Bolt Gauge Distance — g1/g2/g3, Standard Gauges & AISC Table 1-1
Bolt gauge (g) is the transverse center-to-center spacing between bolt holes, measured perpendicular to the member's longitudinal axis. For a column or beam flange, gauge is measured across the flange width. For an angle leg, gauge is measured from the heel (back of the angle) to the bolt hole centerline. Gauge is distinct from pitch (p) — which is the longitudinal center-to-center bolt spacing along the member axis — and from edge distance (Le) — which is the distance from the hole center to the edge of the plate or member.
Bolt hole layout notation:
g = gauge (transverse spacing, perpendicular to member axis)
p = pitch (longitudinal spacing, parallel to member axis)
Le = edge distance (hole center to plate edge, measured normal to edge)
PRELIMINARY — NOT FOR CONSTRUCTION. All content is for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.
Standard Gauges for W-Shape Flanges
AISC Manual Table 1-1 provides the workable gauge (g) for each W-shape. The gauge is constrained by the flange width bf and the need for wrench clearance:
| W-Shape | bf (in) | Workable Gauge (in) | Notes |
|---|---|---|---|
| W8x31 | 8.00 | 5.5 | Single row each side of web |
| W10x49 | 10.00 | 5.5 | Single row each side |
| W12x65 | 12.00 | 5.5 | Two rows possible (g1=5.5, g2=5.5) |
| W14x90 | 14.52 | 5.5 + 5.5 | Two rows: g1=5.5, g2=3.5 or 5.5 |
| W18x76 | 11.04 | 5.5 | Single row |
| W24x104 | 12.75 | 5.5 | Single or double row |
| W30x148 | 10.50 | 5.5 | Single row |
| W36x210 | 12.20 | 5.5 | Single row |
The standard workable gauge of 5.5 inches for most W-shapes is based on providing adequate clearance for 3/4-inch and 7/8-inch bolts with heavy hex heads and impact wrenches. When two rows of bolts are needed (deeper members or higher forces), inner and outer gauges are specified as g1 and g2.
Gauge Notation: g1, g2, g3
For multiple rows of bolts in a flange or plate:
| Symbol | Description | Typical Usage |
|---|---|---|
| g1 | Gauge for innermost row (closest to web/heel) | Standard connection, first row |
| g2 | Center-to-center from row 1 to row 2 | Second row of bolts (heavier loads) |
| g3 | Center-to-center from row 2 to row 3 | Third row (rare, built-up sections) |
Example — W14 flange with 4 bolts transverse (2 per side):
←── g2=3.5" ──→←─ g1=5.5" ─→←── g2=3.5" ──→
○ ○ WEB ○ ○
└─ edge Le ──┘
bf = 16.0" total flange width
Each successive gauge is measured centerline-to-centerline to the next bolt row. The designer must verify that the sum of gauges plus edge distances does not exceed the available flange width.
Standard Gauges for Angles
AISC Manual Table 1-7A lists standard gauges for angle legs:
| Angle Leg (in) | g1 (in) — 1st row | g2 (in) — 2nd row | Maximum rows |
|---|---|---|---|
| 2.0 | 1.125 | 1.375 | 2 |
| 2.5 | 1.375 | 1.625 | 2 |
| 3.0 | 1.75 | 2.0 | 2 |
| 4.0 | 2.5 | 3.0 | 2 |
| 5.0 | 3.0 | 3.0 | 2 |
| 6.0 | 3.5 | 4.0 | 2-3 |
| 8.0 | 4.5 | 5.0 | 3-4 |
Angle gauge is measured from the heel (back of the angle) to the bolt centerline, not center-to-center between adjacent bolts as in flanges. For the outstanding leg of a connected angle, the gauge is measured from the heel of that leg.
Edge Distance Requirements (AISC J3.4)
The bolt gauge must leave sufficient edge distance Le from the outermost hole to the member edge:
| Bolt Diameter (in) | Le_min — Sheared Edge (in) | Le_min — Rolled/ Gas-Cut Edge (in) |
|---|---|---|
| 1/2 | 0.75 | 0.625 |
| 5/8 | 0.875 | 0.75 |
| 3/4 | 1.0 | 0.875 |
| 7/8 | 1.125 | 1.0 |
| 1.0 | 1.25 | 1.125 |
| 1 1/8 | 1.5 | 1.25 |
| 1 1/4 | 1.625 | 1.375 |
These are minimum values per AISC Table J3.4. In practice, standard gauges are larger and provide edge distances well above these minimums to accommodate fabrication tolerances and provide bearing and tear-out capacity.
Frequently Asked Questions
What is the difference between gauge and pitch?
Gauge (g) is the transverse (cross-width) center-to-center bolt spacing. Pitch (p) is the longitudinal (along-member) center-to-center spacing. Together, gauge and pitch define the bolt hole layout grid. Edge distance (Le) is measured from the hole center to the nearest edge, perpendicular to that edge.
Why is the standard workable gauge 5.5 inches for W-shapes?
The 5.5-inch gauge accommodates standard heavy hex bolt heads and nuts (1.25-1.625 in across flats for 3/4-1 in bolts) with impact wrench clearance. A tighter gauge would prevent the wrench from seating properly on the bolt head/nut. The 5.5-inch value has been standardized since the 1920s and allows interchangeable fabrication.
Does gauge affect net section calculations?
Yes. For tensile rupture of the connected element, the net width is the gross width minus the sum of hole diameters plus s²/4g for staggered holes. A wider gauge (larger g) reduces the s²/4g additive term, increasing the net width. Multiple rows of bolts reduce the net section deduction compared to a single row.
Related Terms and Pages
- Pitch Distance — Bolt Pitch, Minimum & Staggered
- Block Shear — Definition & Failure Mode
- Staggered Bolt Pattern — Net Section & s²/4g Rule
- Prying Action — Definition & Bolt Force
- Bolted Connection Calculator — Free Online Tool
- Connection Design Workflow
Educational reference only. Bolt gauge, pitch, and edge distance must comply with AISC 360 Section J3 and the AISC Manual of Steel Construction. All designs must be independently verified by a licensed Professional Engineer.
Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.