Steel Tolerances — Mill, Fabrication & Erection Limits
ASTM A6 mill tolerances, AISC Code of Standard Practice (AISC 303) erection tolerances, and international equivalents. How tolerances affect design calculations.
Why tolerances matter for design
Structural steel is manufactured, fabricated, and erected to specified tolerances — not exact dimensions. A W14x82 column may be 0.5 mm thicker in the flange than nominal, 3 mm out of straightness over its length, and 15 mm out of plumb after erection. These deviations are normal and permitted, but they generate secondary stresses and eccentricities that must be accounted for in design.
Column buckling formulas in all codes implicitly assume an initial out-of-straightness equal to the mill tolerance (L/1000 to L/1500). If the actual column has a greater initial imperfection, the predicted buckling capacity is unconservative. Similarly, beam-to-column connections designed for zero eccentricity must accommodate the actual dimensional variation from fabrication and erection tolerances.
ASTM A6 mill tolerances (key values)
| Property | Tolerance | Notes |
|---|---|---|
| Depth (d) | +/- 3 mm (up to 310 mm depth), +/- 5 mm (over 310 mm) | Measured at center of web |
| Flange width (bf) | +/- 5 mm (bf <= 150 mm), +/- 6 mm (bf > 150 mm) | Per flange |
| Flange thickness (tf) | -0.4 mm to +1.2 mm (tf <= 12 mm), wider range for thicker | Under-tolerance reduces section properties |
| Web thickness (tw) | -0.4 mm to +0.8 mm | Under-tolerance reduces shear capacity |
| Out of straightness | L/1000 (camber and sweep) | Over the full length |
| Cross-section squareness | T/b <= 2.5% | Flange tilt (out-of-square) |
| Weight | -2.5% to +2.5% | Per piece, not per length |
The under-tolerance on flange thickness is critical. A W14x82 with tf = 0.855 in nominal could have tf = 0.839 in (-0.4 mm). This reduces Ix by approximately 2 percent and Zx by approximately 1.5 percent. Standard capacity tables use nominal dimensions, so the mill tolerance is implicitly covered by the phi factor.
AISC 303 erection tolerances
| Condition | Tolerance | Reference |
|---|---|---|
| Column plumb | 1/500 of height, max 25 mm per story, max 50 mm total | AISC 303-22 Cl. 7.13.1 |
| Beam elevation | +/- 5 mm from theoretical | Cl. 7.13.2 |
| Beam horizontal alignment | +/- 5 mm | Cl. 7.13.2 |
| Column splice alignment | 3 mm offset max | Cl. 7.13.3 |
| Anchor bolt position | +/- 6 mm from center of bolt group | Cl. 7.5 |
| Base plate bearing | 3 mm gap max (unless grouted) | Cl. 7.5.3 |
Worked example — eccentricity from column out-of-plumb
A W14x82 column (A992, Fy = 50 ksi) in a 12 ft (3,658 mm) story with factored axial load Pu = 400 kips and factored end moment Mu = 50 kip-ft.
Maximum permitted out-of-plumb = 3,658 / 500 = 7.3 mm = 0.29 in. Additional moment from eccentricity: delta_M = Pu x e = 400 x 0.29 / 12 = 9.7 kip-ft.
Total design moment = 50 + 9.7 = 59.7 kip-ft. This is a 19 percent increase over the nominal moment. For a column near its interaction check limit (Pu/phi*Pn = 0.75), this additional eccentricity can push the combined interaction ratio from 0.95 to 1.05 — meaning the column fails the code check if the tolerance is not considered.
In practice, AISC 360 Direct Analysis Method accounts for this by applying notional loads of 0.002Yi at each level (where Yi is the gravity load), which approximates the effect of a L/500 out-of-plumb. If using the Direct Analysis Method, separate eccentricity calculation is not needed.
International tolerance standards
| Standard | Mill tolerance reference | Erection tolerance reference |
|---|---|---|
| ASTM A6 / AISC 303 | ASTM A6/A6M Table 1 | AISC 303-22 Section 7.13 |
| AS/NZS 3679.1 / AS 4100 | AS/NZS 3679.1 Table 4.1 | AS 4100 Appendix E |
| EN 10034 / EN 1090-2 | EN 10034 Table 1 | EN 1090-2 Table D.2 |
| CSA G40.20 / CSA S16 | CSA G40.20 Table 4 | CSA S16-19 Cl. 28.6 |
EN 1090-2 defines three execution classes (EXC1 to EXC3) with progressively tighter tolerances. EXC2 is standard for buildings. EXC3 is for high-consequence structures (bridges, nuclear). EXC3 erection plumb tolerance is 1/750 of story height versus 1/500 for EXC2.
AS 4100 Appendix E specifies column out-of-plumb as the lesser of L/500 or 10 mm per story, and overall building out-of-plumb as 25 mm maximum. These are tighter than AISC 303.
Common pitfalls
- Assuming nominal dimensions are exact. Section property databases use nominal values. The actual section may be 1-2 percent weaker due to under-rolling. The phi factor covers this statistically, but for critical members at high utilization ratios, consider measuring actual dimensions from the mill certificate.
- Not checking anchor bolt tolerances against base plate hole sizes. AISC 303 permits anchor bolts to be +/- 6 mm from theoretical position. If the base plate holes are standard (bolt diameter + 2 mm), there is no room for bolt misplacement. Oversized or slotted holes in base plates are standard practice.
- Specifying camber tighter than achievable. Minimum practical camber for hot-rolled beams is approximately 19 mm (3/4 in). Specifying 10 mm camber on a short beam results in the fabricator ignoring it or cambering to 19 mm.
- Ignoring cumulative plumb error in multi-story buildings. While each story may be within 1/500 plumb tolerance, the overall building drift accumulates. A 20-story building could be 50 mm out of plumb at the top. Elevator guide rails and curtain wall systems must accommodate this.
Run this calculation
Related references
- Bolt Hole Sizes
- Steel Grades
- Steel Crane Girder
- Surface Finish
- How to Verify Calculations
- bolt capacity calculator
- weld capacity calculator
Disclaimer
This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.