Choosing the wrong purlin can lead to issues such as the inability to install roof panels, failure to support the required loads, or premature failure due to corrosion within just two years. Below, we clarify the differences, suitable applications, and key selection criteria for four common types of cold-formed thin-walled steel purlins.

Quick Identification of Four Purlin Types

 Detailed Characteristics of Each Type

1. C-section Steel (Most Common)

Characteristics: Symmetrical cross-section; simple processing and easy installation.

Application: Simply supported purlins. According to national standard atlas 11G521-1, the applicable span for C-section purlins is 4–9m, with an economical span of 4–6m; specific dimensions are determined by load calculations.

Selection Points: Height (100–300mm), flange width (40–80mm), lip/stiffener (10–20mm), and thickness (1.5–3.0mm). Load-bearing capacity depends primarily on the cross-section height.

2. Z-section Steel (Suitable for Long-span Continuous Purlins)

Characteristics: Top and bottom flanges face opposite directions; lapping allows for the formation of continuous purlins, resulting in superior structural performance.

Application: Continuous roof purlins; spans of 6–9m; commonly used in industrial factory buildings.

Selection Points: Lap length should not be less than 10% of the span (as required by GB 51022-2015) and not less than 600mm; alternatively, use the larger value based on design requirements. Bolted connections at Z-section overlaps must be secure.

3. U-section steel (auxiliary component)

Characteristics: Channel-shaped cross-section, no edge lips, relatively low strength.

Applications: Door frames, window frames, edge trim, struts, connectors, equipment suspension.

Selection notes: Generally not used as primary load-bearing purlins unless specifically verified by a structural engineer; ensure proper anti-corrosion treatment.

4. "几"-section steel (specialized for secondary purlins)

Characteristics: Cross-section resembles the Chinese character "几" (inverted U-shape with flanges); high rigidity; usually arranged perpendicular to primary purlins to form a double-layer purlin system.

Applications: Roof purlins, secondary purlins, photovoltaic panel supports, roof panel supports. Cannot replace primary purlins for supporting heavy roof panel loads.

Selection notes: Common specifications include 41×41 and 52×52 sizes with thicknesses of 1.5–2.5 mm; pay attention to the connection method with primary purlins.

Selection Decision Matrix

Selecting the right purlin involves more than just choosing a cross-sectional shape; it requires a comprehensive assessment of factors such as the building span, loading conditions, roof slope, connection methods, and budget. It is advisable to consult a professional structural engineer during the initial design phase to perform detailed calculations based on regulatory standards, ensuring that every choice can withstand both the applied loads and the test of time.

For more information needed or any inquiry,please feel free to contact Yumisteel team.