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China Cold-Formed Steel Purlins - Quality Suppliers and Factory Direct Design Solutions
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.
Advantages of Cold-formed Steel over Other Materials
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.
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 most common shapes for cold-formed steel purlins?
C and Z sections are the most common shapes available for cold-formed steel purlins, bent according to the respective letters of the alphabet.
What is the difference between C and Z purlins?
Z purlins are point-symmetric and can have unequally manufactured flanges to allow overlapping. C purlins have a slightly lesser vertical section modulus, which makes them more advantageous for roofs with lower slopes.
How are roof purlins typically connected?
Purlins are generally lapped with each other to provide continuity and are connected using machine-grade bolts through the webs of the lapped sections.
Why are torsional effects often neglected in purlin design?
Torsional effects on the member are assumed to be neglected due to the partial lateral and torsional restraints provided by the roof sheathing.
What makes cold-formed steel (CFS) superior to wood and hot-rolled steel?
CFS is more durable than hot-rolled sheets due to cold working processes. Compared to wood, it offers a longer lifespan because it is resistant to mold and vermin. It is also lightweight, easy to install, recyclable, and highly resilient against extreme environmental loads.