Gregory J. Hancock

Cold-formed steel structures

Abstract Cold-formed steel structures are steel structural products that are made by bending flat sheets of steel at ambient temperature into shapes which will support more than the flat sheets themselves. They have been produced for more than a century since the first flat sheets of steel were produced by the steel mills. However, in recent years, higher strength materials and a wider range of structural applications have caused a significant growth in cold-formed steel relative to the traditional heavier hot-rolled steel structural members. This paper contains three components. First, it reviews and summarises the major research developments in cold-formed steel structures over the last 3 years (1999–2001) as published in leading journals on steel structures and thin-walled structures. Secondly, it summarises the development of the North American Specification for the Design of Cold-Formed Steel Structural Members. Finally, it provides a brief summary of the Direct Strength Method being developed by the American Iron and Steel Institute Specification Committee.

Buckling of thin flat-walled structures by a spline finite strip method

Abstract A method of buckling analysis of thin flat-walled structures of finite length subjected to longitudinal compression and bending, transverse compression as well as shear is described. The analysis uses the spline finite strip method and allows for boundary conditions other than simply supported ends as required in the semi-analytical finite strip method of buckling analysis. Convergence studies with increasing numbers of section knots are described for plates in compression, bending and shear, and for long columns with different support conditions subjected to compression. A buckling analysis of a stiffened plate subjected to compression and shear is compared with results from a finite element analysis.

Computer analysis of thin-walled structural members

Abstract The calculation of the stresses and failure modes in thin-walled structural members is a complex procedure. Structural designers will often need help in analysing these types of structures. A vehicle for providing this help is the computer program developed for the microcomputer. In this paper, a computer procedure is described for the cross-section analysis and elastic buckling analysis of thin-walled structural members. The cross-section analysis calculates the section properties, warping displacements, and the longitudinal and shear stresses for thin-walled open and closed cross-sections of any shape. The longitudinal stresses are used to perform an elastic finite strip buckling analysis of thin-walled structural members. The analysis can be done for a number of different buckle half-wavelengths of the member and the load factor and buckled shape are output for each length. The analysis is performed by the user-friendly computer program THIN-WALL, which is also described in the paper.

Plate slenderness limits for high strength steel sections

Abstract The paper describes a test programme on stub columns fabricated from BISALLOY 80 steel plates with nominal yield stress of 690 MPa. The programme comprises 18 box, cruciform and I-section specimens. The slenderness of the component plates are chosen in the vicinity of the yield slenderness limit, beyond which the stub column strength is reduced below the squash load as a result of local buckling. The purpose of the tests was to determine whether the yield slenderness limits used in the Australian Steel Structures Standard AS 4100 are applicable to BISALLOY 80 steel plates. The paper also assesses whether the plate strength rules of the Australian Steel Structures Standard AS 4100, the Load and Resistance Factor Design Specification of the American Institute of Steel Construction (AISC-LRFD), the British Standard BS 5950 Part 1 and the European Convention for Structural Steelwork Eurocode3 are applicable to high strength steel. For this purpose, the plate strength curves of these specifications are compared with the BISALLOY 80 test results and with test results from the USA and Japan on high strength steel plates with nominal yield stress of 690 MPa.

Tests of high strength steel columns

Abstract The paper describes a test programme on columns fabricated from high strength steel plates with nominal yield stress of 690 MPa. The programme comprised 13 box and I-section specimens, including fixed-ended stub columns and pin-ended long columns. For the pin-ended columns, two tests were performed for each length using eccentric and concentric axial loading. The purpose of the test programme was to select a curve for high strength steel columns with a nominal yield stress of 690 MPa from the multiple column curves used in the Australian steel structures standard. It is shown that the α b = −0·5 curve is the appropriate curve for box and I-section columns fabricated from flame-cut high strength steel plate with a nominal yield stress of 690 MPa. This curve is higher than the α b = 0 curve for box and I-section columns fabricated from ordinary steel because the effect of residual stresses is less detrimental to the strength of high strength steel columns than to the strength of ordinary steel columns. The paper also shows a comparison of the tests with column design strengths of the Australian steel structures standard AS4100, the Load and Resistance Factor Design Specification of the American Institute of Steel Construction, The British Standard BS5950: Part 1, and the draft European Committee for Standardisation (CEN) Eurocode3. The design strengths are shown to be in close agreement with the tests except for Eurocode3 which conservatively predicts the test strengths.

Strength design curves for thin-walled sections undergoing distortional buckling

Abstract Certain types of cold-formed sections, notably those composed of high-strength steel, some decking sections and racking sections may undergo a mode of buckling called distortional in which edge and intermediate stiffeners are inadequate to prevent lateral movement of the flanges which they support. The existing design methods in current standards and specifications are often not adequate to account for distortional buckling. This paper describes proposed strength design curves for thin-walled sections undergoing distortional buckling. The design curves are based on testing and the test data base is included in the paper. The influence of local buckling on distortional buckling is also discussed.

Design for distortional buckling of flexural members

Abstract For thin-walled flexural members composed of high-strength steel and/or slender elements in the section, a mode of buckling at half-wavelengths intermediate between local buckling and flexural—torsional or flexural buckling can occur. The mode is most common for edge-stiffened sections such as C- and Z-purlins, and involves rotation of the flange and lip about the flange—web junction. The mode is commonly called distortional buckling. This paper presents a design method for distortional buckling of flexural members recently drafted for use in the Australian/New Zealand Standard for Cold formed Steel Structures. Methods for computing the elastic buckling stress, and design curves for determining the distortional buckling strength are presented. Comparisons of different methods for computing the elastic distortional buckling stress are made with accurate solutions based on the finite strip method of structural analysis.

A theoretical investigation of the column behaviour of cold-formed square hollow sections

Abstract An experimental programme investigating the column behaviour of four sizes of square hollow section was undertaken at the University of Sydney using Australian produced cold-formed square hollow sections. Stub and pinended column tests were performed and detailed measurements of the yield stress and residual stress taken around the sections. A large deflection elastic—plastic finite strip analysis including the measured distributions of yield stress and residual stress is used to investigate the behaviour of the stub and pin-ended columns. In particular, the influence of the measured through thickness residual stress components on the ultimate load and behaviour of the square hollow section columns is demonstrated. The analysis accounts for plate geometric imperfections, the variation of yield stress around a section, the stress—strain characteristics of the material forming the section and the highly complex patterns of residual stress produced by the cold-forming process. Comparison of the analytical results with the test results is provided.

Elastic interaction of local and lateral buckling in beams

A nonlinear finite strip method of analysis is described for the post-local buckling of geometrically imperfect plate assemblies. The method is used to provide an accurate alternative to the winter effective width formula for obtaining the effective section of a simply supported I-Beam in the post-local buckling range of structural response. The effective section of a locally buckled beam with thin flange outstands is used to investigate the resistance of the beam to flexural-torsional buckling. The analytical methods developed to assess the nonlinear interaction of local and lateral buckling are compared with experimental test results. (TRRL)

A nonlinear elastic spline finite strip analysis for thin-walled sections

Abstract A nonlinear elastic analysis based on the spline finite strip method has been developed for studying the post-buckling behaviour of thin-walled sections. The method can handle local, distortional and overall buckling modes in the post-buckling range and the interaction between them. It allows for geometric imperfections, arbitrary loading and non-simple boundary conditions. By comparison, the semi-analytical finite strip method is restricted to simply supported end boundary conditions and a single buckle half-wavelength. The two incremental-iterative strategies adopted in the nonlinear analysis are the arc-length method and the improved iteration method based on displacement control. Switching between load and displacement control can occur automatically as the need arises in an analysis. Numerical examples are presented for comparison with and verification against solutions for the nonlinear behaviour of plates, plate assemblies and cylindrical shells.