2012 Research Reports No R924-R930

Abstract
For cold-formed channel section design in shear, the traditional approach has been to investigate shear plate buckling in the web alone. Recently, an improvement in the elastic buckling stress of the whole thin-walled channel section including flanges and lips in pure shear has been demonstrated. For webs with relatively large depth to thickness ratios, the local buckling mode in shear occurs mainly in web. The structural efficiency of such a web can be improved by adding intermediate stiffeners cold-formed longitudinally in the middle of the webs.

This report presents numerical buckling analyses implemented by means of the Semi-Analytical Finite Strip Method (SAFSM). The shear signature curve from the SAFSM is used in a design proposal for a newly developed Direct Strength Method (DSM) for shear. The DSM was formally adopted in the North American Design Specification in 2004 and in the Australian/New Zealand Standard for Cold-Formed Steel Structures (AS/NZS 4600:2005) in 2005 as an alternative to the traditional Effective Width Method (EWM). The theory and development of the shear signature curve has been clearly presented and discussed by Hancock and Pham (2011). The objective of this report is to apply this methodology to investigate the effect of web stiffeners on the elastic shear buckling stress by varying the number, shapes and locations of the longitudinal web stiffeners. A series of shear signature curves and corresponding buckling mode shapes are studied in three different cases of web stiffener geometry where the variables are stiffener position and dimensions. The results from the analysis are included to identify local and distortional buckling caused by shear stresses. The explanation of the occurrence or disappearance of the minima of the shear signature curves where local or distortional buckling occur is also discussed.

Keywords
Cold-formed steel; Web stiffener; Elastic buckling; Complex channel sections; High strength steel; Direct strength method.

Abstract
Plain C or Z-sections are two of the most common cold-formed steel shapes in use throughout the world. Other shapes are high strength SupaCee® and SupaZed® steel sections which are widely used in Australia as purlins in roof and wall systems. They contain additional return lips and web stiffeners which enhance the bending capacity of the sections. Design methods for these sections are normally specified in the Australian/New Zealand Standard for Cold-Formed Steel Structures (AS/NZS 4600:2005) or the North American Specification for Cold-Formed Steel Structural Members (NAS S100-2007). In both Standards, which include the newly developed Direct Strength Method of design (DSM), the method presented (Chapter 7 of AS/NZS 4600:2005, Appendix 1 of NAS) is developed for beams and columns, including the reliability of the method. This report presents two different test series on both plain C- and SupaCee® sections in pure bending. They were performed at the University of Sydney for the extension of the DSM to include channel section beams with complex stiffeners. Two different section depths and three different thicknesses of high strength lipped channel sections were tested in pure bending. Tests with and without torsion/distortion
restraint straps screwed on the top flanges in the pure bending region were also considered to allow local and distortional buckling to form in the sections respectively. Test results and formulae developed from the DSM are summarized in the report. Three different cases where moments are used in association with yield, inelastic or plastic criteria in the DSM local and distortional strength equations are compared with the test data. By comparisons between cases, a proposed recommendation for DSM inelastic buckling strength design in pure bending with extended non-dimensional slenderness limit for both local and distortional buckling is given in the report.

Keywords
Cold-formed steel; Web stiffener; Elastic buckling; Complex channel sections; High strength steel; Direct strength method.

Abstract
This paper surveys trends in the analysis and design of steel framed structures with reference to design codes such as the US AISC Specification, the UK BS5950, the Australian AS4100, the European EC3, and the Hong Kong Code of Practice .

The paper provides a brief timeline of the development of steel design codes over the past 80 years, summarises the methods of analysis and design now permitted in codes, discusses some of the shortcomings of present design codes, and suggests future areas for improvement.

It is concluded that future design codes might allow the use of purpose-built computer programs which can provide accurate predictions of member strength, and might only describe the characteristics of the methods of structural analysis and the member design strengths which may be used. Such a code would have some of the present member strength inaccuracies and shortcomings removed and allow them to be replaced by the more accurate member strength computer programs.

Keywords
Analysis, codes, design, frames, steel, structures.

Abstract
This report presents an experimental investigation into the behaviour of ultra light-gauge steel storage rack uprights subjected to compression. Two different types of members with varying lengths are tested and while the combined effects of local and distortional buckling are investigated, special attention has been given to longer specimens that fail by flexural-torsional buckling. Deformations experienced during testing by all of the specimens were measured and observations regarding failure modes have been documented. In addition, the geometric imperfections of each member were measured before testing, as were the material properties of the cold-rolled sections and the virgin steel from which the sections were formed. This report details the observed failure modes, the recorded ultimate strengths and load-deflection responses.

Keywords
Interactive Buckling, Cross-sectional Imperfections, Local Buckling, Distortional Buckling, Flexural-Torsional Buckling, Cold Formed Steel, Ultra-Light Gauge Steel, Storage racks

Abstract
The elastic post-torsional-buckling behaviour of a simply supported cruciform column has been analysed, and its strength has been approximated by assuming that it fails when it first yields due to the maximum normal stress induced by the axial compression.

However, torsional shear stresses are induced in the post-buckling regime. The purpose of this note is to investigate the effects of these shear stresses on the first yield strength.

Keywords
Buckling; Columns; Cruciforms; Normal stress; Post-buckling; Shear stress; Steel; Torsion; Yield

Abstract
This report presents a numerical solution to model a simply supported cold-formed steel beam subjected to the pure shear load case. The modelling procedures including the unique concept of pure shear loads in the Finite Element Method (FEM) are discussed thoroughly. The accuracy of the simulation is confirmed by comparison with the available test data. Based on this model, a number of shear strength analyses were conducted on cold-formed channel members with plain webs and various types of web stiffeners. The outcomes are graphically shown in two formats and related discussions are included. Application to the Direct Strength Method (DSM) of design of cold-formed sections is presented.

Keywords
Cold-formed steel; Web stiffener; Shear strength; Pure shear; Complex channel sections; High strength steel; Direct strength method; ABAQUS.