Why Connection Design Matters

Steel connections are often more complex and labor-intensive than the members they join. A well-designed connection transfers loads efficiently with minimum material; a poorly designed or fabricated connection can fail at lower loads than the connected members — causing progressive collapse. AISC 360 (Specification for Structural Steel Buildings) and the AISC Steel Construction Manual provide the design methodology and design aids for bolt and weld connections.

High-Strength Bolts

Most structural steel connections use high-strength bolts: ASTM A325 (now ASTM F3125 Grade A325) with a minimum tensile strength of 120 ksi, or ASTM A490 (now ASTM F3125 Grade A490) at 150 ksi minimum tensile strength. Standard bolt diameters are 3/4", 7/8", 1", and 1-1/8".

Bearing-type connections transfer shear through the bolt bearing against the sides of the bolt hole. The bolt may or may not be pretensioned, and slip of the joint under load is acceptable. This is the most economical connection type and is used for most standard connections.

Slip-critical connections transfer shear through friction between the faying surfaces of the connected plies. The bolts must be fully pretensioned (the bolt is torqued to 70% of its minimum tensile strength) to clamp the surfaces together. Slip-critical connections are required where bolt slippage under service loads would be problematic: vibrating machinery, connections subject to reversal, or connections in seismic force-resisting systems. They are more expensive due to installation requirements and inspection.

Bolt Design Strength

Per AISC 360 Table J3.2, the available shear strength per bolt in a bearing-type connection:

φrn = φ × Fnv × Ab

where Fnv is the nominal shear stress (48 ksi for A325 bolts in double shear), Ab is the bolt cross-sectional area, and φ = 0.75 (LRFD).

For a 3/4" A325 bolt in double shear: φrn = 0.75 × 48 ksi × (π/4 × 0.75²) = 0.75 × 48 × 0.442 = 15.9 kips

Weld Design

Fillet welds are the most common weld type in structural steel connections. A fillet weld is placed in the corner of a lap or T-joint connection. Key parameters:

  • Weld size (w) — the leg size of the fillet weld, measured in 1/16-inch increments
  • Effective throat (te) — for a fillet weld, te = 0.707 × w
  • Electrode strength (FEXX) — E70 electrodes (70 ksi tensile strength) are standard for A36 and A572 steel

AISC design strength of a fillet weld per unit length (LRFD):

φRnw = 0.75 × 0.60 × FEXX × te = 0.75 × 0.60 × 70 × (0.707 × w) = 22.3w kips/inch

For a 3/16" fillet weld (w = 3/16 = 0.1875 inch): φRnw = 22.3 × 0.1875 = 4.18 kips/inch

Minimum and Maximum Weld Sizes

AISC 360 Table J2.4 gives minimum weld sizes based on thicker part joined: 3/16" for material up to 3/4" thick, 1/4" for material over 3/4" thick. Maximum weld size is the thickness of the thinner part minus 1/16" for material ≥ 1/4" thick, or equal to the material thickness for material < 1/4" thick. These limits prevent under-welding and over-welding of connections.

Standard Connection Types

Shear tab (single plate) — a plate welded to the column web, bolted to the beam web. Simple, economical, and flexible. The standard simple shear connection in most beam-to-column framing.

Double angle — two angles bolted to the beam web and welded to the supporting member. Slightly more labor to fabricate than a shear tab but provides redundancy and easier field adjustment.

Moment connection (FR) — beam flanges welded to column flanges with groove welds; web bolted with a shear tab. Transfers both shear and moment. Used at moment frame beam-column joints.