Bolt Spacing & Edge Distance Reference

Reference guide to minimum bolt spacing and edge distance requirements for steel connection design. Always verify with governing code (AISC 360, AS 4100, EN 1993, or CSA S16).

Minimum spacing requirements

Minimum bolt spacing prevents overlapping stress concentrations in connected plates and ensures proper installation access. Spacing is measured center-to-center between adjacent bolt holes. Most codes require minimum spacing of 2.67 times the bolt diameter (approximately 2.7d), which provides adequate bearing and tear-out capacity. For bolts in a line perpendicular to the load direction, spacing may be reduced to 2.0d or 2.2d depending on code and connection type.

Maximum bolt spacing is governed by different criteria: connected member stability, seam tightness, and practicality. For compression members, maximum spacing along a member is typically 24 times the thickness of the thinner connected part (to prevent buckling between fasteners). For tension members and built-up sections, maximum spacing is often 12-15 times the minimum radius of gyration to maintain composite action. Seams subject to weathering or vibration require closer spacing (typically 4-6 inches or 100-150 mm) to prevent leakage and rattling.

Minimum edge distance

Edge distance is measured from the center of the bolt hole to the nearest edge of the connected plate. Minimum edge distance prevents tear-out failure and provides adequate material around the hole for bearing and tension stress distribution. Requirements differ by loading direction: loaded edges are those where tension force acts perpendicular to the edge direction, while unloaded edges are parallel to the load direction.

For standard holes and loaded edges, AISC 360 requires minimum edge distance of 1.5 times bolt diameter (1.5d) but recommends 1.75d or 2.0d for practicality and to accommodate fabrication tolerances. AS 4100 and CSA S16 have similar provisions with slight variations in edge distance multipliers and hole diameter assumptions. Oversize and slotted holes have different edge distance requirements because the hole geometry affects local stress distribution.

Edge distance also affects bearing capacity: smaller edge distances reduce available bearing area and may reduce connection capacity. Some codes provide reduction factors for bearing strength when edge distance is less than the recommended minimum. For high-strength bolts loaded in tension, edge distance influences pryout capacity through the effective thickness of the connected material.

Hole size considerations

Standard hole diameter is typically 1/16 inch (2 mm) larger than bolt diameter for bolts up to 1 inch (25 mm). This allows for fabrication tolerances and bolt installation. Oversize holes (1/4 inch or 6 mm larger than bolt) and slotted holes are used for field adjustment, particularly in base plates where foundation misalignment is common. These hole types have different spacing and edge distance requirements because the larger hole reduces effective bearing area and may cause uneven stress distribution.

Slotted holes can be short-slotted (length parallel or perpendicular to load direction) or long-slotted (length significantly greater than short-slot limits). Long-slotted holes always require washers or plates to prevent bolt head seating in the slot. The orientation of slotted holes relative to load direction affects bearing capacity: slotted holes with load parallel to the slot have reduced bearing strength compared to load perpendicular to the slot.

Bolt spacing and edge distance checklist

When verifying connection detailing or preparing fabrication drawings, confirm the following:

For full verification and documentation workflow, see How to verify calculator results.

Frequently Asked Questions

What is the minimum spacing between bolts in a row? Most codes specify minimum spacing of 2.67 times bolt diameter (2.7d) measured center-to-center. This provides adequate bearing and tear-out capacity. Some codes allow reduced spacing (2.0d to 2.2d) for bolts in a line perpendicular to the load direction. Always check the specific code provision for the connection type.

What is the minimum edge distance for standard holes? For standard holes and loaded edges, the minimum edge distance is typically 1.5 times bolt diameter (1.5d) measured from hole center to plate edge. However, many designers use 1.75d or 2.0d for practicality and to accommodate fabrication tolerances. Edge distance requirements for unloaded edges may be smaller.

How does oversize hole size affect edge distance? Oversize holes (typically 1/4 inch or 6 mm larger than bolt diameter) require increased edge distance compared to standard holes because the larger hole reduces effective bearing area and increases local stress concentration. Some codes specify edge distance based on hole diameter rather than bolt diameter for oversize holes, resulting in slightly larger minimums.

What is the difference between loaded and unloaded edges? A loaded edge is one where tension force acts perpendicular to the edge direction. These edges are critical for tear-out failure and require larger edge distances. Unloaded edges are parallel to the load direction and have less stringent edge distance requirements. In connection design, edges can be loaded in one direction (e.g., bottom flange of a beam connected to a column) and unloaded in another.

What is maximum bolt spacing in steel connections? Maximum spacing depends on the member and connection type. For compression members, maximum spacing along a member is typically 24 times the thickness of the thinner connected part to prevent local buckling between fasteners. For tension members and built-up sections, maximum spacing is often 12-15 times the minimum radius of gyration to maintain composite action. Seams subject to weathering require closer spacing (4-6 inches or 100-150 mm).

How does bolt spacing affect connection capacity? Spacing affects bearing capacity (through tributary area per bolt), tear-out capacity (through edge distance from each bolt), and member behavior (through stiffness distribution). Wider spacing reduces load per bolt but may increase prying action. Closer spacing increases stiffness but may lead to over-design if spacing exceeds minimum practical limits without capacity benefit.

What are gage and pitch in bolted connections? Pitch is the longitudinal spacing between bolt rows parallel to the member length direction. Gage is the transverse spacing between bolt rows perpendicular to the member length direction. In beam flanges, gage is typically measured from the center of the flange to the center of the outer bolt hole row. Standard gages are often 2.75 to 3.5 inches (70 to 90 mm) for W-shape flanges.

When are slotted holes used? Slotted holes are used for field adjustment, particularly in base plates, moment connections with erection tolerances, and connections where thermal expansion or movement is expected. Short-slotted holes (slot length limited to 1.25 times bolt diameter) provide limited adjustment. Long-slotted holes (slot length significantly greater) provide large adjustment ranges but require washers or plates and have reduced bearing capacity.

How do different steel codes compare on spacing requirements? AISC 360, AS 4100, EN 1993, and CSA S16 have similar minimum spacing requirements (approximately 2.5d to 2.7d) but differ in edge distance multipliers and hole size assumptions. AISC uses bolt diameter for spacing and edge distance. AS 4100 and EN 1993 may use hole diameter in some provisions, resulting in slightly different minimums for the same bolt size. Always use the code-specific provision for the project location.

What is the effect of bolt spacing on slip-critical connections? In slip-critical connections, bolt spacing affects slip load through the number of bolts resisting load. Closer spacing distributes load over more bolts and may increase slip resistance, but the primary factor is bolt pretension and faying surface preparation. Spacing that is too tight can cause interference between washers or inadequate wrench clearance for proper tightening.

Run This Calculation

→ Bolted Connections Calculator — bolt group shear, bearing, block shear, and tear-out checks per AISC 360, AS 4100, EN 1993, CSA S16.

→ End Plate Connection Calculator — extended and flush end plate bolt pattern design.

→ Fin Plate Connection Calculator — single plate shear connection with full bolt layout and edge distance checks.

See Also

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