Bolt Hole Sizes — Standard, Oversize & Slotted Holes (AISC 360, EN 1993, AS 4100)

Bolt hole size affects fit-up tolerance, net section capacity, slip-critical detailing, and whether a connection can be erected in the field. This reference provides hole dimensions for all standard bolt sizes across AISC 360 (US), EN 1993-1-8 (Eurocode), and AS 4100 (Australia), plus net area calculations and edge distance adjustments for oversized and slotted holes.

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AISC 360 Table J3.3 — Standard, Oversize & Slotted Holes

Standard Holes

Standard holes provide normal installation clearance and are the default for all bolted connections.

Clearance: standard hole = bolt diameter + 1/16 in (approx 1.5 mm).

Oversized Holes

Oversized holes provide additional erection tolerance where fabrication or erection tolerances accumulate. They require hardened washers under both the bolt head and nut.

Short-Slotted Holes

Short-slotted holes provide one-direction adjustment. The slot is perpendicular to the load direction in bearing connections.

Long-Slotted Holes

Long-slotted holes provide larger movement capacity. In bearing connections, the long dimension must be perpendicular to the load direction.

EN 1993-1-8 Table 11 — Bolt Hole Dimensions

Eurocode bolt hole dimensions for standard and oversized holes:

Clearance: standard hole = bolt diameter + 1 mm for M12-M24, + 2 mm for M27 and larger. Oversized holes add approximately 3-4 mm additional clearance.

AS 4100 — Bolt Hole Dimensions

AS 4100 Clause 9.4.2 uses hole diameter (d_h) for edge distance and spacing calculations:

AS 4100 oversize holes follow the same dimensional convention as EN 1993.

Hole Clearance by Type — Summary

Net Area Calculation — Effect of Hole Type

The hole type directly affects the net area of a tension member. A larger hole reduces the net area and therefore the tension capacity.

Standard Holes

An = Ag - n * dh * t

Where dh = standard hole diameter + 1/16 in (damage allowance for punched holes per AISC B4.3b).

Example: A 1/2 in plate with two 3/4 in bolts in a line:

• Standard hole = 13/16 in. Design hole = 13/16 + 1/16 = 7/8 in.
• An = 6 _ 0.5 - 2 _ (7/8) * 0.5 = 3.0 - 0.875 = 2.125 in^2.

Staggered Holes (s^2/4g Rule)

For zigzag bolt patterns, the net width along a staggered path is:

net width = gross width - sum(dh) + sum(s^2 / (4g))

Where s = longitudinal pitch between staggered holes, g = transverse gage between holes. Check all possible failure paths; the minimum net area governs.

Example: Two 3/4 in bolts staggered at s = 2 in, g = 3 in:

• Standard path: deduct 2 * 7/8 = 1.75 in.
• Staggered path: deduct 2 * 7/8 - 2^2/(4*3) = 1.75 - 0.333 = 1.417 in.
• The staggered path has a LARGER net width (less deduction). Always check both.

Oversized and Slotted Holes

For oversized holes, use the actual hole diameter (not the bolt diameter) for the dh deduction. For slotted holes loaded perpendicular to the slot, use the hole width (not the slot length) for the net area deduction.

Edge Distance Adjustments for Non-Standard Holes

Oversized and slotted holes require increased minimum edge distances per AISC 360 Section J3.4:

Practical impact: A 3/4 in bolt at a sheared edge requires 1-1/4 in edge distance for a standard hole, but 1-3/8 in for an oversized hole. Plan connection geometry accordingly.

Hole Type Restrictions by Connection Category

Long-slotted holes are generally not permitted in slip-critical connections designed to prevent slip under factored loads. Check the specific code clause before specifying slotted holes in slip-critical joints.

Worked Example — Net Area with Standard Holes

Problem: A tension splice plate 8 in x 1/2 in (A992, Fu = 65 ksi) has three 3/4 in A325 bolts in a single line. Calculate the net area and rupture capacity.

Step 1: Design hole diameter

dh = 13/16 + 1/16 = 7/8 in (standard hole for 3/4 in bolt + damage allowance).

Step 2: Net area

An = Ag - n _ dh _ t = (8 _ 0.5) - 3 _ (7/8) * 0.5 = 4.0 - 1.3125 = 2.6875 in^2.

Step 3: Shear lag factor

For a single line of bolts in a plate: U = 1.0 (Case 1, load transmitted directly).

Ae = 1.0 * 2.6875 = 2.6875 in^2.

Step 4: Rupture capacity

phiPn = 0.75 _ 65 _ 2.6875 = 131.0 kips.

Step 5: Compare to yielding

phiPn*yield = 0.90 * 50 _ 4.0 = 180 kips. Rupture (131 kips) governs.

Result: The net section rupture capacity is 131 kips. This is 27% less than the gross section yielding capacity, demonstrating why the net area check is critical.

AISC 360-22 Section J3.2: Bolt Hole Types

AISC 360-22 Section J3.2 defines four types of bolt holes, each with specific dimensions, tolerances, and permitted applications. Selection of hole type affects both constructability and design capacity.

Permitted uses per AISC 360-22 Section J3.2:

• Standard holes: Permitted in all connections (bearing and slip-critical)
• Oversized holes: Permitted only in slip-critical connections. Not permitted in bearing-type connections.
• Short-slot holes: Permitted in slip-critical connections without restriction. In bearing-type connections, short-slot holes are permitted only when the slot is perpendicular to the direction of load (to prevent slip under bearing).
• Long-slot holes: Permitted in slip-critical connections. In bearing-type connections, long-slot holes are permitted only in one ply of the connected parts (the outer ply), and the slot must be perpendicular to the direction of load.

Bolt Hole Dimensions by Diameter

Per AISC 360-22 Table J3.3 and J3.3M, the following dimensions apply to bolt holes for standard imperial bolt sizes:

Metric dimensions per AISC Table J3.3M (for M16 through M36 bolts):

Bearing Strength Implications

Hole type directly affects bearing strength and connection behavior. Per AISC 360-22 Chapter J3.10:

Key AISC 360-22 bearing equations:

• Bearing strength at bolt holes (deformation is a design consideration): phi _ Rn = 0.75 _ 2.4 _ d _ t * Fu (for standard holes, single bolt)
• Bearing strength at bolt holes (deformation is NOT a consideration): phi _ Rn = 0.75 _ 3.0 _ d _ t * Fu (higher capacity allowed if deformation is acceptable)
• Tearout strength: phi _ Rn = 0.75 _ 1.2 _ lc _ t * Fu (where lc = clear distance between edges of adjacent holes or edge of hole and edge of material)

Bolt Hole Installation Guidance

Practical guidance for specifying and inspecting bolt holes per AISC 360-22 and the AISC Code of Standard Practice:

Per the AISC Code of Standard Practice, the fabricator is responsible for hole accuracy. The Engineer of Record specifies hole type, diameter, edge distances, and spacing on the design drawings. Any field reaming for misaligned holes requires written approval from the EOR because reaming increases the hole diameter beyond the specified dimension.

Common Mistakes

1. Using bolt diameter instead of hole diameter for net area. The hole is always larger than the bolt. Use dh from the hole size tables, plus the 1/16 in damage allowance for punched holes.

2. Forgetting the damage allowance. AISC B4.3b requires adding 1/16 in to the nominal hole diameter for punched holes (1/32 in for drilled or milled holes). This is separate from the hole clearance.

3. Not checking all failure paths for staggered bolts. The zigzag path may or may not govern depending on the pitch and gage. Check every possible path through the bolt pattern.

4. Using oversized holes without adjusting edge distance. The code requires increased minimum edge distances for oversized and slotted holes. Using standard-hole minimums is non-compliant.

5. Ignoring slot orientation relative to load. In bearing connections, slotted holes must be oriented with the long dimension perpendicular to the load. An incorrectly oriented slot concentrates bearing stress and can lead to premature failure.

6. Assuming all hole types are permitted in slip-critical connections. Long-slotted holes have restrictions in slip-critical joints. Check the specific code clause before specifying.

Frequently Asked Questions

What is the standard bolt hole size? A standard bolt hole is 1/16 in (1.5-2 mm) larger than the bolt diameter. For a 3/4 in bolt, the standard hole is 13/16 in. This provides normal installation clearance without excessive play.

What is the difference between standard and oversized bolt holes? Standard holes have a clearance of approximately 1/16 in. Oversized holes have a clearance of approximately 1/8 to 3/16 in. Oversized holes require hardened washers and provide additional erection tolerance at the cost of reduced bearing area.

When should I use slotted holes? Use short-slotted holes when one-direction adjustment is needed for erection tolerance or thermal movement. Use long-slotted holes for larger movements. In bearing connections, the slot must be perpendicular to the load direction. In slip-critical connections, slot orientation rules vary by code.

How do bolt holes affect tension member capacity? Holes reduce the gross cross-sectional area to the net area. For a plate with two 3/4 in bolt holes, the net area deduction is approximately 0.875 in^2 per hole per inch of plate thickness. The rupture check on the net area often governs over yielding on the gross area, reducing capacity by 20-40%.

What hole diameter do I use for net area calculations? For AISC: use the nominal hole diameter from Table J3.3 plus 1/16 in for punched holes (or 1/32 in for drilled holes). For EN 1993: use the hole diameter from Table 11 without additional allowance. For AS 4100: use dh as specified in Clause 9.4.2.

Can I use oversized holes in slip-critical connections? Yes, but the slip resistance is reduced. AISC 360 Section J3.8 provides slip-critical design values that account for the hole type. Oversized and slotted holes have lower slip resistance than standard holes because the bolt can shift within the hole before engaging the connected material.

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Related Pages

• AISC Bolt Hole Sizes
• Bolt Capacity Table — A325 & A490
• Bolt Spacing & Edge Distance
• Bolt Grades — A325, A490, 8.8, 10.9
• Bolt Bearing & Tear-Out
• Bolt Pattern Geometry
• Tension Member Design
• Shear Lag Factor U
• Steel Fy & Fu Reference
• Steel Connection Design
• Tools Directory
• Reference Tables Directory
• How to Verify Calculator Results

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