Tolerances & Sizes
Precision roll-forming of GALVASPAN® steel produces purlins to quite close dimensional tolerances. Typical tolerances are:
Depth + 1mm
Hole centres + 2mm
Flange width + 2mm
Camber 1 in 500
Overall length + 5mm
Bow 1 in 250
Tolerances for material thickness are nominated in AS1365: "Tolerances for hot rolled and cold rolled unalloyed low carbon steels (coils and cut lengths)."
Sizes and Dimensions
There are no official standard sizes for purlins, but most purlins made from GALVASPAN® steel come in nominal depths of 100, 150, 200, 250, 300 and 350mm. Not all sizes are available in both C and Z profiles. Sections are generally designated by a code number signifying the nominal depth and material thickness, with a letter prefix to indicate the profile.
Metric or Imperial?
While some manufacturers produce true metric-sized purlins, others produce to superseded imperial dimensions, which are then referred to by their soft metric conversions. Thus, a 200 series purlin could be 200mm or 203mm (eight inches) deep, depending on the manufacturer. While purlin depths are reasonably rationalised, the same cannot be said of other dimensions such as flange width or lip length.
Other Variations Between Manufacturers
It’s important to take into account the variations in detail between sections produced by different roll-formers when comparing their likely performance.
In particular, the effective section rules of AS/NZS4600 require careful attention, as variations in flange width and lip length can have a significant influence on the load capacity of the section.
Other variables can be material strength and thickness. GALVASPAN® steel from BlueScope Steel is a hi-tensile galvanised steel strip that conforms to AS1397-G450 (450MPa minimum yield stress); or for thicknesses less than 1.5mm, AS1397-G500. Clearly, lower strength materials from other steel producers will not deliver the same structural performance.
Steel Thicknesses
Purlins made from GALVASPAN® steel are generally produced in thicknesses of 1.0, 1.2, 1.5, 1.9, 2.4, and 3.0. For design purposes, the relevant material thickness is the base steel thickness before galvanising or painting. Galvanised coatings, in particular, add appreciably to the thickness, but the zinc coating contributes little to strength and is not considered when calculating load capacities.
