Guide to Use
Shapes and sections: which one?
Purlins made from GALVASPAN® steel are roll-formed into two standard shapes – C and Z sections. Both perform effectively, and in many instances the choice comes down to personal preference.
When to use C sections
Generally, C sections are used for simple span construction, with the purlins butted at internal supports. In cases where deflection is a design limitation and the building is of a suitable size, single C sections can be used over two or more bays.
When to use Z sections
Z sections with one broad and one narrow flange are designed to nest neatly, and are intended for lapping at the internal supports. This produces structurally continuous lines of purlins for the length of the building.
Structural continuity results in improved rigidity, but lapping also doubles the thickness of purlin over supports where the bending moments are greatest. This results in a saving of up to 50 per cent – but typically 30 to 40 per cent – of the steel in the purlins, compared with simple span C sections.
Where additional strength is required from the purlins – for example in end bays, or where additional loads are applied locally – Z sections of the same depth but greater thickness can be lapped as required.
Special Applications
In some applications, such as grain or coal handling facilities or wood machining factories, the upturned lip of conventional Z and C section purlins may form a trap for dust and debris. For this reason, Z sections are also available with downturned bottom lips. These sections are normally only used over simple spans with the ends butted rather than lapped.
