Designing Cold-formed Steel Using the Direct Strength Method

The Direct Strength Method is an entirely new design method for cold-formed steel. Adopted in 2004 as Appendix 1 to the North American Specification for the Design of Cold-Formed Steel Structural Members, this paper introduces the Direct Strength Method and details some of the features of a new AISI Design Guide for this Method. The intent of this paper and the Guide is to provide engineers with practical guidance in the application of this new design method. The Direct Strength Method does not rely on effective width, nor require iteration for the determination of member design strength. Instead, the engineer must determine the elastic buckling load in local, distortional, and global buckling. This information along with the load that causes first yield are then employed in a series of simple equations to “directly ” provide the strength prediction. The primary complication with the method lies in determining the elastic local, distortional, and global buckling loads; once these values are determined application of the method is straightforward. Computational tools, such as the freely available open source program CUFSM, can provide the elastic buckling loads that the Direct Strength Method requires. This paper will highlight some of the features of the new Direct Strength Method Design Guide, including design examples, tutorial materials, beam and column charts, and discussion of the finer points and details that could trip up the conscientious engineer when first using the method in design

Stiffened Elements with Multiple Intermediate Stiffeners and Edge Stiffened Elements with Intermediate Stiffeners

Section B5 of the AISI Specification, covering stiffened elements with multiple intermediate stiffeners and edge stiffened elements with intermediate stiffeners, has been entirely replaced in the latest edition of the Cold-Formed Steel Specification (NAS 2001). The new design rules are based primarily on the work of Schafer and Pekoz (1998); however, subsequent to that work and prior to adoption by AISI, additional work was also completed. For elements with multiple intermediate stiffeners consideration of weblflange interaction was added. The resulting expressions are shown to agree well with both experimental and numerical data. New provisions for edge stiffened elements with intermediate stiffeners have also recently been adopted. The logic behind the development of these provisions is discussed herein. Compared with previously used procedures (AISI 1996) the new methods provide a more robust and reliable method for the design of these unique elements

Distortional Buckling Tests on Cold-formed Steel Beams

Laterally braced cold-formed steel beams generally fail due to local or distortional buckling. When the compression flange is not restrained by attachment to sheathing or paneling, such as in negative bending of continuous members (joist, purlins, etc.), members are prone to distortional failures. However, distortional buckling remains a largely unaddressed problem in the main body of the North American Specification (NAS 2001). Only a limited amount of experimental data on unrestricted distortional buckling in bending is available, therefore a new series of distortional buckling tests was completed. The test details are selected specifically to insure that distortional buckling is free to form, but lateral buckling is restricted. Several design methods, including those of the U.S., Canada, Australia, and Europe as well as the Direct Strength Method are compared with the test results. Combined with our previously conducted local buckling tests (Yu and Schafer 2003), we can now provide experimental upper and lower bounds for the capacity of laterally braced cold-formed steel beams in common use in North America. Further, the experimental results have been investigated and extended through the use of non-linear finite element analysis with ABAQUS. This paper covers the setup of the distortional buckling tests, test results, finite element analysis and discussion of current design methods

Laser Scanning to Develop Three-Dimensional Fields for the Precise Geometry of Cold-Formed Steel Members

Geometric imperfections play an important role in the performance and behavior of cold-formed steel members. The objective of this paper is to detail a newly developed imperfection measurement rig, where the full three-dimensional (3D) imperfect geometry of a cold-formed steel member can be measured and reconstructed. The measurement results in a dense three-dimensional point cloud that may be utilized to provide precise knowledge of the basic member dimensions (width, angle, radius including variation along the length), frequency content in the member (waviness, local dents, etc.), or directly as the exact geometry of the member. Practical applications of the data include basic quality control; however, the potential of the data is truly realized when applied to shell finite element models of cold-formed steel members to investigate imperfection sensitivity. The measurement rig set-up (Phase I) consists of three basic parts: a two-dimensional (2D) laser scanner with measurement range up to 304 mm [12 in.]; a linear drive system, allowing the laser to collect measurements of cross sections along the length of the target specimen; and a support beam. The raw point cloud data from the Phase I rig is input into MATLAB where custom postprocessing is employed to develop the full 3D reconstruction of the target specimen. The Phase II rig adds a rotary ring, providing a rotational stage for the laser so that the cross section of the target specimen may be profiled from any direction. This paper provides several examples of full-field imperfection measurements and compares against other methods in current use. The measured imperfections contribute to the database of realized imperfections appropriate for the generation of stochastic imperfections for use in simulation. Accurate knowledge of geometric imperfections is critical to the long-term success of analysis-based design paradigms for cold-formed steel

Local Buckling Tests on Cold-formed Steel Beams

C- and Z-sections are two of the most common cold-formed steel shapes in use today. Accurate prediction of the bending performance of these sections is important for reliable and efficient cold-formed steel structures. Recent analytical work has highlighted discontinuities and inconsistencies in the AISI (1996) design provisions for stiffened elements under a stress gradient (i.e., the web of C- or Z-sections). New methods have been proposed for design, and an interim method has been adopted in the NAS (North American Specification 2001). However, existing tests on Cs and Zs do not provide a definitive evaluation of the design expressions, due primarily to incomplete restriction of the distortional buckling mode. Described in this paper are a series of flexural tests with details selected specifically to insure that local buckling is free to form, but distortional buckling and lateral-torsional buckling are restricted. The members selected for the tests provide systematic variation in the web slenderness (h/t) while varying other relevant non-dimensional parameters (i.e., h/b, b/t, d/t, d/b). Initial analysis of the completed testing indicates that overall test-to-predicted ratios for AISI (1996), S136 (1994), NAS (2001) and the Direct Strength Method (Schafer 2002) are all adequate, but systematic error is observed in AISI and S136 due to web/flange interaction