Question:
We have received CFS material that is slightly below the “minimum thickness” in the thickness table in the SFIA Technical Manual. The supplier is telling me that those tables are conservative and a larger tolerance is acceptable. Are those tables codified, and should we accept this material?
Answer:
The governing requirement for minimum thickness of cold-formed steel members is established by AISI/SDI S100-24, not by SFIA or other industry groups. While the SFIA technical guide publication is not a code document, it does summarize requirements from AISI/SDI standards is a more digestible table.
Per AISI/SDI S100-2024, Section B7.1, the delivered base metal thickness must be at least:
- 95% of the design thickness (t)
This requirement has remained consistent across code editions, although its location within the specification has changed.
Key clarifications:
- The tabulated minimum thickness values published by organizations such as SFIA are based on this AISI requirement and are accurate.
- If a member’s measured thickness is below 95% of the specified design thickness, it does not meet AISI requirements and should be rejected.
- Accepting nonconforming material introduces risk, as it may be rejected later by the owner, inspector, or contractor.
- Design strengths and connection capacities in AISI/SDI S100-24 are based on the assumption that the member meets the minimum 95% thickness requirement. Material below this threshold will have reduced strength and may not perform as assumed in design.
Measurement considerations:
- Thickness requirements apply to base metal only, excluding galvanization
- Field measurements (e.g., calipers) often include the zinc coating, which can add approximately 1 mil total for G60 studs, meaning the actual base metal thickness is less than the measured value.
- Measurements should be taken:
- Away from bends, lips, or cut edges that might have distorted regions caused by fabrication
Its not recommended to accept material below the “minimum thickness” without careful engineering consideration.