A purlin is a secondary element of a structure that supports floors, roofs and walls, and is primarly subjected to bending. They are often called “girts” when used to support walls.
The distinction between these sections and the others, is that they are made of cold-formed Steel, therefore they are thinner and lighter so that sections can be formed with complex geometries such as “C” and “Z” profiles, which are the most commonly used.
Sometimes purlins are subject to axial loads and is necessary to place two sections to prevent buckling, these members are called “strut purlin”.