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.
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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.
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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.
Advantages of Cold-formed Steel over Other Materials
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This article focuses on the use of cold-formed steel (CFS). This type of steel is made from thin gauge sheets and, from the word itself, is made by cold working processes such as pressing and rolling, making it more durable as compared to hot-rolled sheets. In contrast with wood, cold-formed steel offers a longer lifespan as it is resistant to mold or vermin.
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Since it is lightweight, CFS is easier to install and can even be recycled when the roofing system is removed or changed. When correctly constructed and designed, CFS structures can resist extreme environmental loads making it one of the most resilient materials available.
References
• Design Guide for Cold-Formed Steel Purlin Roof Framing Systems (AISI Design Guide D111-09). American Iron and Steel Institute, 2009.
• Segui, William T. Steel Design 6th Edition. Cengage Learning, 2018.
Frequently Asked Questions
What are the main differences between C and Z purlins?
C and Z purlins are named after their shapes. Z purlins are point-symmetric and often have unequal flanges to allow for overlapping, whereas C purlins have a lesser vertical section modulus, making them ideal for lower-sloped roofs.
How are cold-formed steel purlins connected?
They are generally lapped with each other to provide continuity and are connected using machine-grade bolts through the webs of the lapped sections.
Why is cold-formed steel (CFS) more durable than hot-rolled steel?
CFS is made from thin gauge sheets through cold working processes like pressing and rolling, which increases its durability and strength compared to standard hot-rolled sheets.
Is cold-formed steel better than wood for roofing?
Yes, CFS offers a longer lifespan as it is completely resistant to mold, rot, and vermin, which are common issues with traditional wood framing.
Can cold-formed steel purlins be recycled?
Absolutely. CFS is lightweight, easy to install, and fully recyclable when a roofing system is eventually removed or replaced.
What design checks are required for purlins?
Designers mainly check for biaxial bending as loads are applied on the normal and tangential axes. Torsional effects are usually neglected due to the restraints provided by the roof sheathing.
