C-B8B8 Dimensional tolerances

This new section replaces Section M2.7, Dimensional Tolerances, in the 2016 AISC Specification (AISC, 2016).

Dimensional tolerances (and production methods) are used to limit the magnitude of geometric imperfections present in members and structures. In some instances there is a direct relationship between the permitted dimensional tolerances of the member or structure and the design provisions provided in this Specification. Geometric imperfections have the most significant effect relative to the stability of the members and structures. Evaluation of members and structures outside the assumed tolerances falls outside the scope of this Specification and should be based on tests or analysis, subject to the approval of the authority having jurisdiction.

There are a number of conditions that could result in geometric imperfections greater than those assumed in the Specification being present in structural steel elements.

• Engineers sometimes elect to employ materials that are not approved for use. In such cases, the tolerances imposed by the standards for the chosen material may exceed those permitted by comparable ASTM standards or no tolerances may be enforced at all. Engineers must therefore either specify tighter tolerances consistent with those assumed in the Specification or must account for the larger possible tolerances in their designs,
• The AISC Code Standard Practice for Steel Buildings and Bridges (AISC, 2022a), hereafter referred to as the Code of Standard Practice does not address every possible variation that could be encountered in practice. When a tolerance that is not specified in the Code of Standard Practice is required so that the fabricated and erected structural steel satisfy the design intent, the tolerance should be specified in the contract documents. In some instances, it may be impractical to satisfy tolerances comparable to those provided in the Code of Standard Practice or the ASTM standards. In such cases, engineers must account for the larger permitted tolerances in their designs,
• For various reasons, structural steel elements are sometimes unintentionally provided out-of-tolerance. In some instances, the problem may be discovered at a point where repair of the element is impractical or unsafe. In such cases, engineers must account for the larger permitted tolerances in their designs and reinforcement of the member may be an option.

The classification of sections for local buckling in Section B4.1 assumes tolerances and production methods inherent in the ASTM standards approved for use under this Specification in Section A3.1a. The limiting width-to-thickness ratios, $\lambda_{r}$ and $\lambda_{p}$, may have to be adjusted if larger geometric imperfections exist. Chapter C permits the use of any rational method of design for stability as long as the listed effects are considered. Geometric imperfections is one of the effects that must be considered. The effect of geometric imperfections larger than those permitted in the ASTM

standards approved for use under this Specification is not addressed in the effective length method, the first-order analysis method, and the approximate second-order analysis method. The use of these methods where such geometric imperfections exist or are permitted is not consistent with the intent of the Specification. The available compressive strength determined in Chapter E and the minimum strength and stiffness to provide a braced point determined in Appendix 6 also assume that dimensional tolerances provided in the Code of Standard Practice and in the ASTM standards are satisfied.

AISC Design Guide 28, Stability Design of Steel Buildings (Griffis and White, 2013), provides guidance relative to addressing geometric imperfections other than those assumed in the Specification.

There are also conditions for which geometric imperfections may have negligible effect on the strength of the element. Out-of-straightness within reasonable tolerances does not affect the strength of tension members. Applied tension tends to reduce, whereas compression tends to amplify, out-of-straightness. Sometimes cambering of beams can result in distortion of the web. Lateral deflection or out-of-flatness of webs of girders subjected to static loading is of no structural significance.

See also the commentaries to Chapters C, E, and F, and Appendices 1 and 7.