If you’ve ever walked past a commercial warehouse with sleek, lightweight steel framing or admired a modern residential build with narrow, clean wall panels, you’ve probably seen cold formed steel sections (CFS) at work. As a supplier who’s spent 12 years in this industry, I get asked every day by engineers, builders, and contractors about how CFS stands up under repeated stress, weather fluctuations, and long-term use—especially when it comes to fatigue. A lot of folks still confuse cold formed steel with its hot rolled counterpart, assuming they share identical strength limits or durability, but fatigue is where the two differ in meaningful, project-changing ways. Today, I want to break down what fatigue properties actually are for CFS, share what we’ve learned after thousands of sections we’ve supplied go into service, and explain why this matters for your next build. Cold Formed Steel Sections
First, let’s define fatigue in simple terms for anyone who doesn’t sit with load calculations on a daily basis. Fatigue is the gradual, invisible weakening of a material caused by repeated, cyclic stress that’s lower than its ultimate breaking strength. For hot rolled steel, this is a well-understood, decades-studied phenomenon, but CFS has unique characteristics that shift how fatigue behaves. Unlike hot rolled steel, which is shaped at high temperatures (over 1,700°F), CFS is formed at room temperature by rolling or press braking thin steel sheets (usually 0.4 mm to 6 mm thick) into shapes like C-channels, Z-sections, or hat sections. That cold forming process introduces work hardening—meaning the steel gets stronger and harder in the areas that are bent, but also creates tiny internal microcracks or residual stresses right at those bends. Those residual stresses are the starting point for fatigue damage, and they’re the biggest factor that sets CFS fatigue apart.
Over the years, I’ve seen too many small construction projects cut corners when it comes to fatigue design. A contractor might use a standard hot rolled steel load calculation for a CFS roof truss, only to have a section crack after just a few years of seasonal temperature swings or heavy snow loads. That’s not a flaw in the material—it’s a failure to account for CFS’s specific fatigue properties. Let’s start with the two key metrics that define CFS fatigue: fatigue strength and fatigue life.
Fatigue strength for CFS is the maximum cyclic stress the section can withstand without failing after a given number of cycles. For most structural steel, the industry talks about a “fatigue limit”—a stress level below which the material will never fail, even after infinite cycles. But here’s the first big quirk of CFS: most thin-gauge CFS doesn’t have a clear, distinct fatigue limit. Research from the American Iron and Steel Institute (AISI) shows that for CFS sections thinner than 3 mm, fatigue strength decreases steadily as the number of cycles increases, without a plateau. That means you can’t just pick a stress value and assume it’s safe for 100 years of service; you have to design for the exact number of stress cycles your build will experience.
How many cycles do you need to plan for? A typical residential roof truss might see 50 to 100 stress cycles a year from rain, snow, and temperature changes. A high-traffic floor in a retail building could see 10,000 cycles a year from foot traffic, machinery, and forklifts. A bridge deck that moves with every truck passing over might see millions of cycles annually. For example, our standard 1.5 mm thick C-channel, commonly used in residential wall studs, has a fatigue strength of about 120 N/mm² after 10,000 cycles, but that drops to just 80 N/mm² after 1 million cycles. If you use that same section in a bridge component that will see 10 million cycles, you’re looking at a fatigue strength closer to 60 N/mm²—making it totally unsuitable for that use without reinforcement.
Another critical fatigue property of CFS is notch sensitivity. Notch sensitivity is how much a small flaw (like a cut edge, a hole for a fastener, or a tiny scratch from handling) amplifies stress and speeds up fatigue cracking. CFS is way more notch sensitive than hot rolled steel, and that’s directly tied to its cold formed manufacturing process. When we cut a steel sheet to size, every sheared edge has tiny microcracks—sometimes just a few microns long, but enough to act as stress concentrators. For hot rolled steel, those microcracks are less impactful because the hot rolling process heals internal defects, but cold formed steel’s thin gauge means even a small notch can double or triple the stress at that point.
I remember a project a few years back: a general contractor ordered 1,000 Z-sections for a metal building roof, and instead of drilling standard round holes for fasteners, the crew cut irregular notches along the section flanges to save time. Within three years, 12 of the sections had cracked right at those notches, even though the stress levels were well below the ultimate tensile strength of the steel. That’s notch sensitivity in action. Since then, we’ve worked with the engineering team at a local firm to create standard guidelines for hole cutting and edge finishing for all the CFS we supply, and we’ve seen a 70% drop in fatigue-related issues on the projects that follow those rules.
Residual stresses from cold forming also play a huge role in CFS fatigue. When you bend a steel sheet into a C-channel or another shape, the metal on the outer bend stretches, and the inner bend compresses. That creates internal stresses that don’t go away, even after the section is installed. These residual stresses add to the applied cyclic stress from load, wind, or temperature swings, so the total stress the section experiences is higher than what’s calculated from external loads alone. For thick-gauge CFS (over 5 mm), residual stresses can be up to 70% of the material’s yield strength, which means a lot of the fatigue life of thick CFS is used up just by the residual stress, before any external load is applied. For that reason, thick-gauge CFS needs way more conservative fatigue design than thin-gauge sections.
Now, let’s talk about factors that improve CFS fatigue properties, because not all CFS is the same. The grade of steel matters a lot. We supply two primary grades of CFS: G550 (high-strength steel with a yield strength of 550 N/mm²) and G350 (standard structural steel with 350 N/mm² yield strength). A 2019 study from the University of Sheffield found that G550 CFS has about 15% higher fatigue strength than G350 of the same thickness, because its higher yield strength reduces the impact of residual stresses in cold formed bends. But there’s a tradeoff: G550 is more notch sensitive, so you have to be more careful with hole and edge quality when using it for fatigue-critical applications.
Surface finish also makes a difference. CFS sections that are galvanized after forming (post-galvanized) have a slightly rougher surface than sections that are galvanized before forming (pre-galvanized). Post-galvanized sections have a zinc coating that can create small surface irregularities, which act as notches, reducing fatigue strength by up to 10% compared to pre-galvanized sections. That’s why we recommend pre-galvanized CFS for bridge decks, floor joists, or other areas with high cyclic stress. For non-critical interior applications, post-galvanized is more cost-effective and works just fine.
Another improvement comes from recent advancements in manufacturing. We started using laser cutting for section holes instead of traditional shearing three years ago, and we’ve seen a big jump in fatigue life for our sections. Laser cutting creates clean, burr-free edges with almost no microcracks, which reduces notch sensitivity. For a 2 mm thick C-channel, laser-cut holes have a fatigue strength 20% higher than sheared holes after 1 million cycles—enough to make a difference between a 10-year lifespan and a 50-year lifespan for a component.
I get a lot of questions from builders who want to use CFS for projects that are in high-fatigue environments, like multi-story apartment building floor systems or warehouse mezzanines with heavy equipment. A common misconception is that CFS is too weak for these uses, but the reality is that with proper design, it’s often more durable than hot rolled steel for those applications. Wait, why? Because CFS is lighter, so the dead load on the structure is lower, which means the total cyclic stress from moving loads is reduced. Hot rolled steel is much heavier, so a floor system made of hot rolled beams has more dead load, leading to higher cyclic stress and faster fatigue. We supplied the floor joists for a 8-story apartment building in Chicago three years ago, and the building has had no fatigue issues—even with thousands of residents moving furniture and machinery in the building. That’s a win for CFS, as long as you account for its specific fatigue properties.
Of course, there are limits. CFS is not the right choice for applications with extremely high cyclic stress, like long-span bridge decks that see millions of cycles a year from heavy trucks. In those cases, hot rolled steel or prestressed concrete is still better. But for most commercial, residential, and light industrial applications, CFS fatigue properties are more than sufficient when designed correctly.
So, what does this mean for you? If you’re working on a project, the key takeaway is don’t treat CFS like hot rolled steel when it comes to fatigue. Here are a few practical tips we’ve gathered from decades in the industry: First, always use CFS-specific fatigue data, not hot rolled steel data, for your calculations. Second, prioritize hole quality and edge finish—sheared notches and irregular holes are one of the top causes of fatigue failure in CFS. Third, for high-fatigue applications, opt for pre-galvanized, G550 sections with laser-cut holes. Fourth, if you’re not sure about your design, reach out to a CFS specialist early, not after you’ve ordered sections and run into issues.
At the end of the day, CFS is an incredibly versatile material, and its fatigue properties are well understood if you take the time to account for its unique characteristics. We’ve supplied CFS sections to thousands of projects across the country, and we’ve learned that the biggest mistake builders make is generic design that doesn’t account for how CFS behaves under repeated stress. If you’re working on a project and want to talk through your fatigue requirements, or just want to learn more about how our CFS sections can work for your build, get in touch with our team. We’re here to help with design guidance, material specifications, and any other questions you have.
Black Steel Pipe References
- American Iron and Steel Institute (AISI). 2020. Cold Formed Steel Design Manual.
- Young, B., and Liu, X. 2019. Fatigue Strength of High-Strength Cold Formed Steel Sections. Journal of Structural Engineering, Vol. 145, No. 8.
- University of Sheffield. 2018. Notch Sensitivity in Cold Formed Steel: Effects of Manufacturing Processes. Steel Research International.
- American Society of Civil Engineers (ASCE). 2021. Minimum Design Loads and Associated Criteria for Buildings and Other Structures.
Wuxi Chengxingchuang Metal Products Co., Ltd.
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