Bolted Connection Design Guide — AISC 360 Section J3

Complete bolted connection design reference per AISC 360-22 Section J3. Covers bearing-type versus slip-critical connections, bolt grade selection (A307, A325, A490, F3125), all limit states including bolt shear, bearing, tearout, and block shear, plus edge distance and bolt spacing requirements.

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.

Overview of Bolted Connection Design per AISC 360

Bolted connections transfer forces between structural steel members through high-strength bolts loaded in shear, tension, or combined shear and tension. AISC 360 Chapter J governs the design of all structural steel connections, with Section J3 specifically addressing bolted connections. The design methodology follows the Load and Resistance Factor Design (LRFD) philosophy, where factored loads are compared to design strengths using resistance factors (φ).

The two fundamental categories of bolted connections are bearing-type and slip-critical. Bearing-type connections allow the connected parts to slip into bearing against the bolt shank at service loads. The bolt transfers load through shear on the bolt cross-section and bearing on the connected plate material. These are the most common connection type for typical building construction where slight movement at service loads is acceptable.

Slip-critical connections rely on the clamping force generated by pretensioned bolts to develop friction at the faying surfaces. The connection resists slip through friction alone, and no bearing stress develops at the bolt hole. Slip-critical connections are mandatory for connections subject to fatigue, for connections with oversize or slotted holes loaded parallel to the slot, for column splices in certain seismic systems, and where slip would cause structural distress or impair serviceability.

Bolt Grade Selection and Mechanical Properties

The selection of bolt grade directly determines the available shear and tensile capacity. Per ASTM F3125 (which consolidates the former A325/A490 standards into a single specification), the primary high-strength structural bolt grades are:

A325 bolts are the workhorse of structural steel construction. They provide a minimum tensile strength of 120 ksi for diameters up to 1 inch (105 ksi for diameters over 1 inch through 1-1/2 inches). A490 bolts offer 25% higher strength (150 ksi) but cannot be hot-dip galvanized due to hydrogen embrittlement risk — use mechanically galvanized or weathering steel alternatives.

The Type 1 designation denotes plain carbon or alloy steel bolts. Type 3 designates weathering steel bolts (e.g., ASTM A588) for use in unpainted applications where atmospheric corrosion resistance is needed. A307 bolts are ordinary machine bolts used for secondary connections, bearing stiffeners, and non-structural attachments — they are never used as pretensioned or slip-critical fasteners.

Limit States for Bolted Connections

AISC 360 Section J3 defines the following limit states for bolted connections. Each must be checked independently; the lowest capacity governs the design.

Bolt Shear (J3.6): The nominal shear strength per bolt is rn = Fnv × Ab, where Fnv is taken from Table J3.2 and Ab is the nominal bolt area. For a 3/4-inch A325-N bolt (threads included in shear plane): Fnv = 54 ksi, Ab = 0.442 in², rn = 23.9 kips, φrn = 0.75 × 23.9 = 17.9 kips per shear plane. When the threads are excluded from the shear plane (A325-X), Fnv increases to 68 ksi, giving φrn = 22.5 kips per shear plane — a 26% increase. Specifying thread exclusion matters for single-shear connections where bolt length can be controlled.

Bolt Tension (J3.6): Nominal tensile strength rn = Fnt × Ab, where Fnt is 90 ksi for A325 and 113 ksi for A490. For combined shear and tension per Table J3.2, the available tensile stress is reduced based on the required shear stress: Fnt' = 1.3Fnt - (Fnt/φFnv) × frv, limited to Fnt.

Bearing at Bolt Holes (J3.10): When deformation at the bolt hole is a design consideration: rn = 2.4d × t × Fu for standard holes, where d = bolt diameter, t = plate thickness, Fu = tensile strength of the connected material. When deformation is not a concern, rn = 3.0d × t × Fu. For a 3/4-inch bolt through a 3/8-inch A36 plate (Fu = 58 ksi): rn = 2.4 × 0.75 × 0.375 × 58 = 39.2 kips, φ = 0.75 gives φrn = 29.4 kips per bolt.

Tearout (J3.10): When the bolt is close to an edge and the plate material shears out, rn = 1.2Lc × t × Fu ≤ 2.4d × t × Fu, where Lc is the clear distance from the bolt hole edge to the plate edge. For a 3/4-inch bolt with 1-1/4-inch edge distance: Lc = 1.25 - 0.875/2 = 0.813 inches, rn = 1.2 × 0.813 × 0.375 × 58 = 21.2 kips. Tearout commonly governs over bearing for bolts near plate edges.

Block Shear (J4.3): Block shear rupture involves a combination of shear failure on one plane and tension failure on a perpendicular plane. Rn = 0.60Fu × Anv + UbsFu × Ant ≤ 0.60Fy × Agv + UbsFu × Ant, where Anv is the net area in shear, Ant is the net area in tension, Agv is the gross area in shear, and Ubs = 1.0 for uniform tension stress distribution (typical for bolted connections). This limit state frequently controls the design of coped beams and gusset plate connections.

Tension Rupture (J4.1): For connecting elements subject to tension, the net section rupture strength is Rn = Fu × Ae, where Ae is the effective net area accounting for shear lag per Table D3.1.

Bearing-Type Versus Slip-Critical Design

The fundamental distinction between these two connection types affects which limit states apply and how the connection is detailed and erected.

Bearing-type connections: Bolts are installed snug-tight (full effort of a worker using an ordinary spud wrench, or a few impacts of an impact wrench). At service loads, the connected parts slip into bearing against the bolt shank. The connection strength comes from bolt shear and plate bearing. Advantages include simpler erection (no pretensioning inspection required), lower labor costs, and adequacy for most building applications. Bearing-type connections must still meet all bolt spacing and edge distance requirements of Section J3.3 through J3.5.

Slip-critical connections: Bolts are pretensioned per Table J3.1 to a specified minimum tension (28 kips for 3/4-inch A325, 35 kips for 3/4-inch A490). The clamping force develops friction at the faying surfaces. The slip resistance per bolt is Rn = μ × Du × hf × Tb × Ns, where μ is the mean slip coefficient (0.30 for Class A surfaces, 0.50 for Class B), Du = 1.13 (accounts for pretension uncertainty), hf is the hole factor (1.0 for standard holes, 0.85 for oversize/short-slot, 0.70 for long-slot perpendicular), Tb is the minimum bolt pretension, and Ns is the number of slip planes. Design slip resistance φRn uses φ = 1.00 for standard holes at service load.

Slip-critical design also requires all bearing limit states to be checked at factored load levels per Section J3.8. The connection must satisfy slip resistance at service load and bearing/shear capacity at factored load.

Bolt Pretensioning Methods

AISC 360 Section J3.1 references the RCSC Specification for four approved pretensioning methods:

1. Turn-of-nut method: The most common field method. After snug-tight condition, the nut is rotated an additional 1/3 to 1 turn (depending on bolt length and diameter). No special equipment beyond a calibrated wrench is required.

2. Calibrated wrench method: An impact wrench or torque wrench calibrated to deliver the required pretension. Requires periodic recalibration and verification.

3. Twist-off type tension control bolts: Bolts with a splined end that shears off at a predetermined torque, confirming pretension visually. These are increasingly popular for critical connections.

4. Direct tension indicator (DTI) method: Compressible washers with protrusions that flatten at the required tension. Feeler gauge inspection confirms pretension was achieved.

Edge Distance and Spacing Requirements

Proper bolt layout is essential for both structural performance and constructability. Table J3.4 provides minimum edge distances based on bolt diameter and edge condition. The minimum center-to-center bolt spacing is 2-2/3d per Section J3.3, with a preferred spacing of 3d. Maximum edge distance per Section J3.5 is 12 times the thickness of the connected part but not more than 6 inches. These maximum limits prevent moisture intrusion between plies.

For a practical bolt layout: a 3/4-inch bolt requires minimum edge distance of 1-1/4 inches (sheared edge) and preferred spacing of 2-1/4 inches. A typical 3-bolt shear tab at 3-inch spacing with 1-1/2-inch edge distances requires a plate width of 1.5 + 3 + 3 + 1.5 = 9 inches minimum. The plate thickness is governed by bolt bearing and block shear checks.

Worked Example: Simple Shear Connection

Problem Statement: Design a bolted double-angle shear connection for a W16×26 beam (A992, Fy = 50 ksi) transferring a factored shear of Vu = 40 kips to a column flange. Use 3/4-inch A325-N bolts in standard holes. Beam web thickness tw = 0.250 inches. Assume 3/8-inch clip angles (A36, Fu = 58 ksi).

Step 1 — Determine number of bolts based on shear: Single shear capacity per bolt φrn = 0.75 × 54 × 0.442 = 17.9 kips per shear plane. With double angles providing two shear planes per bolt, each bolt resists 2 × 17.9 = 35.8 kips. Required number: 40 / 35.8 = 1.1 bolts. Minimum practical bolt count for stability is 2, but 3 bolts provide better rotational behavior for standard shear connections per AISC Manual Part 10.

Step 2 — Check bearing on the beam web (deformation a concern): Bearing strength per bolt φrn = 0.75 × 2.4 × 0.75 × 0.250 × 65 = 21.9 kips. Total for 3 bolts: 3 × 21.9 = 65.7 kips > 40 kips. OK.

Step 3 — Check bearing on the clip angles: Plate thickness = 0.375 inches, Fu = 58 ksi. φrn = 0.75 × 2.4 × 0.75 × 0.375 × 58 = 29.4 kips per bolt. For 3 bolts through the outstanding legs in double shear: capacity = 3 × 29.4 × 2 angles = 176.4 kips >> 40 kips. Not governing.

Step 4 — Check tearout on beam web: Assume Le = 1.5 inches to beam end. Clear distance: Lc = 1.5 - (13/16)/2 = 1.094 inches. φrn = 0.75 × 1.2 × 1.094 × 0.250 × 65 = 16.0 kips per bolt. For 3 bolts: 48.0 kips > 40 kips. OK, but note tearout governs over bolt shear.

Step 5 — Check block shear on beam web: Shear plane length = 1.5 + 3 + 3 = 7.5 inches (to bolt centerline). Gross shear area Agv = 7.5 × 0.250 = 1.875 in². Net shear area Anv = (7.5 - 2.5 × 0.875) × 0.250 = 1.328 in². Tension plane length = 1.5 inches. Gross tension area Agt = 1.5 × 0.250 = 0.375 in². Net tension area Ant = (1.5 - 0.5 × 0.875) × 0.250 = 0.266 in² (one bolt in tension plane). Using AISC Equation J4-5: Ubs = 1.0. Rn = 0.60 × 65 × 1.328 + 1.0 × 65 × 0.266 = 51.8 + 17.3 = 69.1 kips. Upper limit: 0.60 × 50 × 1.875 + 1.0 × 65 × 0.266 = 56.3 + 17.3 = 73.6 kips. φRn = 0.75 × 69.1 = 51.8 kips > 40 kips. OK.

Design Summary: 3 rows of 3/4-inch A325-N bolts through double angles 2L 3×3×3/8 at 3-inch bolt spacing, with 1-1/2-inch edge distance to beam end. Beam web tearout controls at 48.0 kips > Vu = 40 kips. All limit states pass.

Engineering Best Practices

• Check all six limit states for every bolted connection: bolt shear, bolt tension, combined shear and tension, bearing, tearout, and block shear. Do not stop at the first check.
• The EOR specifies connection loads on the design drawings. The fabricator designs the connections per AISC Code of Standard Practice. The EOR must review and approve all connection designs before fabrication.
• For beam end connections, consider the effect of cope dimensions on block shear capacity. A deeper cope reduces the shear plane length and may trigger block shear failure.
• For slip-critical connections, specify the faying surface condition (Class A or B) and pretension method on the drawings.
• Long slotted holes require washers under the bolt head or nut and suffer a 30% slip resistance penalty (hf = 0.70).
• For connections with oversized or slotted holes loaded perpendicular to the slot direction, no slip reduction applies, but washers are still required in the outer ply.

References

• AISC 360-22 Chapter J — Design of Connections
• AISC Steel Construction Manual, 16th Edition — Part 7 (Bolts) and Part 9 (Connections)
• RCSC Specification for Structural Joints Using High-Strength Bolts (2020)
• ASTM F3125 — Standard Specification for High Strength Structural Bolts and Assemblies

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