Part 1 — Mill Tolerances (ASTM A6/A6M)
Mill tolerances govern the as-rolled shape of W-shapes, channels, angles, HSS, and plates as delivered from the producing mill. These are the starting point for all subsequent fabrication tolerances — a good fabricator can compensate for mill variation, but the mill tolerance defines what the fabricator is buying.
W-Shape Cross-Section Tolerances
| Parameter | Permitted Variation | How Measured |
|---|---|---|
| Depth (d) — up to 12 in | ±1/8 in (±3.2 mm) | Overall depth including flange thickness |
| Depth (d) — over 12 in | ±3/16 in (±4.8 mm) | Overall depth |
| Flange width (bf) — up to 6 in | ±1/8 in (±3.2 mm) | Each flange independently |
| Flange width (bf) — 6 to 12 in | ±3/16 in (±4.8 mm) | Each flange independently |
| Flange width (bf) — over 12 in | ±1/4 in (±6.4 mm) | Each flange independently |
| Web off-center (e) — all sizes | ±1/4 in (±6.4 mm) max from centerline | Eccentricity of web relative to flange centerline |
| Flange out-of-square (T) | 1/4 in per inch of flange width max | Tilt of flange relative to web |
| Web thickness (tw) | Not less than 0.01 in below nominal | Undertolerance only; overtolerance unlimited |
W-Shape Straightness Tolerances
| Parameter | Permitted Variation | Notes |
|---|---|---|
| Camber (sweep upward) | 1/8 in per 10 ft of length | Measured with beam on its side; natural mill camber |
| Sweep (horizontal bow) | 1/8 in per 10 ft of length | Measured with beam standing on flanges |
| End out-of-square | 1/64 in per inch of depth (≈ 1°) | Maximum gap between beam end and square |
For a W24×84 beam at 40 ft length:
- Maximum natural camber or sweep: 40 × (1/8) / 10 = 0.50 inches
- Maximum end out-of-square: flange width of 9.02 in → out-of-square at beam end ≤ 9.02/64 = 0.14 inches
Plate Tolerances (ASTM A6, for built-up sections)
| Parameter | Permitted Variation |
|---|---|
| Thickness — up to 1/2 in | ±0.010 in undertolerance; overtolerance varies (0.030 in for carbon, 0.050 in for alloy) |
| Thickness — 1/2 to 1 in | ±0.010 in undertolerance; overtolerance 0.030–0.060 in |
| Thickness — over 1 in | ASTM A6 Table 16, typically 0.020–0.060 in undertolerance |
| Width — mill edge | +3/4 in, -0 in (sheared: +1/2 in, -1/8 in) |
| Width — universal mill plate | +3/4 in, -0 in (for widths up to 48 in) |
| Flatness (1/2 in plate and thinner) | 1/2 in max deviation from flat in any 8 ft length |
HSS (Hollow Structural Sections) Tolerances (ASTM A500)
| Parameter | Permitted Variation |
|---|---|
| Outside dimension — 2-1/2 in and under | ±0.020 in, measured across flats |
| Outside dimension — 2-1/2 to 3-1/2 in | ±0.025 in |
| Outside dimension — 3-1/2 to 5-1/2 in | ±0.030 in |
| Outside dimension — over 5-1/2 in | 1% of nominal, ± |
| Wall thickness | -10% minimum; overtolerance not specified |
| Straightness | 1/8 in per 3 ft of length (× 5 for 15 ft maximum) |
| Twist (torsional straightness) | 1/16 in per foot of length, measured as out-of-plane corner |
| Squareness of sides | 90° ± 2° for each adjacent side |
| Corner radii (R) | Not specified; R ≤ 3× wall thickness is a practical limit |
Part 2 — Fabrication Tolerances (AISC 303, Section 6)
Fabrication tolerances govern the accuracy of shop operations: cutting to length, drilling/reaming holes, cambering, welding, and assembly of built-up members. These are the tolerances the structural engineer relies on for fit-up in the field.
Member Length (AISC 303 Section 6.1.1)
| Condition | Permitted Variation |
|---|---|
| Length ≤ 30 ft, member with both ends detailed for connection to other steel | ±1/32 in |
| Length ≤ 30 ft, member without both ends detailed | ±1/16 in |
| Length > 30 ft | ±1/8 in |
The distinction matters: a column that lands on a base plate and connects to a beam above is "detailed at both ends" and must meet the tighter ±1/32 in tolerance. A filler beam that rests on a seat angle (one end detailed) gets the ±1/16 in tolerance.
Straightness of Fabricated Members (AISC 303 Section 6.2.3)
| Member Type | Permitted Variation |
|---|---|
| Individual rolled beam or column (as-received) | ASTM A6 per Part 1 above |
| Welded built-up column or beam (shop-fabricated) | 1/1000 of length between points of lateral support |
| Truss chords (individual segment between panel points) | 1/1000 of segment length |
| Compression members (single-angle struts) | 1/1000 of length, ≤ 1/4 in |
For a welded plate girder column, 30 ft tall: maximum straightness deviation = 360 in / 1000 = 0.36 in (approximately 3/8 in). This is more generous than mill tolerance for a single rolled section because welding distortion must be accounted for.
Camber, Sweep, and Beam Alignment (AISC 303 Section 6.5)
Camber is the intentional upward curvature fabricated into a beam to offset dead load deflection. AISC 303 addresses the tolerance on the as-fabricated camber — how close the actual camber must be to the specified camber.
| Specified Camber | Tolerance on As-Delivered Camber |
|---|---|
| Any specified camber | +1/2 in, -0 in |
| No camber specified (natural mill camber only) | As permitted by ASTM A6 (1/8 in per 10 ft) |
The +1/2 in, -0 in tolerance is important: over-cambering by up to 1/2 in is acceptable because it produces a conservative beam (more upward bow = less dead load deflection), but under-cambering (less camber than specified) is not permitted because it could produce the appearance of sag under dead load alone.
Bolt Hole Location (AISC 303 Section 6.2.1)
| Parameter | Permitted Variation |
|---|---|
| Individual hole location from theoretical | ±1/16 in |
| Outermost holes in group, center-to-center | ±1/32 in per foot of distance between holes, ≤ ±1/16 in total |
| Standard hole diameter vs nominal bolt diameter | +1/16 in over bolt diameter (for bolts ≤ 1 in); +1/8 in (for bolts > 1 in) |
| Oversized hole diameter | Per RCSC Table 3.1: +3/16 in over bolt dia for ≤ 7/8 in bolts, +1/4 in for 1 in bolts, +5/16 in for ≥ 1-1/8 in bolts |
| Short-slotted hole width | +1/16 in over bolt diameter |
| Short-slotted hole length | +1/16 in over (bolt diameter + allowable slot length per RCSC) |
Welded Built-Up Members (AISC 303 Section 6.4)
| Parameter | Permitted Variation |
|---|---|
| Out-of-straightness, welded plate girder | 1/1000 of length, ≤ 3/8 in for spacing ≤ 30 ft |
| Flange tilt (out-of-square) | 1/4 in per inch of flange width, ≤ 1/4 in total |
| Web flatness (panel between stiffeners) | D / 165, where D = web depth (e.g., 36 in web / 165 = 0.22 in max bow) |
| Web flatness (panel with longitudinal stiffener) | D / 330 |
| Stiffener fit (gap at web contact) | ≤ 1/16 in gap before welding |
| Stiffener straightness | 1/1000 of stiffener length |
| Cross-sectional area / weight | Per ASTM A6 or contract specification |
Part 3 — Erection Tolerances (AISC 303, Section 7)
Erection tolerances govern the accuracy of the completed steel frame in its final position. These are the tolerances that matter for curtain wall fit-up, elevator rail alignment, and overall building geometry. They are the end result of accumulated mill tolerances + fabrication tolerances + erection tolerances.
Column Plumbness (AISC 303 Section 7.13.1.1)
| Condition | Permitted Variation |
|---|---|
| Individual column, single story | 1:500 of story height |
| Building column line, cumulative (H ≤ 300 ft) | 1:500 of total height, ≤ 2 in |
| Building column line, cumulative (H > 300 ft) | 1:500 of total height, ≤ H/300 (but ≤ 6 in) |
| Exterior column, adjacent column offset at any floor | 1 in, except as may be required by the building envelope (curtain wall attachment) |
Example: 20-story building, 240 ft tall (12 ft per story):
- Individual column per story: 144 in / 500 = 0.29 in per floor
- Cumulative at roof: 2,880 in / 500 = 5.76 in total, but H > 300 ft does not apply, so the 2 in cap governs. The 240 ft building roof column line must be within 2 in of theoretical location.
Column Base Plate — Level and Alignment (AISC 303 Section 7.11)
| Parameter | Permitted Variation |
|---|---|
| Top of base plate elevation | +0 in, -3/4 in from specified elevation |
| Centerline location in plan | ±1/4 in from specified location |
| Level (from horizontal) | 1:500 of the base plate dimension |
Anchor bolts are set by the general contractor (not the steel erector) to tolerances specified in AISC 303 Section 7.5. This division of responsibility is a common source of disputes and must be explicitly addressed in the contract.
Beam Alignment and Elevation (AISC 303 Section 7.13.1.2)
| Parameter | Permitted Variation |
|---|---|
| Beam elevation at column connection | ±1/4 in from specified |
| Beam elevation at mid-span (relative to column connection) | ±(span/500), ≤ 1 in |
| Beam horizontal alignment (plan location) | ±1/2 in from specified centerline |
| Beam end bearing — gap at top flange | ≤ 3/16 in between top flange and column flange (for directly welded moment connections) |
Column Splice Alignment (AISC 303 Section 7.13.1.3)
| Parameter | Permitted Variation |
|---|---|
| Offset of abutting sections, milled splice faces | 6% of thinner section thickness, ≤ 1/8 in |
| Offset of abutting sections, non-milled | As permitted by connection design (typically ≤ 1/4 in) |
| Gap between abutting faces, milled for full bearing | 1/16 in max, no shimming permitted without engineer approval |
| Gap between abutting faces, partial penetration weld | As required for the specified weld detail |
Part 4 — Coordination: How Tolerances Stack Up
The three tolerance categories compound. A 40 ft W24×84 beam, cambered 1.5 in at the shop, installed at the 20th floor of a building, has the following accumulated uncertainty:
| Source | Contribution to Final Position Uncertainty |
|---|---|
| Mill: sweep tolerance | 40 × 0.125/10 = 0.50 in |
| Mill: depth tolerance (±3/16 in) | 0.19 in (affects beam-to-beam alignment) |
| Fabrication: camber tolerance | +0.50 in (over-camber permitted) |
| Fabrication: length tolerance | ±0.125 in (> 30 ft) |
| Fabrication: hole location (±1/16 in) | 0.06 in (at the specific connection) |
| Erection: column plumbness at floor | 0.29 in (12 ft story) |
| Erection: beam elevation | ±0.25 in |
| Root-sum-square (RSS) combined | √(0.50² + 0.19² + 0.125² + 0.06² + 0.29² + 0.25²) = √(0.25 + 0.036 + 0.016 + 0.004 + 0.084 + 0.063) = √0.453 = 0.67 in |
This 0.67 in total uncertainty is why standard bolted connections use 1/16 in oversized holes (providing 1/16 in of adjustment in each direction at each bolt), why beam-to-column flange connections provide a total erection clearance of up to 1/16 in in the bolted or welded configuration, and why steel construction tolerances are fundamentally designed to permit fit-up without field modification for 99.7% of connections.
Inspection and Verification
Measuring Straightness
Beam straightness is measured with the member supported near its ends, with a taut wire or laser line stretched between measurement points. A ruler or feeler gauge measures the maximum deviation from the straight line at any point along the member length. The member is rotated to verify sweep in both major and minor axes.
Measuring Plumbness
Column plumbness is verified using a plumb bob, transit, or laser plummet at two orthogonal faces — typically the flange tips or web centerline for W-shapes. Plumbness must be checked before and after final bolt tightening because the tightening sequence can shift column alignment by 1/16 to 1/8 in.
Measuring Bolt Hole Location
A calibrated steel tape or coordinate measuring system (for critical connections) verifies hole center-to-center distances. Go/no-go gauge pins of the nominal bolt diameter verify hole size and roundness.
When Tolerances Are Exceeded
Per AISC 303 Section 8.1, work that exceeds the specified tolerances is not automatically rejectable — it is subject to evaluation by the engineer of record. The evaluating engineer must determine whether the deviation:
- Impairs the strength or serviceability of the completed structure
- Creates unacceptable fit-up problems for subsequent trades (curtain wall, elevator, MEP)
- Can be corrected at reasonable cost
If the deviation is accepted as-is, the acceptance must be documented in writing, including any compensating adjustments to the design (e.g., reduced column capacity due to increased eccentricity). If rejected, the erector/fabricator must submit a written repair proposal for engineer approval before proceeding.
Quick Reference — Key AISC 303 Tolerances
| Tolerance Category | Maximum Permitted Variation |
|---|---|
| Beam length ≤ 30 ft | ±1/16 in |
| Beam length > 30 ft | ±1/8 in |
| Beam straightness (mill) | 1/8 in per 10 ft |
| Column straightness (fabricated) | 1/1000 of length |
| Fabricated camber | +1/2 in, -0 in |
| Bolt hole location | ±1/16 in from theoretical |
| Column plumbness per story | 1:500 of story height |
| Column plumbness cumulative (≤ 300 ft) | ≤ 2 in |
| Base plate elevation | +0 in, -3/4 in |
| Beam elevation at support | ±1/4 in |
| Beam mid-span elevation | ±(span/500), ≤ 1 in |
| Column splice offset (milled) | ≤ 1/8 in |
References
- AISC 303-22 — Code of Standard Practice for Steel Buildings and Bridges
- ASTM A6/A6M-22 — General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling
- ASTM A500/A500M-21 — Cold-Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes
- RCSC Specification for Structural Joints Using High-Strength Bolts (2020)
- AISC Steel Construction Manual, 16th Edition — Part 1 (Dimensions and Properties)
- AWS D1.1/D1.1M:2020 — Structural Welding Code — Steel, Sections 5.22–5.24
- MBMA Metal Building Systems Manual — Tolerances for Pre-Engineered Metal Buildings