be used to adequately predict the load-carrying capacity of continuous beams. The DSM is also used in the investigation presented in two papers; one by Casafont et al., which stemmed from a cooperation between the Universitat Politècnica de Catalunya (Barcelona, Spain) and Cornell University (Ithaca, New York); and the other by Silvestre et al., originating from the Technical University of Lisbon. The first paper aims at predicting the load-carrying capacity of perforated storage rack columns and provides evidence that a DSM approach can be readily used to adequately estimate the ultimate strength of columns failing in distortional and global (flexural-torsional) modes; however, this approach is ineffective in predicting failure loads associated with predominantly local modes. On the other hand, the second paper revisits the design of equal-leg angle with short-to-intermediate lengths (i.e., those buckling in flexural-torsional modes) and develops distinct DSM-based procedures for fixed- and pin-ended columns that lead to accurate ultimate strength estimates in a wide slenderness range. These procedures are shown to provide accurate ultimate strength predictions while retaining the simplicity and LRFD resistance factor ($\varphi =0.85$) of the current DSM.

The remaining four papers in this group deal with experimental investigations. Moen et al. (Virginia Tech, Blacksburg, Virginia) explore experimentally the load-deformation response and capacity of cold-formed steel C-section joists with discrete unstiffened web holes susceptible to distortional buckling. The web holes amplify the distortional deformations in the joist and reduce its load-carrying capacity, thus evidencing the need for a forthcoming new DSM design approach. The next paper, coauthored by Dubina et al. (Politehnica University of Timisoara, Timisoara, Romania), reports the results of an experimental study involving pallet rack uprights (with and without perforations), with the intention of evaluating their ultimate load erosion stemming from distortional-flexural buckling interaction. The test values are subsequently used to calibrate a design approach based on the erosion of critical bifurcation load concept. Next, Afshan and Gardner, from Imperial College (London), coauthor the only paper of this special issue dealing with cold-formed stainless steel. The work displays experimental results concerning material, cross-section, and member tests involving several steel grades and including rigorous measurements of the local and global initial geometrical imperfections exhibited by the test specimens. These results are then used (1) to assess the applicability of the current European and North American design provisions to ferritic stainless steel structural components, and (2) to compare the structural performance of this relatively low-cost stainless steel grade with that of those more commonly used. Finally, the last paper in this group by Ádány et al. reports the results of experimental studies carried out at the Budapest University of Technology and Economics (Hungary) on panels built from deep trapezoidal sheeting with perforated webs. The purpose of these studies, which also include panels without perforations, is to assess how the perforations erode the panel stiffness and/or resistances associated with response under a variety of loads; i.e., bending, shear, and web crippling. On the basis of the test results, reduction factors for stiffness and resistance, intended to be used with the design rules prescribed in Eurocode 3, are proposed.

Teh and Gilbert (University of Sydney and Gold Coast Griffith University, Australia) examine the accuracy of equations specified by the North American and Australasian steel codes for predicting the net section tension capacity of channel braces bolted at the web. The strength depends chiefly on three effects; i.e., in-plane shear lag, out-of-plane shear lag, and bending moment as a result of connection eccentricity. After concluding that no existing equations are adequately reliable, the authors propose a novel equation that incorporates explicitly the aforementioned three effects and outperforms the existing expressions. The second paper, coauthored by Bolandim et al. (University of São Paulo, São Paulo, Brazil), reports experimental results concerning the net section rupture of bolted connections in angles and channels. The test results are used to evaluate connection strength predictions according to the Brazilian and North American specifications. Errors in the predictions are incorporated into a reliability analysis that suggests further calibration of the strength equations for bolted connections is needed. The last paper in this group comes from The University of Hong Kong and is the only one in this special issue to address temperature effects. The paper details experimental research work carried out by Yan and Young on double shear bolted connections subjected to elevated temperatures, which may fail in bearing, tear out, net section tension, bolt shear, and/or material failures. It is shown that the current North American, Australian/New Zealand, and European specifications, based on reduced material properties (because of the elevated temperature), consistently underestimate the experimental ultimate strength values.

Finally, it is worth mentioning that the technical note included in this special issue, which is coauthored by Yu and Xu (University of North Texas, Denton, Texas), also deals with connections. The note concerns a parametric experimental study to assess the sheet shear strength in bolted connections involving oversized holes. The parameters considered are the sheet thickness, connection type (single or double shear), and bolt diameter. The best shear strength predictions are achieved by using the method prescribed in the previous version of American Iron and Steel Institute (AISI) Specification AISI S100, provided that newly proposed resistance factors are adopted.

On the basis of extensive analytical, numerical, and experimental studies dealing with various behavioral features exhibited by walls framed from studs and tracks that rely on traditional sheathing materials as primary bracing against compressive loads, Vieira and Schafer (Johns Hopkins University, Baltimore) develop a robust design method to estimate the ultimate strength of such walls. Unlike its predecessors, the proposed method is able to handle dissimilar sheathing attached to the stud flanges, and is shown to provide consistently accurate estimates of the values obtained from experimental tests. In the work of Ozaki et al. (Nippon Steel, Japan) the notion of replaceable fuses for dissipating seismic demands—an idea that has been successfully applied to hot-rolled steel structures—is shown to be valid and useful for cold-formed steel framed structures with shear walls as the primary lateral force resisting system. The fuses are utilized inside novel shear wall hold-downs and are shown to significantly increase energy dissipation and decrease peak drift demands. Traditional sheathing materials adopted in shear walls can only provide lateral capacities within certain limits. As a result, flat and corrugated steel sheet shear walls have recently been investigated. The shear buckling that occurs in the sheet can lead to large boundary/connector forces, a result that has presented a number of challenges in the research. Shimizu et al. (Nippon Steel and Toyko University of Technology) have studied this problem and developed details ensuring that corrugated steel sheet shear walls fail in a stable, nicely dissipative, end failure mechanism. This mechanism appears to be very promising in providing improved strength and ductility to corrugated steel sheet diaphragms and shear walls. Meanwhile, Shamim et al. (McGill University, Montreal) performed single and multistory shake table tests to acquire further insight into the behavior of steel sheet sheathed shear walls. The testing demonstrates the importance of a careful capacity-based design philosophy for detailing the chord studs, and provides a full characterization of the nonlinear response of steel sheet sheathed shear walls appropriate for nonlinear time history modeling.

The development of cold-formed steel systems in China is now proceeding at a rapid pace. Li et al. (Tongji University, Shanghai, China) provide a rare summary of the extensive research that has been conducted over the last 5 years in order to update the Chinese cold-formed steel code. The paper reports on efforts in basic element behavior, concentrically and eccentrically loaded compression members, flexural members, reliability analysis, screw connections, shear walls, trusses, and seismic performance of full buildings. In Dao and van de Lindt’s work (University of Alabama, Tuscaloosa, Alabama) the focus shifts from code development to innovation in new systems for light steel framing. The authors explore a novel system including floor trusses, open panels, V-braced panels, columns, and connections. Energy dissipation in the walls primarily relies on bending of the gusset plate at the end of the V-bracing, and is shown to be a reliable mechanism in tests. In the final paper in this group, Rasmussen and Gilbert (University of Sydney and Gold Coast Griffith University) provide a glimpse into the future for analysis-based design of cold-formed steel frames, with the focus being on storage racks. Based on the new Australian Standard for Steel Storage Racks, guidance is provided for analysis-based design, enabled by material and geometric nonlinear analysis, for both conventional beam-element models and full-shell finite-element models that include cross-section distortion.

Before closing, the authors of the papers included in this special issue are thanked for the quality of their work, and also for their patience and support with the preparation and revision of their manuscripts. It was a real pleasure to interact and cooperate with each of them. Likewise, the help of the reviewers is greatly appreciated because their valuable criticisms and suggestions contributed decisively to improving the technical content. Within this context, Dr. Roger LaBoube, Director of the Wei-Wen Yu Center for Cold-Formed Steel Structures, deserves a special acknowledgment because he provided excellent reviews for several papers. Finally, sincere gratitude is also expressed to Dr. Sherif El-Tawil, Chief Editor of the Journal of Structural Engineering, who suggested, and enthusiastically supported, the publication of this special issue.