9.3 Hole Types, Dimensions, Spacing & Edge Distance Verification

The integrity of a bolted connection relies not just on the strength of the bolts, but on the geometry of the steel plates they connect. The placement and size of the holes are governed by strict AISC parameters to prevent the steel from tearing, yielding, or fracturing under load. As a special inspector, verifying hole dimensions, spacing, and edge distances against the approved details and AISC standards is a primary responsibility.

Standard and Oversized Holes

The bolt diameter dictates the hole diameter. Using the wrong hole size can compromise the joint's capacity or its ability to achieve required pretension.

Standard Holes (STD)

For the majority of structural connections, standard holes are used.

• Dimension: For bolts up to 7/8 in., a standard hole is 1/16 in. larger than the bolt. For 1 in. and larger bolts, AISC 360-16 (15th Edition) uses d + 1/8 in., while AISC 360-10 and RCSC 2014 use d + 1/16 in.
• Example: A 7/8 in. bolt uses a 15/16 in. standard hole in both editions. A 1 in. bolt uses a 1-1/8 in. hole under AISC 360-16 and a 1-1/16 in. hole under AISC 360-10 and RCSC 2014.
• Application: Standard holes can be used in any ply of any bolted joint.

Oversized Holes (OVS)

Oversized holes are used to accommodate fabrication tolerances and make field erection easier.

• Dimension: 5/8 in. for a 1/2 in. bolt; 3/16 in. larger than the bolt for 5/8 through 7/8 in. bolts; 1-1/4 in. for a 1 in. bolt; and d + 5/16 in. for 1-1/8 in. and larger bolts.
• Application: Because the hole is larger, there is a risk of the bolt head or nut pulling through the plate or losing tension. Therefore, oversized holes are only permitted in slip-critical connections, and an ASTM F436 hardened washer must be installed over an oversized hole in an outer ply (an extra-thick 5/16 in. F436 washer for A490 or F2280 bolts over 1 in.). They are not allowed in bearing-type joints where the bolt must bear against the edge of the hole to transfer load.

Slotted Holes

Slotted holes allow for movement or adjustment in one direction.

• Short-Slotted (SSL): Can be used in any joint type, but in bearing joints, the slot must be perpendicular to the direction of the load.
• Long-Slotted (LSL): Permitted in only one connected part at an individual faying surface. In bearing-type connections the slot must be normal to the load; slip-critical connections allow any direction. A long slot in an outer ply needs a plate washer or continuous bar (RCSC Table 6.1).

Minimum Pitch and Spacing

The distance between bolts is critical. If bolts are placed too close together, the steel web or plate between the holes can tear out (block shear or net section fracture) before the bolts themselves reach their capacity.

Minimum Spacing (Pitch)

The distance between the centers of adjacent bolt holes is known as the pitch (longitudinal spacing along the line of force) or gage (transverse spacing).

• AISC Requirement: The AISC Specification states that the absolute minimum distance between the centers of standard, oversized, or slotted holes is 2-2/3 times the nominal bolt diameter (2.67d).
• Preferred Spacing: While 2.67d is the absolute minimum, AISC heavily recommends a preferred minimum spacing of 3 times the bolt diameter (3d).
• Example Calculation: For a 3/4" diameter bolt:
  • Minimum Spacing = 2.67 x 0.75 inches = 2.0 inches.
  • Preferred Spacing = 3.0 x 0.75 inches = 2.25 inches.
• Inspection Focus: If an inspector measures the center-to-center spacing and finds it is less than the 2.67d absolute minimum, this is a non-conformance that must be reported to the EOR.

Clear Distance

In addition to center-to-center spacing, the clear distance between the edges of adjacent holes must also be considered, particularly for bearing calculations. The clear distance is the center-to-center spacing minus one hole diameter. AISC 360-16 Section J3.3 requires this clear distance to be not less than the bolt diameter, d.

Edge Distance Verification

Edge distance is the distance from the center of a bolt hole to the nearest edge of the steel plate or structural shape. This dimension is crucial to prevent the bolt from tearing through the edge of the steel (tear-out failure) under bearing loads.

Minimum Edge Distance (AISC Table J3.4 / J3.4M)

In both Manual editions on the S1C reference list, AISC 360 Table J3.4 gives one minimum edge distance per bolt size, measured from the center of a standard hole to the edge of the connected part:

AISC Specifications before 2010 had a second, larger column for sheared edges; for a 7/8 in. bolt it required 1-1/2 in. at a sheared edge versus 1-1/8 in. at a rolled or thermally cut edge. The 2010 Specification dropped that column because the bearing and tearout checks govern regardless of how the edge was cut. A sheared-edge value in a practice question is pre-2010 practice.

Table J3.4 footnotes matter in the field:

• Lesser edge distances are permitted when the bearing, tearout and block shear provisions (Sections J3.10 and J4) are satisfied, but edge distances less than one bolt diameter are not permitted without engineer of record approval.

• For oversized or slotted holes, add the increment from Table J3.5.

• Inspection Focus: Measure from the center of the hole to the nearest edge. If the field measurement is less than the detailed value or the Table J3.4 minimum, report it for engineer of record evaluation.

Maximum Edge Distance and Spacing

While minimum distances prevent structural failure, maximum distances are specified to prevent environmental issues and ensure the plates behave as a single unit.

• Maximum Edge Distance: AISC limits the maximum edge distance to 12 times the thickness of the connected part, but not more than 6 inches. If the edge distance is too large, the plates can separate at the edges, allowing moisture to enter and cause corrosion (pack rust), which expands and can eventually pop the bolts.
• Maximum Spacing: For unpainted weathering steel, the maximum spacing between bolts is 14 times the thickness of the thinnest part, not exceeding 7 inches, to ensure a tight seal against moisture.

By meticulously verifying these geometric parameters—hole sizes, center-to-center spacing, and edge distances—the inspector confirms that the physical steel detailing provides the necessary strength to support the specified fasteners without premature failure of the base metal.