9.2 Bolted Joint Callouts, Bolt Sizes & Quantities on Drawings

A structural special inspector must be fluent in the language of structural steel drawings. The connection details and bolt schedules are the instruction manuals for assembling the steel frame. Accurately deciphering these callouts ensures that the correct fastener assembly is used in the right location, in the proper quantity, and installed to the required tension standard.

Deciphering the Bolt Callout

A typical bolt callout on a structural detail might look like this: 6 - 7/8" Ø A325 SC-A. Every component of this string provides critical information that dictates the inspection requirements.

Fastener Quantity and Diameter

The first part of the callout (e.g., 6 - 7/8" Ø) specifies the total number of bolts in the connection and their nominal diameter.

• Quantity: The inspector must count the bolts installed in the field and verify it matches the drawing. A missing bolt drastically reduces the connection's capacity.
• Diameter: Common structural bolt diameters in building construction are 3/4", 7/8", 1", and 1-1/8". The bolt diameter dictates the required hole size, the minimum edge distance, the minimum spacing, and the required pretension value if the joint is not snug-tight. Mixing up a 3/4" bolt for a 7/8" bolt is a major non-conformance.

Fastener Grade (ASTM Specification)

The next segment (e.g., A325) identifies the material grade of the bolt. Under the consolidated ASTM F3125 specification, the two primary grades of high-strength structural bolts are:

• Grade A325 (or F3125 Grade A325): These are medium-carbon steel bolts, heat-treated for high strength. They are the workhorse of structural steel connections.
• Grade A490 (or F3125 Grade A490): These are alloy steel bolts, heat-treated to a higher strength than A325s. Because of their higher strength and different metallurgical properties, A490 bolts are more susceptible to hydrogen embrittlement and stress corrosion cracking. Consequently, A490 bolts are prohibited from being hot-dip galvanized and must not be reused under any circumstances.

The inspector must check the head markings on the bolts in the field to verify the grade matches the drawings.

Joint Classification Callouts

The final part of the callout dictates how the joint must be assembled and tensioned. The Research Council on Structural Connections (RCSC) defines three main joint types, which are typically abbreviated on drawings.

Snug-Tightened (ST)

• Callout: Often shown by the absence of a pretension note, or explicitly as "ST" or "Snug". The "N" (threads included in the shear plane) and "X" (threads excluded) suffixes describe thread location and can appear with any joint type. AISC 360 Section J3.1 requires bolts to be tightened beyond snug-tight only where the design drawings clearly identify them.
• Definition: The snug-tight condition is defined as the tightness that is attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the plies into firm contact.
• Inspection: The inspector verifies that the correct bolts are used and that the plies are in firm contact. No pretension verification is required, and the RCSC commentary says installed snug-tight bolts should not be inspected for actual pretension or arbitrated.

Pretensioned (PT)

• Callout: Indicated by "PT" following the bolt grade and size.
• Definition: These joints require the bolts to be tightened to a specific minimum tension, typically 70% of the specified minimum tensile strength of the bolt. Pretensioned joints are used where tension loads are applied, where fatigue is a concern, or in certain seismic applications.
• Inspection: The inspector must observe the pre-installation verification (PIV) testing in the Skidmore-Wilhelm device and then monitor the installation method (Turn-of-Nut, Calibrated Wrench, Twist-Off Control, or DTI) to ensure the minimum tension is achieved in the structure.

Slip-Critical (SC)

• Callout: Indicated by "SC". This is usually followed by a class designation, such as "SC-A" or "SC-B".
• Definition: Slip-critical joints are pretensioned joints that also require specific preparation of the faying surfaces (the mating surfaces of the steel plies). They are designed to prevent the joint from slipping into bearing under service loads, relying entirely on the friction between the steel plates clamped together by the bolt tension.
• Inspection: In addition to verifying the pretensioning process, the inspector must verify the faying surfaces.
  • Class A surfaces are unpainted clean mill scale or blast-cleaned surfaces with Class A coatings (slip coefficient = 0.30).
  • Class B surfaces are unpainted blast-cleaned surfaces or surfaces with Class B coatings (slip coefficient = 0.50).

The inspector must ensure no paint, oil, dirt, or overspray compromises these surfaces before assembly.

Bolt Length and Grip Verification

While the nominal diameter is explicitly called out, the required bolt length is rarely listed directly on the connection detail. Instead, the erector determines the length based on the total thickness of the connected parts (the "grip"), plus allowances for washers, the nut, and required thread stick-through.

Calculating Grip

The grip is the total thickness of all the steel plies clamped together by the bolt.

• Example: A 3/8" thick shear tab bolted to a 5/16" thick beam web has a grip of 3/8" + 5/16" = 11/16".
• Inspection Focus: The inspector must verify that the bolt length used is appropriate for the actual grip. If the bolt is too short, there will not be enough thread engagement in the nut. The RCSC requires that the end of the bolt must be at least flush with the outer face of the nut. If the bolt is too long, the nut may reach the thread runout (the unthreaded shank) before the joint is properly tightened, preventing firm contact or proper pretensioning.

Shop-Bolted vs. Field-Bolted Symbols

Structural drawings use specific symbols to differentiate between work done in the fabrication shop and work done on the construction site (the field).

• Shop Bolts: Typically drawn as open circles or standard hex head symbols on the detail plan. The fabricator installs these, and they are usually inspected in the shop environment.
• Field Bolts: Differentiated by being filled in (solid black circles) or having an "x" through the circle, and are usually accompanied by a note.

For the field inspector, understanding this distinction is crucial for defining the scope of work. When reviewing a connection detail, the solid symbols indicate the fasteners the field crew is responsible for installing and the field inspector is responsible for verifying. If a shop-installed bolt requires tightening in the field, the drawings must explicitly state this.