2008 Research Reports No R888-R893

Abstract
This report describes research carried out at the University of Sydney to study the interaction of local and overall buckling in stainless steel compression members.

A finite element model was developed taking into account the specific material properties of stainless steel: non-linear stress-strain behaviour, anisotropy and enhanced corner properties as a result of cold-working. The model was verified against experimental data and subsequently used in parametric studies. Simulations were run for lipped channels, plain channels, SHS and I-sections, covering the practical ranges of overall and cross-sectional slenderness values.

The current Australian/North-American and European design guidelines were assessed using the available experimental and numerical data. Finally, a direct strength approach for stainless steel compression members was proposed.

Keywords
Stainless steel, interaction buckling, local buckling, overall buckling, non-linear material, finite element, cold-formed sections, lipped channels, plain channels, SHS, back-to-back channels, design guidelines, Direct Strength Method.

Abstract
High strength cold-formed steel sections are commonly used in a variety of applications including residential construction.  These steel sections typically have a nominal yield stress of 550 MPa and the use of such high strength material allows for a reduction in thicknesses.  With this reduction in thickness, the high strength steel can be manufactured into complex shapes including stiffeners.  Such complex shapes exhibit structural instabilities such as local, distortional and flexural-torsional buckling modes, and in some cases, interaction of the local and distortional buckling modes may occur.

This paper describes the design and testing of web-stiffened high strength steel cold-formed lipped channel columns.  In order to be able to apply the Direct Strength Method in Section 7 of the Australian Standard AS/NZS 4600:2005 to design, the steel sections must be pre-qualified as a compression member.  The section chosen is pre-qualified and has nearly coincidental local buckling and distortional buckling loads as well as a flexural-torsional mode which varies with length.

A series of compression tests was carried out in a 300 kN capacity SINTEC testing machine over a range of lengths with fixed ended conditions.  The varying lengths were chosen so as to observe the buckling modes and the possibility of interaction between them.  The effect of the different type of failure modes is also discussed in this paper.  The experimental results are then compared with design methods in the existing design standards.  The methods include the Effective Width Method (EWM) and the Direct Strength Method (DSM) as described in the Australian Cold-Formed Steel Structures Standard AS/NZS 4600.  It is shown that the existing standards are unconservative and new proposals for dealing with this are made.

Keywords
HighCold-formed; high strength steel; local buckling; distortional buckling; interaction buckling; postbuckling behaviour; intermediately stiffened section

Abstract
Single equal angle steel beams are often used as lintels to support eccentric loading acting normal to one flange.  This loading causes combined biaxial bending and torsion, which is not allowed for in most steel design codes.  Instead, approximate methods based somewhat loosely on past research studies have been used to develop design approximations and tables.

This paper reviews past research on single equal angle beams used as lintels and develops an improved method of predicting their strengths which includes the effects of initial twist rotations, eccentric loads, and large twist rotations, and utilizes the plastic capacities of compact beams.  The strengths predicted are significantly higher than those of previous approximations.  More accurate strength approximations are proposed, and suggestions are made for serviceability design.

Keywords
Angles, beams, bending, buckling, design, elasticity, member strength, moments, serviceability, steel, torsion.

Abstract
Steel design codes do not provide sufficient information for the efficient design of steel structures against out-of-plane failure, and what is provided is often overly conservative.  The method of design by buckling analysis corrects this situation for beams, but the extension of this method to columns is only suggested, while there is no guidance on how to apply this method to the design of beam-columns and frames.

Beam design by buckling analysis uses the design code formulation for the member nominal design strengths in terms of the section moment capacities and the maximum moments at elastic buckling, accurate predictions of which may be determined by available computer programs.  Column design by buckling analysis is similar to beam design, in that it uses the design code formulation for the column nominal design strengths in terms of the section compression capacities and accurate predictions of the elastic buckling loads which may also be obtained from computer programs.

However, design codes do not provide formulations for the direct buckling design of beam-columns, but instead use the separate results of beam design and column design in interaction equations.  The further extension to frames is not directly possible, because frames are not designed as a whole (except through the rarely used methods of advanced analysis), but as a series of individual members.  This paper shows how the method of design by buckling analysis can be used to design beam-columns and frames as well as beams and columns.  Two example frames are designed and very significant economies are demonstrated when the method of design by buckling analysis is used.

Keywords
Beams, beam-columns, bending, buckling, columns, compression, design, frames, member strength, moments, steel.

Abstract
The elastic buckling stress of the web of a thin-walled section member in shear is generally improved by the presence of flange and lips. However, for webs with relatively large depth-to-thickness ratios, the local buckling mode in shear occurs mainly in the web. The structural efficiency of such webs can be improved by adding an intermediate stiffener cold-formed longitudinally in the middle of the web.

In this report, the computational modelling of thin-walled steel sections is implemented by means of a spline finite strip analysis to determine the elastic buckling stresses of channel sections subject to pure shear. Lipped channels with an intermediate web stiffener are studied where the main variables are the dimensions of the stiffener in both depth and width directions. Results and comparisons of analyses are included in this reports.

Keywords
Shear buckling, Intermediate web stiffener thin-walled channel sections, Lipped and unlipped channel sections, Spline finite strip method, Shear buckling capacity, Twisting and lateral buckling mode.

Abstract
This report describes the setup, procedure, and results of scaffold Cuplok joint tests. The aims of the tests are to investigate the joint stiffness for rotations about vertical and horizontal axes in various joint configurations, and carry out statistical analyses of the experimental results. The tests were performed in the laboratory of the School of Civil Engineering at the University of Sydney from second-hand Cuplok scaffold pars provided by Boral Formwork and Scaffolding Pty Ltd. A total of 172 tests were carried out on various joint configurations, bending axes, loading directions, types if material (galvanised or painted components), and degree of tightening of the joints. The results are shown graphically in terms of moment-rotation curves. Since there was substantial variation in Cuplok joint stiffness, a statistical analysis on the results was performed. These experimental studies are especially useful for modelling and performing probabilistic analysis the Cuplok scaffold systems.

Keywords
Joint tests, Joint stiffness, Probabilistic analysis, Support scaffold systems, Falsework.