Although both are "C" - shaped, their cross - sectional details and structural strengths are quite different, which directly affects their load - bearing capacities and application scopes.

The cross - section of C Channel is a hot - rolled integral structure. Its web (the vertical part of the "C") is thick (usually 6mm - 16mm), and the flanges (the two horizontal sides) are wide and have a certain slope (to facilitate hot - rolling processing). This design makes the cross - section have strong bending resistance and torsional rigidity. For example, a 10# C Channel (with a height of 100mm) has a web thickness of 5.3mm and flange widths of 48mm, which can easily bear the weight of floors or walls in the main structure.

C Purlin, on the other hand, is formed by cold bending of thin steel plates. Its cross - section is more "slim": the web thickness is only 1.5mm - 4mm, and the flanges are narrow and often have small folds (called "reinforcing ribs") on the edges. These reinforcing ribs are designed to improve the local stability of the thin flanges and prevent deformation under small loads. However, due to the thin material, the overall torsional resistance of C Purlin is weak. For instance, a common C160×60×20×2.5 C Purlin (height × flange width × web height × thickness) has a total weight of only about 5.5kg per meter, which is far lighter than the 10# C Channel (about 12.7kg per meter).