Keywords: buckling, cold-formed, computer, design, program, purlin.

Cold-formed purlins are used throughout the world as thin-walled beams to support thin profiled steel sheeting subjected to wind load. Purlins are usually thinner than hot-rolled sections and have modes of failure which are not commonly encountered in hot-rolled structural steel design. The individual plate elements in cold-formed sections are normally thin compared with their width and so local buckling may occur before section yielding. Also, purlins may undergo flexural-torsional buckling because of their low torsional stiffness. Purlins which are braced against flexural-torsional buckling may undergo a mode of buckling called distortional buckling. The design capacity of purlins depends very much on their resistance to these three different types of buckling failure modes.

The correct design of cold-formed purlins requires a thorough understanding of the buckling behaviour of these types of sections. The finite strip buckling analysis program THIN-WALL [1,2] can be used to study the local, distortional and flexural-torsional buckling behaviour of cold-formed purlins in uniform bending. The analysis can be done for a number of different buckle half-wavelengths and the load factor and buckled shape are output for each length. The buckling stresses calculated by THIN-WALL can be used to design cold-formed purlins.

The design of cold-formed steel structures in Australia and New Zealand is performed according to the Australian/New Zealand Cold-Formed Steel Structures Standard AS/NZS 4600 [3]. This paper describes the design rules in AS/NZS 4600 for cold-formed purlins. This standard is in limit states format and incorporates rules for yielding, local buckling, distortional buckling and flexural-torsional buckling of the whole cross-section, and yielding and buckling of webs subjected to shear, and combined moment and shear.

AS/NZS 4600 uses the effective width method to calculate the section capacity of cold-formed members. In this method, the widths of compression plates prone to local buckling are reduced and new effective section properties are calculated. The calculation of the effective section properties is a very tedious procedure if access to a computer program is not possible. Steel designers will require help in the efficient implementation of the complex design rules for cold-formed steel structures. A computer program is an essential tool for the economical design of cold-formed purlins.

The computer program PURLIN [4] allows the structural designer to design cold-formed purlins quickly and efficiently. The program first performs a linear elastic beam analysis of the continuous purlin and then a finite element flexural-torsional buckling analysis [5] on the purlin. The results of the flexural-torsional buckling analysis and the distortional buckling stress are used to calculate the member capacity of the purlin. The local buckling stress and the effective width method are used to calculate the section capacity of the purlin. The section and member capacities of the purlin are checked with the maximum moment and shear force induced in the purlin by the applied loads.

In PURLIN, the section to be designed is chosen from a library of standard sections. This library can be edited by the user so that new sections can be added. The user can select purlin configurations from one to twelve spans with cantilevers at either or both ends. Purlin systems with laps over supports can also be designed. In addition, up to three rows of bridging can be included in each span. The effects of load height and restraint caused by roof sheeting on flexural-torsional buckling are also taken into account. The structure can be analysed for different combinations of point and uniformly distributed load acting at different heights from the centroid of the purlin section.

The program PURLIN also creates a formatted report which has all the design data, equations and results. Bending moment diagrams, shear force diagrams and the deformed shape of the purlin can also be displayed on the screen. The program is very user-friendly and has an on-line help facility that explains the input data.

A new edition of AS/NZS 4600 is due to be released in late 2005. The new edition allows the direct strength method [6] to be used in the design of cold-formed purlins. The direct strength method uses elastic buckling stresses to design purlins, and avoids complicated effective width calculations. It is possible to obtain higher design load capacities by the direct strength method, especially for purlins in which flexural-torsional buckling is the governing failure mode. The program PURLIN allows the option of using the effective width method or the direct strength method to design cold-formed purlins.

1

J.P. Papangelis and G.J. Hancock, "Computer Analysis of Thin-Walled Structural Members", Computers and Structures, 56(1), 157-176, 1995. doi:10.1016/0045-7949(94)00545-E

2

University of Sydney, "THIN-WALL - Cross-Section Analysis and Finite Strip Buckling Analysis of Thin-Walled Structures", 2005.

3

Standards Australia/Standards New Zealand, "AS/NZS 4600 - Cold-Formed Steel Structures", 1996.

4

University of Sydney, "PURLIN - Analysis and Design of Cold-Formed Purlins According to AS/NZS 4600", 2005.

5

J.P. Papangelis, N.S. Trahair and G.J. Hancock, "Elastic Flexural-Torsional Buckling of Structures by Computer", Computers and Structures, 68(1-3), 125-137, 1998. doi:10.1016/S0045-7949(98)00037-6

6

G.J. Hancock, T.M. Murray and D.S. Ellifritt, "Cold-Formed Steel Structures to the AISI Specification", Marcel Dekker, New York, 2001.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £135 +P&P)