In structural steel fabrication, bolt holes aren't just clearance features — they're defined in the AISC Specification (AISC 360) and the RCSC (Research Council on Structural Connections) specification, which governs high-strength bolt assemblies. The hole size and type affect how load is transferred between connected members, how bolts can be pre-tensioned, and what inspection criteria apply.
A structural engineer specifying a bolt connection will indicate the hole type (standard, oversized, slotted) and sometimes the bolt grade on the drawing. The fabricator is responsible for producing holes that meet these specifications within the tolerances defined by the AISC. Holes that are too small prevent bolt installation. Holes that are too large reduce the net section capacity and may not meet the connection design assumptions.
The AISC Table J3.3 defines standard hole diameter as bolt diameter plus a specific clearance. For bolt diameters up to 7/8": standard hole = bolt diameter + 1/16". For 1" bolts: standard hole = bolt diameter + 1/16". For bolts over 1": standard hole = bolt diameter + 1/8".
Examples:
A 3/4" bolt goes in a 13/16" (0.8125") standard hole. A 7/8" bolt goes in a 15/16" (0.9375") standard hole. A 1" bolt goes in a 1-1/16" (1.0625") standard hole.
These are nominal drill sizes. In practice, standard structural hole production uses the listed drill size — a 13/16" drill for 3/4" bolts, etc. The finished hole is typically very close to drill nominal; structural tolerances are relatively generous compared to precision machining, and subpunch/sub-drill or punch methods are also commonly used.
The AISC defines three additional hole types for connections where some adjustment or accommodation of dimensional variation is needed:
Oversized holes (OVS): Larger than standard clearance. For bolts up to 7/8", oversized = bolt + 5/16". For 1" bolts = bolt + 5/16". These are used when member fit-up is difficult or where the connection needs some adjustment during erection. Oversized holes require hardened washers under the bolt head and nut to span the larger hole.
Short-slotted holes (SSL): Slotted holes perpendicular to or parallel to the load direction, with slot width = standard hole width and slot length = bolt diameter + 5/16" for bolts up to 7/8". Used where moderate movement during erection is needed. Require washers per the RCSC specification.
Long-slotted holes (LSL): Longer slots, slot length = bolt diameter + 1-3/8" for bolts up to 7/8". Used in connections designed for significant thermal movement or where relatively large erection adjustment is required. LSL connections have more restrictive pretension requirements and load transfer assumptions than standard connections.
Three main methods produce bolt holes in structural steel shop fabrication:
Subpunching and reaming. The most common method for high-volume structural fabrication. The hole is first punched 3/16" smaller than final diameter using a hydraulic or mechanical punch press, then reamed to final diameter. Punching is fast; reaming refines the quality and final size. This method requires that the steel thickness is less than the bolt diameter per AISC Section J3.2, and that the material meets ductility requirements for punching.
Drilling to final size. Drilling directly to bolt hole diameter in a radial arm drill, drill press, or CNC drill is common for plates too thick to punch and for post-assembly or field work. Drilling produces the cleanest hole quality and is required when the material can't be punched (thicker than bolt diameter, higher-hardness alloy, or connection design specifying drill-only).
Thermal cutting (plasma or oxy-fuel). Permitted by AISC for standard holes in lower-strength grades (A36, A572 Gr 50) provided the hole is finished to final diameter by reaming. Thermal cutting alone is not acceptable as a bolt hole fabrication method for most structural connections — the heat-affected zone and surface quality aren't consistent enough for reliable connection behavior.
Because structural hole tolerances are relatively wide compared to machined parts, drill selection is straightforward: use the drill closest to the target hole size. For standard 3/4" bolt holes (13/16" = 0.8125"), use a 13/16" drill. For standard 1" bolt holes (1-1/16" = 1.0625"), use a 1-1/16" drill.
In practice, the most common structural bolt diameters are 3/4", 7/8", 1", and 1-1/8". The corresponding standard drill sizes (13/16", 15/16", 1-1/16", 1-3/16") are all common jobber and drill set sizes. Keep a dedicated set of these sizes in good condition for structural work — they'll see consistent use on any structural fab job.
Cobalt HSS drills are worth the investment for structural hole drilling. A36 and A572 Gr 50 are mild steels that cut easily, but structural fabrication typically involves high volumes of holes, often with mill scale on the entry surface, and sometimes in plate that's been galvanized or primed. The improved edge retention of M42 cobalt over standard M2 HSS shows up in holes-per-resharpen on high-volume structural work.
AISC requires that drilled holes be free of burrs that prevent solid seating of connected parts. In practice, this means deburring is required — a pass with a deburring tool, countersink, or flap disc to remove the raised edge at the top and bottom of each hole. Burrs in structural connections can prevent proper bolt bearing and introduce hydrogen corrosion sites in galvanized or weathering steel connections.
Punch-produced holes also need inspection for cracks or tears in the heat-affected material around the hole perimeter, particularly in higher-strength steels. The AISC permits punching with the requirement that punched holes be inspected for these defects; holes with significant tearing or cracking must be reamed or rejected.
For drilled structural holes, the entry and exit surface quality is typically adequate without additional finishing. The concerns are burr removal and, for critical connections, verification of hole diameter and location within the specified tolerances (typically ±1/32" for hole position in most structural detailing standards).
Structural fab burns through 13/16", 15/16", and 1-1/16" cobalt drills fast — especially with mill scale at entry. Keep your structural drill sizes on a standing resharpening program.
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