Secondary framing is an important component of many pre-engineered metal buildings. Also referred to as “secondary structurals”, this type of framing runs in between primary framing elements, creating a structure-within-a-structure, much like cross-beams in a wooden building.
The purpose of secondary framing is to distribute loads from the building’s surfaces to the main framing and the foundation. Secondary framing can add longitudinal support that helps resist wind and earthquakes. And it can provide lateral bracing for compression flanges that are part of the primary framing, increasing overall frame capacity.
Secondary framing components are known as girts and purlins, and they work like this:
- Girts provide additional support for walls. They work in conjunction with columns and wall panels to support vertical load, improving both strength and stability. They also help attach and support wall cladding.
- Purlins provide additional support for the roof. They create a horizontal “diaphragm” that supports the weight of your building’s roof deck – whatever material you use for the roof itself. They also help make your entire roof structure more rigid. Because they add mid-span support, purlins allow longer spans, enabling you to create a wider building.
- Eave struts are another kind of secondary framing. Also called eave girts or eave purlins, these are essentially a combination of the two. They’re used where sidewalls intersect with the roof, using a top flange that helps support the roof and a “web” that helps support the walls.
Secondary framing comes in two configurations, named for their shape: CEE and ZEE. They’re shaped on a bending press, to create a web with two flanges. They come in a variety of sizes; for instance, purlins can run over 30 feet in length.
Girts, purlins and eave struts are almost always made of cold-formed steel. It’s more affordable and easier to work with, but it also presents some structural stability issues that must be considered as part of your metal building framing options and overall design. In particular, local or distortional buckling or lateral displacement can occur, in which portions of the compression flange, web or connectors can buckle or shift from their initial position.