The design code used for cold-formed thin-walled steel is GB50018-2002 (implemented from January 1, 2003), while the design code used for hot-rolled steel is GB50017-2003 (implemented from December 1, 2003). The reason why cold formed thin-walled steel sections need to have separate design specifications is mainly because they have significant differences in performance.

1. Due to different production methods, residual stress is formed by cold bending of cold-formed steel, while residual stress is formed by welding and temperature changes of hot-rolled steel. The magnitude and distribution of residual stress in the cross-section are different.

2. The material properties of cold-formed steel sections change after cold bending, and there is a cold bending effect at the corners, which means that the strength increases and the plasticity decreases; The strength of the flat plate has also been improved, and the strength increase value after cold bending can be utilized in design.

3. The width=100% to thickness ratio of cold-formed steel section plates is large, and there is no limit to the width=100% to thickness ratio in the design, allowing for local buckling and utilizing the post buckling strength of the plates; Local buckling is not allowed in hot-rolled steel structures, which limits the width=100% to thickness ratio of cross-section plates. Cold formed steel components utilize the strength after local buckling of the cross-section plate, and consider the impact of local buckling on overall buckling using an effective cross-section. However, hot-rolled steel sections are not allowed to experience local buckling of cross-sectional plates before the overall buckling of the components, by limiting the width=100% to thickness ratio of the plates or setting stiffeners.

4. Cold formed steel generally has a large number of uniaxial symmetric open sections and weak torsional resistance. Due to the fact that the centroid of a uniaxial symmetric section generally does not coincide with the shear center, if the load does not pass through the shear center, the compression bar often undergoes bending and torsional buckling control; The transverse bending component undergoes torsion, resulting in double moments.

5. The instability modes of the pressure bar vary. The overall buckling modes are basically the same, including bending buckling, bending torsional buckling, and torsional buckling. However, local buckling does not occur in hot-rolled steel sections, and there are two local buckling modes for cold-formed steel sections:local buckling and distorted buckling.

6. Cold formed steel is prone to bending of the web under concentrated force. Hot rolled section steel generally does not appear.

7. The difference in connection methods between cold-formed steel components and hot-rolled steel components. In hot-rolled steel components, welding and high-strength bolt connections are mainly used, while in cold-formed steel components, welding is less commonly used (damaging the zinc coating), and welding thin plates requires high welding technology. Its unique welding types include bell welding and resistance spot welding. Cold formed steel sections are sometimes connected with bolts, but the failure mode is different from that of hot-rolled steel sections. Cold formed steel plates are often subjected to compression failure rather than bolt shear failure due to their thin thickness. Cold formed steel mainly adopts new connection methods such as self tapping and self drilling screw connection, rivet pulling, and nail shooting connection. The cold formed steel components are also mechanically connected (engaged, buckled) without connectors.

8. For cold-formed steel with thin walls, the GB50018-2002 specification stipulates that the calculation of component sections does not consider plastic development and takes edge yielding as the criterion; The hot-rolled steel section can consider partial plastic development. In addition, the cross-sectional characteristics of cold-formed steel members can be calculated using the linear method.

9. The production equipment for cold formed steel is simple, with low investment. The cold rolling mill can replace the rollers to produce new products, with a wide variety of types; The production equipment for hot rolled section steel is complex and requires a large investment. The hot rolling mill cannot change the rollers, and the product is single, only producing standard sections of specifications.