Purlin systems

If the roof cladding cannot be laid from girder to girder due to the span width, a purlin system is used as a secondary supporting structure. Single-span purlins and multi-span purlins in the form of continuous, articulated or coupled purlins are used as purlin systems. Multi-span purlin systems are basically more economical because of the redistribution of moments, but they increase the hall construction because they are laid above the truss level. Since the centre support of two-span beams carries 25% more load than the outer supports, systems with more than two spans should always be chosen because of the more even load transfer to the substructure.
This also applies to trapezoidal steel sheets that are tensioned in the longitudinal direction of the hall. Alternatively, if two-span beams are chosen, the centre supports can be staggered alternately. For larger roof pitches, the purlins are laid according to the roof pitch – this stresses them to double bending – or else hung plumb between the main trusses as single-span purlins. In the second case, the purlins are chamfered to accommodate the roof cladding according to the roof pitch. This service should be shown separately in the tender. For normal spans, the girders are connected by means of commercially available small iron fasteners, which must, however, have building authority approval if no static proof is available in the individual case.
Coupling purlins made of rough-sawn squared timber are extremely economical. Although these purlins are very economical for small purlin spacing, they are not architecturally appealing due to the external appearance and cracking of solid wood. Coupling purlins are mainly used in conjunction with fibre-cement corrugated sheets in the agricultural sector.
When using sandwich panels, glulam should always be chosen because of the higher spacing and the occurrence of leaks due to twisting of the substructure.