4.3 Bolt Holes, Clearances & Washer Requirements

4.3 Bolt Holes, Clearances & Washer Requirements

The physical preparation of the steel components—specifically the size, shape, and quality of the bolt holes, as well as the correct application of washers—is foundational to a compliant structural steel connection. The ICC Structural Steel and Bolting Special Inspector must be intimately familiar with the dimensional tolerances and fabrication standards governing these elements, as defined by the RCSC Specification and the AISC 360 standard.

Types of Bolt Holes

Structural steel design utilizes several distinct types of bolt holes, each serving a specific structural or constructability purpose. The nominal dimensions of these holes are strictly tied to the diameter of the bolt being used, per AISC 360.

These are AISC 360-16 Table J3.3 (15th Edition) values. RCSC 2014 Table 3.1 and AISC 360-10 use d + 1/16 in. for the standard hole and slot width of bolts 1 in. and larger (1-1/16 in. for a 1 in. bolt); the other values match. RCSC Table 3.1 permits an upper tolerance of 1/32 in. on these nominal dimensions and accepts the slightly conical hole that punching produces.

Standard Holes (STD)

Standard holes are the default requirement for all structural connections unless the design drawings explicitly specify otherwise.

• Dimension: For bolts up to 7/8 in., they are 1/16 in. larger than the bolt. For bolts 1 in. and larger, AISC 360-16 uses d + 1/8 in., while RCSC 2014 and AISC 360-10 use d + 1/16 in.
• Example: For a 3/4-inch diameter bolt, a standard hole is fabricated to 13/16 inch. This small clearance allows for minor misalignment during erection while ensuring that the bolt will bear evenly against the steel in a bearing-type connection without excessive slip.

Oversized Holes (OVS)

Oversized holes provide greater tolerance for steel erection, making it easier to plumb and square the building frame.

• 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.
• Restrictions: AISC 360 Section J3.2 permits oversized holes in any or all plies of slip-critical connections but not in bearing-type connections, and RCSC Section 3.3 makes them subject to the engineer of record's approval. Because the hole is large, the joint must rely on friction to prevent slipping; if a bearing joint used oversized holes, the structure could shift significantly before the bolts engaged the steel.

Short-Slotted (SSL) and Long-Slotted (LSL) Holes

Slotted holes are elongated in one direction to accommodate significant field adjustments or thermal expansion.

• Short-slotted holes: The width equals the standard hole; the length is 3/16 to 5/16 in. longer than the bolt for 1/2 through 1 in. bolts, and d + 3/8 in. for larger bolts.
• Long-slotted holes: The length is 2.5 times the bolt diameter.
• Orientation Restrictions: In bearing-type connections, slotted holes must be oriented so that the long direction of the slot is perpendicular to the direction of the applied load. If the slot were parallel to the load, the connection would simply slide along the slot when force was applied. In slip-critical connections, slot orientation may be parallel to the load, provided the connection is designed for the corresponding reduction in slip resistance.
• Ply and approval limits: Long slots are permitted in only one of the connected parts at an individual faying surface (AISC 360 J3.2; RCSC 3.3.4). RCSC Section 3.3 requires engineer of record approval for oversized holes, short slots not perpendicular to the load, and long slots in any direction.

Hole Fabrication and Quality

Bolt holes are typically fabricated in the steel shop, though field modification is sometimes necessary. The method of fabrication impacts the quality of the hole and the performance of the connection.

Drilling and Punching

Drilling is the most precise method and is always acceptable. Punching (forcing a punch through the steel into a matched die) is efficient and leaves a slightly conical hole, which RCSC Table 3.1 accepts. Older AISC Specifications limited punching by material thickness; the AISC 360-16 commentary explains that the Specification now defines acceptable hole quality instead of the method used to form the hole, leaving tool limits to the equipment manufacturer.

Thermal Cutting

Thermal cutting (such as oxy-fuel or plasma cutting) is permitted for creating bolt holes, but it is heavily regulated. The primary concern is that thermal cutting can leave a rough, jagged edge or cause localized hardening of the steel.

• Where permitted: RCSC Section 3.3 permits thermally cut holes produced by mechanically guided means in statically loaded joints. Free-hand thermally cut holes require engineer of record approval, as do thermally cut holes in cyclically loaded joints.
• Roughness limits: The surface roughness profile may not exceed 1,000 microinches (ASME B46.1). AISC 360 Section M2.5 states the same limit and also permits water-jet-cut holes.
• Gouges: Occasional gouges not more than 1/16 in. deep are permitted. Inspectors must carefully check thermally cut holes, particularly those made in the field with hand torches, to ensure they meet these smoothness criteria.

Reaming and Enlargement

When steel arrives on site and the holes do not align, ironworkers may need to enlarge them. This process is heavily restricted. Holes may be slightly enlarged by reaming (using a rotating cutting bit) to accommodate the bolts. However, using a thermal torch to blindly blow out a hole to make a bolt fit is a serious violation that damages the structural integrity of the connection. Enlarged holes must be evaluated by the Engineer of Record to ensure edge distances and bearing capacities remain adequate. AISC 360 Section M2.5 states that drift pins used during assembly must not distort the metal or enlarge the holes and that poor matching of holes is cause for rejection, and the RCSC commentary treats a hole that exceeds its Table 3.1 limits as the next larger hole type.

ASTM F436 Washer Requirements

Washers in structural bolting are not mere spacers; they distribute the high clamping forces over a larger area of the steel and prevent the hardened nut or bolt head from galling (digging into) the softer structural steel. The RCSC Specification dictates exactly when and where standard ASTM F436 hardened washers, or specialized washers, must be used.

Snug-Tightened Joints (RCSC Section 6.1)

Washers are not required in snug-tightened joints except:

• Sloping surfaces: an F436 beveled washer where an outer face slopes more than 1:20 from a plane normal to the bolt axis (6.1.1); and
• Slotted holes in an outer ply: an F436 washer or a 5/16 in. thick common plate washer, as needed to completely cover the hole (6.1.2).

Pretensioned and Slip-Critical Joints (RCSC Section 6.2)

In addition to the two rules above:

• A490 or F2280 bolts in material with Fy below 40 ksi: F436 washers under both the bolt head and the nut, except no washer is needed under the head of a round-head F2280 twist-off bolt (6.2.1).
• Calibrated wrench method: an F436 washer under the turned element (6.2.2).
• Twist-off bolts: an F436 washer under the nut as part of the assembly (6.2.3).
• Direct tension indicators: an F436 washer under the turned element and, when the DTI sits under the turned element, between that element and the DTI (6.2.4).
• Turn-of-nut with A325 bolts: no washer is required unless one of the rules above applies.

Oversized and Slotted Holes in an Outer Ply (RCSC Table 6.1)

Plate washers and bars must be structural-grade steel but need not be hardened. Several washers stacked to reach 5/16 in. do not satisfy the extra-thick requirement; alternatively, a 3/8 in. plate washer with an ordinary-thickness F436 washer may be used. Every washer must be large enough to completely cover the hole (6.2.5), and the washer under the head of a round-head tension-control bolt is waived when its bearing circle meets ASTM F1852 or F2280.

Beveled Washers

Structural steel shapes, such as the inner flanges of American Standard Beams (S-shapes) or channels, often have sloped surfaces. If the slope of the surface under the bolt head or nut exceeds 1:20 (a 5% slope), an ASTM F436 beveled washer must be installed to compensate for the slope. This ensures the bolt head or nut sits squarely against a flat surface, preventing severe bending stresses from being introduced into the bolt shank during tensioning.

By verifying precise hole dimensions, monitoring fabrication quality, and enforcing strict washer placement rules, the special inspector ensures the physical geometry of the connection matches the engineered design.