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Common Types of Cold-Formed Roof Purlins

C and Z sections are the most common shapes available for cold-formed steel purlins. As their names imply, these sections are bent according to the letter in the alphabet (refer to Figure 2). Z Purlins are point-symmetric, and sometimes their flanges are manufactured slightly unequally to allow overlapping. Meanwhile, C Purlins have a slightly lesser vertical section modulus, making them more advantageous for roofs with lower slopes. Each shape has its own pros and cons; it’s up to the engineer to assess the appropriate purlin based on the project requirements.

When installed on the roof, purlins are generally lapped with each other, providing continuity in the section. It is usually connected using machine-grade bolts through the webs of lapped purlins.

When it comes to its design, loads are applied on the normal and tangential axes of the section’s centroid. Thus, biaxial bending is mainly checked. Torsional effects on the member are assumed to be neglected due to the partial lateral and torsional restraints provided by the roof sheathing.

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