5.1Mill Materials

The fabricator may purchase materials in stock lengths, exact lengths, or multiples of exact lengths to suit the dimensions shown in the structural design documents. Such purchases will normally be job-specific in nature and may not be suitable for use on other projects or returned for full credit if subsequent design changes make these materials unsuitable for their originally intended use. The fabricator should be paid for these materials upon delivery from the mill, subject to appropriate additional payment or credit if subsequent unanticipated modification or reorder is required. Purchasing materials to exact lengths is not considered fabrication.

Dimensional tolerances for rolled mill material are set forth in ASTM A6/ A6M. Likewise, dimensional tolerances for HSS and pipe material are set forth in the applicable ASTM standard specifications. Normal variations in the cross-sectional geometry of standard structural shapes must be recognized by the designer, the fabricator, the steel detailer, and the erector (for example, see Figure C-5.1). Such tolerances are mandatory because roll wear—thermal distortions of the hot cross section immediately after leaving the forming rolls and differential cooling distortions that take place on the cooling beds—are all unavoidable. Geometric perfection of the cross section is not necessary for either structural or architectural reasons if the tolerances are recognized and provided for.

Figure description:

Subject: Mill tolerances for the cross section of a W-shape steel beam, comparing theoretical vs. actual cross sections.

Key Dimensions Defined:

• $d$: Theoretical depth
• $d_1$: Actual depth
• $d_{max}$: Total depth including flange tilt ($d + T + T'$
• $b_f$: Theoretical flange width
• $b^+, b^-$: Flange width segments from the centerline
• $T, T'$: Flange tilt measurements

U.S. Customary Units Tolerances:

• Flange-tilt ($T + T'$: 1/4 in. for $d \leq 12$ in.; 5/16 in. for $d > 12$ in.
• Actual depth ($d_1$: $d \pm 1/8$ in.
• Actual flange width:
  • $b^+ = b_f/2 \pm 3/16$ in.
  • $b^- = b_f/2 \mp 3/16$ in.
  • $b_{max} = b_f + 1/4$ in. or $- 3/16$ in.

Metric Units Tolerances:

• Flange-tilt ($T + T'$: 6 mm for $d \leq 300$ mm; 8 mm for $d > 300$ mm.
• Actual depth ($d_1$: $d \pm 3$ mm.
• Actual flange width:
  • $b^+ = b_f/2 \pm 5$ mm
  • $b^- = b_f/2 \mp 5$ mm
  • $b_{max} = b_f + 6$ mm or $- 5$ mm

Fig. C-5.1. Mill tolerances on the cross section of a W-shape.

ASTM standard specifications also stipulate tolerances for straightness that are adequate for typical construction. However, these characteristics may be controlled or corrected to closer tolerances during the fabrication process when the added cost is justified by the special requirements for an atypical project.