Pre-engineered steel building is portal frame construction comprised of primary and secondary framing and bracing system. A combination of these three elements and weather covering sheeting results in stable steel buildings instead of individual frames.
Primary frames or members are the main load carrying and support members of pre-engineered building. The primary frame members include columns, rafters and other supporting members. The shape and size of these members vary based on the application and requirements. The frame is erected by bolting the end plates of connecting sections together. Secondary structural framing consists purlins, girts, eave struts, wind bracing, flange bracing, base angles, clips and other miscellaneous structural parts. Secondary framing consists of elements that support the roof & wall sheeting and transfer the load to the primary framing.
Secondary framing is an important component of Pre-Engineered Buildings, also referred to as secondary structural, this type of framing runs in between primary framing elements, creating a structure within a structure. The purpose of secondary framing is to distribute loads from the building’s surface to the main framing and the foundation. Secondary members have two functions: they act as struts that help in resisting part of the longitudinal loads that are applied on the building such as wind and earthquake loads, and they provide lateral bracing to the compression flanges of the main frame members thereby increasing frame capacity.
Roof Purlin - Wall Girts - Eave Struts are three key secondary structural members that are used to support the wall and roof panels. Purlins are used on the roof; Grits are used on the walls and Eave Struts are used at the intersection of the sidewall and the roof.
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 is 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.
Because they add mid-span support, purlins allow longer spans, enabling to create a wider building. In their design life, purlins are subjected to dead load (e.g self weight of sheeting materials and accessories), live load (e.g. during maintenance services and repairs), and environmental loads (e.g. wind and snow load).
Purlin manufacturers commonly produce two types of purlins- C and Z purlins.
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. As one side of this purlin is plain, it is also preferred for cladding.
Z shaped Purlins
The shape of Z purlins is similar to that of the Z alphabet; they are stronger than C shaped 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.