Technical standard for cold-formed steel structures
1Key Takeaways
1.0.1 This standard is formulated to ensure that the design and construction of cold-formed steel structures comply with national technical and economic policies, achieving safety, applicability, economic rationality, advanced technology, and guaranteed quality. 1.0.2 This standard applies to the design and constructio…
2Expert Interpretation
GB/T 50018-2025, the "Technical Standard for Cold-Formed Steel Structures," has been comprehensively upgraded to cover cold-formed steel with wall thicknesses ranging from 0.6 to 20 mm. It adds S280/S350/S550 steel, seismic design, and the direct strength method, driving technological innovation in the industry. This article provides an in-depth analysis of the standard's core revisions, design indicators, seismic requirements, and implementation recommendations.
Standard Overview and Scope
The "Technical Standard for Cold-Formed Steel Structures" GB/T 50018-2025 will be officially implemented on September 1, 2025, replacing the original "Technical Specification for Cold-Formed Thin-Walled Steel Structures" GB50018-2002. This standard, compiled by the Ministry of Housing and Urban-Rural Development, applies to the design and construction of cold-formed steel structures with a thickness of 0.6mm~20mm in building engineering. It is no longer limited to "thin-walled" and supplements relevant provisions for thick-walled (6mm~20mm) and ultra-thin-walled (0.6mm~2mm) structures.
Comparison of Main Revisions
| Revision Direction | Original Standard 2002 Version | New Standard 2025 Version |
|---|---|---|
| Scope of Application | Wall thickness 2mm~6mm (thin wall) | Wall thickness 0.6mm~20mm, divided into ultra-thin wall, thin wall, and thick wall |
| Material Grade | Q235, Q345 (later changed to Q355) | Added Q390 and S280, S350, S550 cold-rolled steel plates |
| Calculation Method | Effective width method as the main approach | Effective width method + direct strength method (Appendix C) |
| Seismic design | Not covered | New performance-based seismic design method (Chapter 7) |
| Connection calculation | Simple provisions | Includes self-tapping screw crest connection, spliced section connection, etc. |
Material strength design index
The new standard adds strength design values for Q390 and S280, S350, and S550 steels, and takes values for different thickness ranges.
The following is a partial table of design values for the tensile, compressive, and bending strengths of major steel materials:
| Grade | Thickness t (mm) | Yield Strength fy (N/mm²) | Tensile/Compressive/Bending f (N/mm²) | Shear fv (N/mm²) |
|---|---|---|---|---|
| Q235 | 2≤t≤16 | 235 | 205 | 120 |
| Q355 | 2≤t≤16 | 355 | 300 | 175 |
| Q390 | 2≤t≤16 | 390 | 345 | 200 |
| S280 | 0.6≤t≤2 | 280 | 240 | 135 |
| S350 | 0.6≤t≤2 | 350 | 300 | 175 |
| S550 | t≤0.6 | 530 | 455 | 260 |
Resistance partial factor: 1.125 for Q390 steel, 1.165 for others; and specifies the reduction requirements for the strength design value of welds, bolts and self-tapping screws.
Core points of component calculation
The new standard provides detailed calculation formulas for axially compressed, bent, and compression-bending members, and introduces the direct strength method as an optional method. Key changes include:
- >Effective width-to-thickness ratio of compression plates:The calculation formulas for stiffened, partially stiffened, and unstiffened plates are unified, and the plate group constraint coefficient is considered.
- Composite Section:For the cohesive box section shown in Figure 5.2.9, the stability bearing capacity can be multiplied by a reduction factor of 0.7 or simplified by using an amplification factor. Distortion Buckling:Three cases where distortion buckling can be omitted are specified to reduce complex calculations.
New Seismic Design Regulations
The new standard systematically incorporates seismic design (Chapter 7) for the first time, adopting a performance-based design method and setting four performance levels (performance 1~4) and three ductility levels (levels I~III). The design ground motion parameters are based on GB/T50011, and the bearing capacity is verified according to the design earthquake, and the inter-story drift angle is controlled: 1/250 for frequent earthquakes, 1/100 for design earthquakes, and 1/50 for rare earthquakes. The plastic energy dissipation zone of the components must meet the plate width-to-thickness ratio limit (Table 7.1.5-2), and the joints must have rotational capacity (0.03 rad for Class I, 0.02 rad for Class II). Frame beam-column joints must meet the requirements for strong column-weak beam and strong joint, and a table of connection coefficients is provided.
Implementation Recommendations
1. Understanding Wall Thickness Classification:During design, the corresponding strength indicators and structural requirements should be selected based on the actual wall thickness of the components, especially the newly added provisions for thick walls (6~20mm) and ultra-thin walls (<2mm).
2. Selecting Appropriate Analysis Methods:For components with complex cross-sections or sensitive to distortion buckling, it is recommended to use the direct strength method in Appendix C, which can more accurately assess the bearing capacity. 3. **Emphasis on Seismic Design:** For buildings with a seismic intensity ≥ 7 degrees, the performance-based design process in Chapter 7 should be strictly followed, the ductility level should be rationally selected, and the joint connections should be ensured to meet the plastic rotation capacity. 4. **Corrosion and Fire Protection:** The galvanized layer or coating system should be selected according to the classification of environmental erosion effects (Appendix G); hot-dip galvanizing should be preferred for thin-walled components, and fireproofing can be achieved by covering with fireproof boards. 5. **Attention to Connection Details:** When self-tapping screws are used for corrugated joints, the load-bearing capacity test should be conducted according to Appendix E; the screw spacing at the spliced section should meet the maximum spacing requirement of Clause 6.1.10.
Standard Evolution and Technological Outlook
This standard evolved from GBJ18-87 and GB50018-2002. This revision reflects the industry's progress in cold-formed steel from "thin-walled" to "full-thickness" coverage. It also introduces internationally advanced direct strength methods and performance-based seismic design, and is consistent with the design specifications for hot-rolled steel structures. In the future, with the widespread application of high-strength steel (such as S550), the application of cold-formed steel in multi-story residential buildings and industrial plants will become even more extensive.