Design Issues
Meeting the codes
The design of cold formed sections made from GALVASPAN® steel is subject to the requirements of AS/NZS4600, the SAA Cold formed steel structures code. The code contains the rules by which the physical design of the section should be carried out and its load capacities calculated. It is intended as a supplement to, and should be used in conjunction with, AS4100 (the SAA Steel structures code) to carry out the structural design.
Any part of a steel structure must be designed for loads laid down in a number of codes specified in AS4100. Purlin loads are generally established from AS1170, the SAA Loading code (Part 1: Dead and live loads, and Part 2: Wind forces). In practice, the loads are dead load, roof live load and wind load and, in some areas, snow load. The nett load effect may act inward or outward (often the latter under typical Australian wind conditions) and the purlins may need to be analysed for both cases.
Load Tables
For design ease, some manufacturers provide detailed load tables that are either calculated in accordance with AS/NZS4600, or established by test (as permitted under the code).
Bridging
Light gauge cold formed purlins are capable of carrying loads over quite large spans in relation to their depth (commonly 40D and more).
But because of this, they have a tendency to twist or buckle laterally. In the case of C sections, this is due to the shear centre lying outside the section; with Z sections, it’s because the loading axes do not coincide with the principal axes.
This problem can be overcome by using lateral bracing (bridging). Running transverse to the purlins, bridging braces and stabilises the purlin webs, usually at mid span or approximately third points.
Boltless Bridging Systems
Some manufacturers provide bridging that is designed to hook into and lock the purlins in position quickly and easily, without the need for nuts, bolts or tie rods. These boltless bridging systems can reduce installation and construction costs, as well as increasing the safety for riggers installing the purlins.
