CFS Bracing Design Using Combinations of Discrete and Sheathing Bracing

Final Project ReportThe objective of this report is to summarize efforts related to establishing the behavior and design of cold-formed steel wall assemblies potentially braced by discrete steel bracing and wall sheathing. Cold-formed steel wall assemblies are typically designed with discrete steel braces; however, the predicted accumulation of brace forces in these systems can lead to demands at odds with past successful practice. At the same time, testing on sheathing braced wall studs demonstrate the effectiveness of sheathing bracing, but questions persist about their effectiveness in extreme conditions. It is hypothesized that a combined bracing system could provide adequate strength from the steel system under extreme (environmental) conditions and benefit from the effectiveness of sheathing bracing in service and other conditions. A focused series of experiments on wall stud assemblies in compression with different combinations of discrete bracing and sheathing bracing were completed. The testing indicated that sheathing bracing dominates the braced stud response and that brace forces in discrete braces do not accumulate when sheathing is present. A series of spreadsheets were developed to provide engineers with a clear and efficient means to predict the strength of studs with both discrete bracing and sheathing bracing. The proposed strength calculations agree with the limit states observed in past and current testing; however, unlike in previous work where predicted strengths were conservative, in the testing conducted in this project the predicted strength is from 7 to 19% unconservative depending on the details of the tested specimen. Enabling combined bracing in cold-formed steel design requires modifications to AISI S100 and AISI S240 and a series of ballots that would be necessary for adoption of the new methods are detailed and priorities for adoption given. It is recommended that additional testing and design aids be developed.American Iron and Steel Institute (AISI), Steel Framing Industry Association (SFIA

Design of Metal Building Roof Purlins Including System Reliability Effects

Final Project ReportThis report provides a framework to incorporate structural system reliability effects in the design of roof purlins in a typical metal building. Today every roof purlin is considered as a separate component and the effect of spatial variation in the demand loads and potential redistribution and load sharing in the roof system capacity are ignored in design. Component reliability is established by first-order reliability methods implemented through load and resistance factor design. Based on recent work in loading bearing cold-formed steel framing systems the load and resistance factor design framework is extended from components to systems through an additional resistance factor to account for system influence. An archetypical metal building is designed and selected for this study. Monte Carlo simulations of a segment of the metal building roof are performed with consideration of both randomness in the demands and capacity and employing geometric and material nonlinearity in the response model of the roof. The simulations indicate that the system effect in metal building roofs is beneficial, and increases in the design capacity when evaluated against demands may be appropriate. Sensitivity to the target reliability (allowed probability of failure), deflection limits, and modeling assumptions are observed and discussed. Preliminary factors to account for roof system reliability are provided.Metal Building Manufacturers Association (MBMA

Distortional Buckling of Cold-Formed Steel Columns

Research on the distortional buckling of cold-formed steel columns, primarily C and Z shapes is summarized in this document.American Iron and Steel Institut

Sheathing Braced Design of Wall Studs

This report provides a summary document and final report for the multi-year project on Sheathing Braced Design of Wall Studs conducted at Johns Hopkins University. This project examined the axial behavior and axial + bending behavior of cold-formed steel stud walls braced solely by sheathing connected to the stud and track flanges. This report is a practical summary of the work: the history of sheathing braced design, derivations of new analytical methods, testing on components and full sheathing-braced walls, modeling of sheathing-braced members, and the development of the proposed design method are all provided in accompanying documents. Here the focus is on the proposed design method and its application.American Iron and Steel Institut

Experimental Study on the in-plane behavior of standing seam roof assembly and its use in lateral bracing of rafters

Final Test ReportThe standing seam roof (SSR) system is the most commonly used roof system for metal buildings due to its superior durability, water tightness, and energy efficiency. In this type of system, SSR panels attach to Z-shaped or C-shaped purlins with clips, and the purlins are in turn connected to rafters (i.e. roof beams). For the design of the rafters against lateral torsional buckling, bottom flange braces provide torsional bracing to the rafter and the SSR system provides some lateral bracing. However, the degree to which the SSR system can restrain the rafter against lateral movement has not previously been studied. The objective of this study is to quantify the in-plane strength and stiffness of the SSR system and identify how this can be used to provide lateral bracing to the rafter. A total of 11 full-scale standing seam roof specimens were tested to investigate the effects of different standing seam roof configurations (SSR panel type, clip type, thermal insulation, and purlin spacing) on the in-plane stiffness and strength of the SSR system. The resulting stiffness and peak strength of the specimens were tabulated and compared for different SSR configurations. Results showed that the in-plane load-deformation behavior of SSR systems was governed by clip deformations and that variations in the type of SSR panel or clip can have a major impact on the strength and stiffness of the specimens. A specimen with vertical rib panels was shown to have 16 times more stiffness than a similar specimen with trapezoidal rib panels because the vertical ribs restrain the clip deformation. However, even a small standoff was found to reduce the stiffness of vertical rib SSR assemblies with more than three-fold drop in stiffness as the standoff was increased from 0 in. to 0.4 in. Trapezoidal rib SSR assemblies had consistent strength stiffness with fixed clips having standoff of 0 in. or 0.5 in., but with floating clips the stiffness decreased with increasing standoff. Addition of blanket insulation and thermal blocks were found to result in 60% to 350% increase in stiffness. A method for using these experimental results in calculations of required bracing for metal building rafters is described. An example is also provided which demonstrates that the SSR roof can contribute to bracing of the rafter and may reduce spacing or size of discrete/point torsional braces.American Institute of Steel Construction (AISC), American Iron and Steel Institute (AISI), Steel Deck Institute (SDI), Steel Joist Institute (SJI), Metal Building Manufacturers Association (MBMA), National Science Foundation (NSF

Analytical Study on Rotational Restraint of Sheathing

Building upon a previous set of experiments performed to determine connection rotational restraint provided by sheathing in cold-formed steel floor joists, this study investigates the reliability of reported connection rotational stiffness values. Key assumptions made by the experimental researchers during testing and post processing of experimental data are explored. Effects observed but not measured during testing, including fastener pullout failure and construction flaws, are also examined

Hysteretic shear response of fasteners connecting sheathing to cold-formed steel studs

The series of experiments reported here aims to characterize the hysteretic behavior of the connection between cold-formed steel (CFS) studs and sheathing when subject to in- plane lateral demands. This connection provides the key energy dissipating behavior in wood sheathed CFS shear walls, and provides bracing to the studs under gravity and out- of-plane loads. A testing rig is developed consisting of two CFS lipped channels facing toe-to-toe connected on the flanges by sheathing (oriented strand board, or gypsum board) and cycled such that the 8 connecting fasteners experience shear. Sheathing configuration, fastener spacing, steel thickness, and fastener type are varied to determine connection performance. The dominant role of sheathing type and stud thickness is highlighted in the results. The hysteretic behavior of the experimental results is summarized for further use in the analysis of shear walls and under gravity and lateral load. The work serves as a supplement to North American efforts to advance seismic performance-based design of CFS structures and is part of a larger effort to better understand CFS lateral force resisting systems.National Science Foundatio

Inelastic Bending Capacity in Cold-Formed Steel Members

The enclosed research report has been prepared for the AISI as supporting material for proposed additions to the AISI Specification (AISI-S100-07) with respect to inelastic bending of cold- formed steel flexural members. In particular, an extension to the Direct Strength Method of Appendix 1 of AISI-S100-07 is proposed which allows for design capacities to exceed My (and approach Mp) as a function of the slenderness in the local-global or distortional modes

Moment-Rotation Characterization of Cold-Formed Steel Beams

The objective of this study is to provide a prediction method for characterizing the complete moment-rotation (M-!) response of cold-formed steel (CFS) members in bending. The work is an ancillary effort related to the National Science Foundation funded Network for Earthquake Engineering Simulation (NEES) project: CFS-NEES (www.ce.jhu.edu/bschafer/cfsnees). The goal of CFS-NEES is to enable performance-based seismic design for cold-formed steel framed buildings. A basic building block of performance-based seismic design is nonlinear structural analysis. For cold-formed steel members, which suffer from local and distortional buckling, existing codes provide peak strength and approximations for stiffness loss prior to peak strength, but no estimation of post-peak M-! behavior. Complete M-! response is necessary for nonlinear structural analysis of CFS framed buildings. In this research, existing data, obtained by experiments and finite element analysis, are processed to examine the complete M-! response in cold-formed steel beams. Using a modification of the simplified model introduced in ASCE 41 for pushover analysis, the M-! response is parameterized into a simple multi-linear curve. The parameters include the initial stiffness, fully effective limit, reduced pre-peak stiffness, peak moment, post-peak plateau, and post-peak rotation at 50% of the peak moment. It is shown herein that the parameters of this multi-linear M-! curve may themselves be readily predicted as a function of either the local slenderness or distortional slenderness of the cross-section, as appropriate. Accuracy of the proposed M-! approximation is assessed. The impact of utilizing the full M-! response in a single and multi-span CFS beam is demonstrated. The proposed prediction method for M-! provides a necessary step in the development of nonlinear structural analysis of CFS systems