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Metroll Newcastle
Metroll Newcastle

STRUCTURAL STEEL » PURLINS & GIRTS » installation

 

C Section
| Z Section | Corrosion Protection | Cladding | Design Issues
| Installation
Applications
| GALVASPAN® | Standards | Warranty

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Installation

Purlins need to be bolted to the primary frame by cleats welded to the rafters or columns by a qualified steel fabricator. The cleats and the associated hole geometry have been standardised in the AISC Standardised Structural Connections.

The bolts are usually M12 Grade 4.6 requiring snug tightening to make an effective connection.
To allow for minor variations in frame alignment, purlins made from GALVASPAN® steel generally have quite large clearance holes-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 do not affect structural performance.

When Z purlins are lapped, additional holes are provided to ensure structural continuity. Purlins are engineered for M12 bolts and in these cases, M16 should not be used. With lapped Z Section Purlins in some of the thicker gauges, the holes cover one another sufficiently to prevent assembly with M16 bolts unless they are forced.

To minimise the tendency of the sections to rotate between supports or bridgings, purlins need to be installed with the top flange facing up the slope from the cleat:

  • 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.

While purlin fixing to cladding is quite straightforward, the sections are very flexible until they become part of the total sheeted system. The aim should be to maintain a stable framework by fitting the bridging as the purlins are attached.

Bundles of roof sheeting should not be placed on unsheeted purlins, as this can cause overloading and result in permanent deformation of the sections.

 




 
   
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