AISC Bolt Spacing and Edge Distance Requirements
AISC 360-22 specifies minimum and maximum bolt spacing and minimum edge distances to ensure structural adequacy, prevent material damage, and allow proper bolt installation and inspection. These requirements appear in Sections J3.3, J3.4, and J3.5 of the specification. Bolt spacing that is too close can cause material splitting between holes; spacing that is too wide can allow plate buckling or moisture intrusion. Edge distances that are too small can cause tearout failure or edge splitting during tightening.
This page provides complete tables for minimum spacing, preferred spacing, edge distances, and maximum limits per AISC 360-22.
Quick access:
- Minimum Bolt Spacing (Section J3.3)
- Minimum Edge Distance (Table J3.4)
- Maximum Bolt Spacing (Section J3.5)
- Maximum Edge Distance
- Practical Design Guidelines
- Worked Examples
- Frequently Asked Questions
Minimum Bolt Spacing — AISC 360 Section J3.3
The minimum center-to-center spacing between bolts in any direction is 2-2/3 times the nominal bolt diameter (2.67d). A spacing of 3d is preferred for practical reasons — it provides room for the wrench to turn during installation and for inspection of the tightened bolt.
| Bolt Diameter (in) | 2.67d Minimum (in) | Preferred 3d (in) | Bolt Diameter (mm) | 2.67d Minimum (mm) | Preferred 3d (mm) |
|---|---|---|---|---|---|
| 1/2 | 1-5/16 | 1-1/2 | M16 | 43 | 48 |
| 5/8 | 1-5/8 | 1-7/8 | M20 | 53 | 60 |
| 3/4 | 2 | 2-1/4 | M22 | 59 | 66 |
| 7/8 | 2-5/16 | 2-5/8 | M24 | 64 | 72 |
| 1 | 2-5/8 | 3 | M27 | 72 | 81 |
| 1-1/8 | 3 | 3-3/8 | M30 | 80 | 90 |
| 1-1/4 | 3-5/16 | 3-3/4 | M36 | 96 | 108 |
| 1-3/8 | 3-5/8 | 4-1/8 | |||
| 1-1/2 | 4 | 4-1/2 |
Why 3d is Preferred
The code minimum of 2.67d is the absolute lower bound. In practice, 3d spacing is preferred because:
- Installation: Standard impact wrenches and spud wrenches need room to fit between adjacent bolt heads
- Inspection: The turn-of-nut method requires enough space to see the nut turn relative to the bolt
- Material integrity: Closer spacing increases the stress concentration between holes, especially in tension members
- Gap washers: In some configurations, the bolt head or nut may overlap the adjacent hole if spacing is too tight
Most detailers use 3d as the default spacing. Use 2.67d only when space is genuinely constrained (e.g., short connection legs on small angles).
Minimum Edge Distance — AISC 360 Table J3.4
The minimum edge distance is the distance from the center of the bolt hole to the nearest edge of the connected part. It depends on the bolt diameter and the edge condition (sheared vs. rolled/milled/thermally cut).
At Sheared Edges
Sheared edges have a rougher surface and residual stress from the shearing process, requiring larger minimum edge distances.
| Bolt Diameter (in) | Min Edge Distance (in) | Bolt Diameter (mm) | Min Edge Distance (mm) |
|---|---|---|---|
| 1/2 | 7/8 | M16 | 22 |
| 5/8 | 1-1/8 | M20 | 28 |
| 3/4 | 1-1/4 | M22 | 30 |
| 7/8 | 1-1/2 | M24 | 32 |
| 1 | 1-3/4 | M27 | 36 |
| 1-1/8 | 2 | M30 | 40 |
| 1-1/4 | 2-1/4 | M36 | 48 |
| Over 1-1/4 | 1.75 x d | Over M36 | 1.75 x d |
At Rolled, Milled, or Thermally Cut Edges
These edges are smoother and have less residual stress, so smaller minimum edge distances are permitted.
| Bolt Diameter (in) | Min Edge Distance (in) | Bolt Diameter (mm) | Min Edge Distance (mm) |
|---|---|---|---|
| 1/2 | 3/4 | M16 | 19 |
| 5/8 | 7/8 | M20 | 22 |
| 3/4 | 1 | M22 | 25 |
| 7/8 | 1-1/8 | M24 | 27 |
| 1 | 1-1/4 | M27 | 32 |
| 1-1/8 | 1-1/2 | M30 | 35 |
| 1-1/4 | 1-5/8 | M36 | 40 |
| Over 1-1/4 | 1.25 x d | Over M36 | 1.25 x d |
Note: "Thermally cut" includes gas-cut, plasma-cut, and laser-cut edges. These are treated the same as rolled or milled edges for minimum edge distance purposes.
Edge Distance Formula Summary
| Edge Condition | Formula for d > 1-1/4" |
|---|---|
| Sheared edges | 1.75 x d |
| Rolled/milled/thermal | 1.25 x d |
Maximum Bolt Spacing — AISC 360 Section J3.5
Maximum spacing limits prevent moisture intrusion, plate buckling between bolts, and paint failure in exposed conditions.
| Condition | Maximum Spacing |
|---|---|
| Connected parts in contact (painted or unpainted) | Lesser of 24t and 12 in |
| Unpainted weathering steel subject to corrosion | Lesser of 14t and 7 in |
| Along a line of elements in compression | Lesser of 127/sqrt(F_y) and 12 in |
Where:
- t = thickness of the thinnest outside plate or shape (in inches)
- F_y = specified minimum yield stress of the compression element (in ksi)
Maximum Edge Distance
| Condition | Maximum Edge Distance |
|---|---|
| General | Lesser of 12t and 6 in |
Where t = thickness of the connected part.
Maximum Spacing Examples
Example 1: A 3/8" plate (t = 0.375") in a painted structure:
Max spacing = lesser of 24 x 0.375 = 9.0" and 12" → 9.0"
Max edge distance = lesser of 12 x 0.375 = 4.5" and 6" → 4.5"
Example 2: A 1/2" plate (t = 0.50") in unpainted weathering steel:
Max spacing = lesser of 14 x 0.50 = 7.0" and 7" → 7.0"
Max edge distance = lesser of 12 x 0.50 = 6.0" and 6" → 6.0"
Example 3: A 5/8" plate (t = 0.625") with F_y = 50 ksi in a compression member:
Max spacing (compression) = lesser of 127/sqrt(50) = 18.0" and 12" → 12"
Max spacing (general) = lesser of 24 x 0.625 = 15.0" and 12" → 12"
Both controls give 12" in this case.
Practical Design Guidelines
Gage Distance
The gage distance (g) is the transverse spacing between bolt lines. Standard gages for common shapes:
| Shape Type | Typical Gage (in) | Notes |
|---|---|---|
| W-shape flanges | Standard gage from AISC Manual Table 1-1 | Varies by flange width |
| Angles | 2-1/2 to 3-1/2 | Depends on leg length |
| HSS walls | Centered or as needed | Limited by wall width |
| Channels | Standard gage from AISC Manual | Typically 1-1/2 to 2-1/2" from web |
Common Bolt Patterns
Single row: Used for light connections (purlin clips, sag rods, small angles). Minimum 2 bolts per connection.
Double row (staggered): Used for moderate connections. Bolts are staggered to fit within the available gage while maintaining minimum pitch.
Rectangular pattern: Used for heavy connections (beam-to-column, moment end plates). Rows and gages are standardized.
Edge Distance Tips
- Always check both sheared and rolled edge distances. Many plates are sheared (not milled), so the larger sheared-edge minimum applies.
- Standard angle leg minimums: For a 3/8" thick angle with 3/4" bolts at a sheared edge, the minimum edge distance is 1-1/4". A 3-1/2" leg can accommodate this; a 2-1/2" leg is marginal.
- Flange gage on W-shapes: The standard gage from the AISC Manual Table 1-1 typically satisfies minimum edge distance requirements. If the gage is wider than the standard, check the edge distance to the flange tip.
Worked Examples
Example 1 — Bolt Pattern for a Beam Web Connection
Design a bolt pattern for a W18x35 web (t_w = 0.300") using 3/4" A325 bolts.
Minimum spacing: 2.67 x 0.75 = 2.0" (use 3" for preferred spacing)
Minimum edge distance (sheared web): 1-1/4" (Table J3.4 for 3/4" bolt at sheared edge)
Note: W-shape webs are rolled, not sheared, so the rolled edge distance of 1" technically applies. However, many engineers use 1-1/4" as a conservative default.
Maximum spacing: lesser of 24 x 0.300 = 7.2" and 12" → 7.2"
A 4-bolt pattern at 3" spacing:
Bolt 1: 1-1/2" from top of web
Bolt 2: 4-1/2" from top (3" below Bolt 1)
Bolt 3: 7-1/2" from top (3" below Bolt 2)
Bolt 4: 10-1/2" from top (3" below Bolt 3)
Check: spacing = 3" > 2.0" minimum ✓, 3" < 7.2" maximum ✓
Example 2 — Edge Distance on a Short Angle Leg
A L4x3-1/2x5/16 is connected through the 3-1/2" leg with 7/8" bolts. The edge is sheared.
Minimum edge distance: 1-1/2" (Table J3.4 for 7/8" bolt at sheared edge)
Available edge distance: 3.5" - 1.5" = 2.0" from the last bolt to the far edge
This works — there is 2.0" available, which exceeds the 1.5" minimum.
If the angle were L4x3x5/16 (3" leg): Available edge distance: 3.0 - 1.5 = 1.5" — just meets the minimum. Marginal.
Example 3 — Maximum Spacing Check
A 5/16" plate with bolts at 8" spacing, painted structure.
Maximum spacing: lesser of 24 x 0.3125 = 7.5" and 12" → 7.5"
8" > 7.5" — FAILS the maximum spacing check. Reduce spacing to 7-1/2" or increase plate thickness to 3/8".
At 3/8" thickness: max spacing = 24 x 0.375 = 9.0" > 8" ✓
Calculator
Check bolt spacing, edge distance, bearing, tearout, and bolt capacity automatically:
- Bolted Connections Calculator — Full AISC 360-22 connection check with spacing and edge distance validation
- Bolt Spacing and Edge Distance Reference — Multi-code comparison (AISC, AS 4100, EN 1993, CSA S16)
FAQ
Q: What is the minimum bolt spacing for a 3/4" bolt? A: The AISC 360 code minimum is 2.67 x 3/4 = 2.0 inches. The preferred spacing is 3 x 3/4 = 2.25 inches. Most detailers use 3 inches as a practical minimum for 3/4" bolts.
Q: What is the minimum edge distance for a 3/4" bolt? A: At a sheared edge, 1-1/4 inches. At a rolled, milled, or thermally cut edge, 1 inch. The sheared edge distance is more conservative due to the rougher surface finish and residual stress from the shearing process.
Q: Can I use 2.67d spacing instead of 3d? A: Yes, 2.67d is the code minimum and is permitted. However, 3d is preferred for constructability — it provides room for wrenches, inspection, and reduces stress concentration between holes. Use 2.67d only when space is genuinely constrained.
Q: What is the maximum bolt spacing? A: For connected parts in contact (painted or unpainted), the maximum is the lesser of 24t and 12 inches, where t is the thickness of the thinnest outside plate. For unpainted weathering steel, the maximum is the lesser of 14t and 7 inches (to prevent moisture trapping and corrosion).
Q: Does edge distance apply to the bolt head or the bolt hole? A: Edge distance is measured from the center of the bolt hole to the nearest edge of the connected part. It is not measured from the edge of the hole or from the bolt head.
Q: What edge distance do I use for a plasma-cut edge? A: Plasma-cut edges are classified as "thermally cut" and use the same minimum edge distances as rolled or milled edges (the smaller values). The smooth finish of thermal cuts does not require the extra clearance that sheared edges need.
Q: Do maximum spacing rules apply to all connections? A: Maximum spacing rules (Section J3.5) apply to prevent plate buckling between bolts and to control moisture intrusion in exposed connections. They are particularly important for compression members and weathering steel structures.
Related: AISC 360-22 Steel Design Overview | AISC Standard Hole Sizes | AISC Block Shear — Section J4.3 | Bolt Bearing and Tearout | Bolt Grades | Bolt Capacity Table