Installation
Purlins are bolted to the primary frame by cleats welded to the rafters or columns by the steel fabricator.
The cleats and the associated hole geometry have been standardised in the AISC Standardised Structural Connections. (Most manufacturers comply with the standard and the hole centres, as shown in the diagram below.)
Bolts
Bolts are usually M12 Grade 4.6 requiring snug tightening to make an effective connection. These can be ordered from purlin manufacturers, some of whom supply special types, such as bolts with short 16mm plain shanks and M12 threads, nuts and bolts with integral washer faces, etc.
Clearance Holes
To allow for minor variations in frame alignment, purlins made from GALVASPAN® steel generally have quite large clearance holes; typically 18mm diameter holed for 12mm diameter bolts. Some manufacturers produce 18mm x 22mm slots for greater adjustment. These generous clearances make for easier assembly, but won’t affect structural performance.
When Z purlins are lapped, additional holes are provided to ensure structural continuity. It’s here that the extra clearances are especially useful. Purlins are usually engineered for M12 bolts and in these cases, M16 should not be used. In fact, with lapped Z purlins in some of the thicker gauges, the holes cover one another sufficiently to prevent assembly with M16 bolts unless they are forced.
Top Flange Facing Up
Purlins should always be installed with the top flange facing up the slope from the cleat, ie:
- C sections should be fitted on the high side of the cleat, open face facing up the slope
- Z sections should be fitted with the web on the low side of the cleat, with the top flange above it.
This is to minimise the tendency of the sections to rotate between supports or bridgings.
Fitting the Bridging and Cladding
While purlin fixing is quite straightforward, the sections are very flexible until they become part of the total sheeted system. The aim, therefore, should be to maintain a stable framework by fitting the bridging as the purlins are attached.
Ideally, the cladding should also be progressively fixed, although this isn’t always practical if the jobs are handled by different sub-contractors.
Bundles of roof sheeting should not be placed on unsheeted purlins, as this can cause overloading and result in permanent deformation of the sections.
