ground mounted PV system C purlin

I. Cold-Formed Thin-Walled Steel Most people associate steel with hot-rolled I-beams and channel steel, which are heavy and thick. Inward-rolled C-shaped steel, however, belongs to cold-formed thin-walled steel—it is produced at room temperature by continuously bending coiled steel plates or strips through several sets of rollers, like folding paper, to gradually create complex cross-sections such as C-shapes and Z-shapes. This process is heat-free, relying on "cold working."   Why is this noteworthy? Because the cold bending process produces a work hardening effect: the yield strength of the steel increases by 10% to 20% compared to the raw material. In other words, the same material becomes "stronger" after cold bending. Furthermore, it allows for the creation of large cross-sections with very thin wall thicknesses (commonly 1.5 to 3.0 mm), resulting in extremely high material utilization. Compared to hot-rolled steel, cold-formed steel can save approximately 25% to 30% of steel. This is one of the core technologies that allows photovoltaic brackets to reduce costs while maintaining strength.     II. Purlins vs. Main Beams Many people, when looking at support system drawings, call all C-shaped steel beams "purlins," but they actually have different functions. In a photovoltaic support system:   Purlins are the horizontal members that directly "support" the photovoltaic modules. The modules are fixed to the purlins by clamps or bolts, and the purlins are responsible for collecting wind and snow loads transmitted from the modules.   Main beams (also called diagonal beams) are the inclined load-bearing members that support the purlins. One end connects to the column, and the other end connects to the diagonal brace or another column, transferring the force from the purlins to the column.   To put it simply: purlins are like the rafters on a roof, while main beams are like the main beams on a load-bearing wall. A single inward-curving C-shaped steel beam can be used as either a purlin or a main beam; the only difference is the magnitude of the load and the direction of its arrangement. During the design phase, structural calculations are needed to determine the specifications of each component—typically, the main beam's cross-section is one size larger than the purlin's.   （The photo is from the 微信公众号-机电原理）   lll. Hot-dip galvanizing thickness and lifespan Photovoltaic brackets require a lifespan of over 25 years, making corrosion prevention paramount. The most common corrosion protection method is hot-dip galvanizing: immersing C-shaped steel in molten zinc at approximately 445°C to form a zinc-iron alloy layer and an outer pure zinc layer. But how thick is sufficient?   Empirical data tells us: Rural or general inland environments: A double-sided galvanized layer of 40-50 μm (approximately 275-350 g/m²) can support 15-20 years.   Industrial areas or lightly polluted areas: 50-65 μm (approximately 350-450 g/m²), corresponding to 20-25 years.   For coastal areas within 2km or in high-humidity, high-salt-spray environments: a coating thickness of 80μm or more (approximately 550~600g/m²) is required to achieve a service life of over 25 years.   It's important to note that a thicker coating is not always better—excessive thickness increases coating brittleness, reduces adhesion, and dramatically increases costs. Therefore, a reasonable design involves selecting an appropriate coating weight based on the corrosion level of the project site. The recently popular zinc-aluminum-magnesium plating (containing 3.5%~11% aluminum and 1%~4% magnesium) represents a technological upgrade: its corrosion resistance is 3~10 times that of pure zinc, and the cut edges are self-healing; even if scratched during installation, recoating is unnecessary, making it particularly suitable for coastal and acidic/alkaline environments.   （The photo is from the 微信公众号-机电原理）   IV. Why does inward curling improve strength? This question best demonstrates the ingenuity of engineering mechanics. When an open C-shaped steel beam is subjected to pressure, the most likely outcome is not strength failure, but instability—like crushing an empty soda can. The flanges (the two straight edges) of a C-shaped steel beam tend to twist outward or inward under pressure; this type of failure is called local buckling.   The function of the inward-curved edge is to add an elastic constraint to the flange edge. The rolled edge acts like a "small baffle," preventing the flange from twisting freely. This significantly increases the critical buckling stress of the flange, allowing the component to maintain stable load-bearing capacity even with thinner wall thicknesses. In technical terms, it improves the distortion buckling and local buckling bearing capacity of the section.   To illustrate: imagine a thin piece of paper; it's easy to bend when held flat; but if you fold a small edge on each side, it becomes much stiffer. The inward-curved edge is that "folded edge," with an immediate effect. This is why photovoltaic C-shaped steel must have rolled edges, and not just an open U-shaped groove.   V. Load Transfer Path: From Module to Ground, Not a Single Interruption is Allowed The core safety logic in photovoltaic power plant design is the integrity of the load transfer path. The inward-curved C-shaped steel section occupies a central position on this path. Let's walk through the process from top to bottom:   Wind or snow acts on the surface of the photovoltaic modules.   The modules transfer the load to the purlins (inwardly rolled C-shaped steel) via clamps or bolts.   The purlins then transfer the load to the main beam (which may also be C-shaped steel).   The main beam transfers the load to the columns (usually C-shaped steel or round pipes).   The columns transfer the load to the foundation (cast-in-place piles, helical piles, etc.).   The foundation ultimately transfers the load to the ground (soil or rock).   The failure of any node along this path—such as loose connecting bolts, local buckling of the C-shaped steel, or weld rust—will lead to the collapse of the entire structure. Therefore, photovoltaic support design must not only calculate the strength of each steel section but also verify the load-bearing capacity of the connection nodes and ensure that the coating of all components is continuous at the nodes (e.g., using galvanized bolts, spring nuts, etc.). The long mounting holes on the back of the inwardly rolled C-shaped steel are for easy position adjustment and to allow sufficient slack for bolt connections.   VI. Why Avoid On-Site Welding? In some small-scale photovoltaic projects or temporary power stations, construction teams may cut and weld C-shaped steel on-site for convenience. This is a major taboo for three reasons:   First, the galvanized layer is burned off. During welding, the local temperature can reach over 1500℃, causing the galvanized layer to evaporate or oxidize instantly. The zinc layer around the weld point will also fail due to the high temperature. This point will become a "breakthrough" for corrosion, rusting through from the inside within a few years and becoming irreparable.   Second, welding causes deformation. The steel cools and contracts after being heated locally, causing the C-shaped steel to bend and twist. What was designed to be no more than 1mm straightness per meter may become 5mm per meter after welding. Photovoltaic modules are glass products and are extremely sensitive to flatness; deformation of the support structure can directly lead to microcracks or breakage of the modules.   Third, the strength of the heat-affected zone decreases. The work hardening effect of cold-formed steel is eliminated under the welding thermal cycle, resulting in a lower yield strength near the weld point than the original base material.   Therefore, standard photovoltaic support systems all use bolted connections: prefabricated connectors, bolts, spring nuts, and anti-loosening washers are used for on-site assembly, similar to building blocks. This ensures continuous corrosion resistance, facilitates disassembly and adjustment, and better meets the quality requirement of a 25-year service life.