Steel Connection Design Guide — Bolted and Welded Connections

Steel connections transfer forces between members. Inadequate connection design is a leading cause of structural failures — connections must be designed for every limit state, not just shear. This guide covers the major connection types, governing limit states, design equations, and practical detailing rules for bolted and welded structural steel connections per AISC 360-22.

Connection Types Overview

Bolt Limit States (AISC 360-22 Chapter J)

For every bolted connection, check all applicable limit states:

Bolt Shear (Section J3.6)

φRn = φ × Fnv × Ab × n

Where:
  φ = 0.75
  Fnv = nominal shear stress = 54 ksi (A325, threads in shear plane)
                              = 48 ksi (A325, threads excluded)
                              = 68 ksi (A490, threads in shear plane)
                              = 60 ksi (A490, threads excluded)
  Ab = gross bolt area = π/4 × d²
  n = number of shear planes (1 for single shear, 2 for double shear)

Bolt Tension (Section J3.6)

φRn = φ × Fnt × Ab

  φ = 0.75
  Fnt = 90 ksi (A325 / F1852)
       = 113 ksi (A490 / F2280)

Combined Shear and Tension (Section J3.7)

F'nt = 1.3 × Fnt − (Fnt / (φFnv)) × frv  ≤ Fnt

Where frv = required shear stress on bolt

Bearing on Connected Material (Section J3.10)

For standard holes, deformation not a design consideration:
  φRn = φ × 2.4 × Fu × d × t

For standard holes, deformation is a consideration:
  φRn = φ × 1.2 × lc × Fu × t ≤ φ × 2.4 × Fu × d × t

  φ = 0.75
  Fu = tensile strength of connected material
  d = nominal bolt diameter
  t = thickness of connected material
  lc = clear distance in direction of force

Weld Limit States (Section J2)

Fillet Weld Shear (Section J2.4)

φRn = φ × 0.6 × FEXX × te × (1 + 0.5 sin^1.5(θ))

  φ = 0.75
  FEXX = electrode tensile strength (70 ksi for E70)
  te = 0.707 × weld leg size
  θ = angle of load to weld axis (0° longitudinal, 90° transverse)

Base Metal Limit State — Shear Yielding (Section J4.2)

φRn = φ × 0.6 × Fy × Agv

  φ = 1.00
  Agv = gross area subject to shear

Base Metal Limit State — Shear Rupture (Section J4.2)

φRn = φ × 0.6 × Fu × Anv

  φ = 0.75
  Anv = net area subject to shear

Plate and Element Limit States (Chapter J)

Every connection plate must be checked for:

Block shear rupture commonly governs in thin plates with bolt patterns near edges. The failure mode involves simultaneous tension fracture on a cross section and shear fracture or yielding on the perpendicular section.

Shear Tab (Single Plate) Connection

The shear tab is the most common simple beam connection in steel construction. Designed for shear only; the beam is assumed to rotate freely at the connection.

Design checks for a typical shear tab:

1. Bolt shear capacity — φRn = φFnv×Ab×n (bolt shear governs for fewer, larger bolts)
2. Bolt bearing on tab — φRn = 1.2lc×Fu×t ≤ 2.4Fu×d×t
3. Bolt bearing on beam web — same formula, use tw
4. Shear yielding of tab — φ×0.6Fy×(depth of tab)×t
5. Shear rupture of tab — φ×0.6Fu×Anv
6. Block shear in tab — combined tension/shear path
7. Weld to column — fillet weld each side of tab (E70 min)
8. Tab flexure — bending check for eccentric shear

Typical proportions:

• Tab thickness: 3/8 in to 1/2 in (A36 or A572 Gr50)
• Minimum edge distance: 1.25d for standard holes
• Bolt pitch: 3d (minimum), 2.67d for tightened end rows

Moment Connection Design (Extended End Plate)

Extended end plates (4-bolt and 8-bolt unstiffened/stiffened) are the most common rigid connection in mid-rise buildings.

Design components:

AISC Design Guide 4 provides complete procedures for 4ES, 4ES-S, 8ES, and 8ES-S configurations.

Minimum Edge Distance and Spacing (Section J3.4, J3.3)

Minimum edge distance from center of bolt hole to edge of connected part:

Minimum bolt spacing (Section J3.3): 2.67d (preferred 3d = 3 times nominal diameter).

Maximum spacing (Section J3.5): 12t or 6 in (whichever is less) for painted members; 14t or 7 in for unpainted weathering steel.

Frequently Asked Questions

What is block shear and when does it govern? Block shear is a failure mode where a block of material tears out of a connection, involving simultaneous tension fracture on one plane and shear on a perpendicular plane. It commonly governs for shallow plates, angles, or gussets with bolt patterns close to edges. Always check block shear — it is easy to miss and can be the controlling limit state.

Do I always need to check bearing? Yes, for every bolted connection. Bearing on the connected material (not just the bolt itself) must be checked. The thinner of the two connected parts governs. For short slots or deformation-sensitive connections, use the conservative 1.2lc×Fu×t formula.

Should connections be designed for a minimum force? AISC recommends designing connections for the greater of the required force or a minimum force. AISC 360 Section B3.6b: connections shall be designed for the actual force but not less than the minimum required force (varies by connection type). For gravity beam connections, a common rule is to design for 1/2 the beam shear capacity as a minimum.

When are column stiffeners required? Column stiffeners (continuity plates) are required when the concentrated flange force from a beam flange exceeds the local web yielding capacity (AISC Section J10.2) or web crippling capacity (J10.3), or when local flange bending capacity is insufficient (J10.1). In practice, stiffeners are often required at moment connections and should be checked routinely.

When does block shear govern over bolt shear in a connection? Block shear tends to govern when the connected plate or angle is thin, the bolt group is shallow relative to the edge distance, or the number of bolt rows is small. In a single-row bolt pattern near a free edge — such as a shear tab with two or three bolts — block shear can control because the net tension area Ant is small while the shear path is short. Bolt shear typically governs for large bolt groups in thick material. Designers should always check block shear per AISC 360-22 Section J4.3 and not assume bolt shear or bearing will always be the critical limit state; the difference can be significant for connections with long bolt rows and small edge distances.

What is the difference between a bearing-type and a slip-critical bolted connection? Bearing-type connections rely on the bolt shank bearing against the hole walls to transfer load; some slip is permitted before the connection engages bearing. Slip-critical connections are pretensioned so that clamping force between the connected plies develops friction sufficient to transfer the design load without any slip. Slip-critical connections are required where slip would be detrimental — at connections subject to load reversal, fatigue, or where oversized or slotted holes are used in the load direction. Per AISC 360-22 Section J3.8, slip-critical connections are designed using a slip resistance φRn = φ × µ × Du × hf × Pt × ns, where µ is the mean slip coefficient (Class A = 0.35, Class B = 0.50) and Pt is the minimum bolt pretension from Table J3.1.

How do you determine the number of bolts required for a shear connection? The required number of bolts n is found by dividing the factored shear demand Vu by the governing bolt design strength φRn per bolt. The governing strength is the minimum of: bolt shear capacity (φFnv × Ab), bolt bearing on the thinner connected element (φ × 2.4Fu × d × t for standard holes), and the corresponding plate limit states checked globally. For a typical W-shape beam framing to a column using 3/4-in A325 bolts in single shear through a 3/8-in shear tab, each bolt provides approximately φRn ≈ 0.75 × 54 × 0.442 ≈ 17.9 kips in shear; a beam carrying 60 kips of factored shear would require at minimum n = 60/17.9 ≈ 4 bolts, subject also to bearing, block shear, and tab flexure checks.

Run This Calculation

→ Base Plate Calculator — column base plate bearing, anchor bolt tension, and weld design per AISC 360 / ACI 318.

→ Bolted Connections Calculator — bolt shear, bearing, and block shear per AISC 360, AS 4100, EN 1993, CSA S16.

→ Welded Connections Calculator — fillet weld capacity and weld group analysis.

→ Beam Capacity Calculator — member capacity for connection demand check.

Related pages

• Bolt Hole Sizes Reference — AISC Table J3.3 hole dimensions
• Bolt Torque Chart — A325/A490 pretension and tightening
• Fillet Weld Size Chart — minimum/maximum weld sizes
• Weld Symbol Chart — reading welding symbols on drawings
• HSS Connection Design — K/T/Y/X connection types for HSS
• Reference tables directory
• How to verify calculator results
• steel beam allowable load tables
• structural steel weight per foot table

Design equations per AISC 360-22 LRFD. All connections must be designed, detailed, and inspected in accordance with the applicable code, project specifications, and AISC Code of Standard Practice. Consult a licensed structural engineer for final connection design.

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