Purlins / Girts

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Purlins are steel elements which are used as support structures in pre engineered buildings, mainly below the roofs. Purlins provide extra roof support, creating a horizontal diaphragm that provides support to the weight of the roof and deck. Purlins are installed parallel to the building eave and are supported by rafters or walls. The raw material used to fabricate purlins are cold formed steel. It can span a length of upto 30 feet or longer. The length and width depends on the dimensions of the primary frame of the building, its usage and engineering design. Added stiffness can be given to purlins to prevent loss of structural strength, as required. Purlins are therefore an important part of the roof structure.

In pre engineered buildings, steel purlins are widely used. They are lightweight, durable structures that are built to precision, having consistency in dimensions. Purlins expand and contract in summer and winter respectively. 

Let’s look at the two most widely used forms of purlins:

C shaped

C shaped Purlins

As the name itself suggests, these purlins bear shape similar to alphabet C. They are commonly used to support walls and floors. C purlins are also sometimes referred to as channel section purlins when they have flange stiffeners and U-sections when they do not. C-sections are those which are mono-symmetrical. These sections cannot be lapped, but the stable shape ensures easy packaging and transportation. The C-section purlins are widely used in clear span design due to their high stability factor. 
Z shaped

Z shaped Purlins

The Z-shaped purlins are the other type of purlins, which are stronger than C purlins. Z-purlins are most commonly used in joints and overlaps. These purlins are placed between the roofing sheet and wall and provide optimum support to the primary framing system. Z-purlins can be lapped, which is done by rotating one Z-purlin 180 degrees and having it fitted to another one.

Doing away with instability in purlins and girts

There are two kinds of lateral instability that purlins and girts feature, which are deflection and rotation. Both these can be reduced significantly by adopting certain engineering methods like bridging and attaching the sheet by fastening screws to provide a lateral brace to the flange to which it is attached. The process of bridging can help do away lateral deflection of the free flange and rotation of the section.

Purlins are therefore an important part of PEB. They are instrumental in providing structural support to the primary frame of the building.