Steel Connection Types — Shear, Moment & Brace Connections

Stiffness classification, moment capacity tables, and a capacity estimator for shear tab, end plate, moment, and splice connections.

Overview

Steel connections transfer forces between members and are classified by the type of force they resist: shear (simple) connections, moment (rigid) connections, and bracing connections. Connection type affects structural behavior, fabrication cost, and erection sequence. AISC 360, AS 4100, EN 1993-1-8, and CSA S16 each provide design provisions for these connection categories.

Stiffness classification

Design codes classify connections by their rotational stiffness, which determines how forces and moments are distributed in the frame:

AISC 360-22 Section B3.4 defines Type FR (fully restrained) and Type PR (partially restrained) connections. A connection is FR when its stiffness is sufficient to maintain the angle between members essentially unchanged under loading.

EN 1993-1-8 Clause 5.2 provides explicit numerical stiffness boundaries: a connection is rigid when its initial rotational stiffness S_j,ini exceeds 25EI_b/L_b for braced frames or 8EI_b/L_b for unbraced frames. Semi-rigid falls between these limits and the pinned boundary at 0.5EI_b/L_b.

Shear connections (simple)

Shear connections transfer vertical reaction only and are assumed to allow free rotation at the beam end. Common types include:

• Single plate (shear tab) — one plate shop-welded to the column and field-bolted to the beam web. The most economical and widely used shear connection in U.S. practice. Standard configurations handle reactions up to approximately 150 kip for typical beam depths. Design per AISC Manual Table 10-10.
• Double angle — two angles bolted or welded to both sides of the beam web and to the support. Provides symmetric load transfer, good ductility, and higher capacity than single-plate connections.
• Seated connection — an angle or tee supporting the beam bottom flange, combined with a top clip angle for stability. Unstiffened seats handle up to ~60 kip; stiffened seats handle significantly more.
• Single angle — one angle bolted to the beam web and the support. Simplest and most economical but limited to light loads (typically < 50 kip).

Moment connections (rigid)

Moment connections transfer both shear and moment, maintaining the angle between connected members:

• Extended end plate — a plate welded to the beam end and bolted to the column flange. Configurations include 4-bolt (4E), 4-bolt stiffened (4ES), and 8-bolt stiffened (8ES) per AISC Design Guide 4/16.
• Bolted flange plate (BFP) — plates shop-welded to the column flange and field-bolted to the beam flanges. Prequalified for seismic per AISC 358 Chapter 7.
• Welded unreinforced flange — welded web (WUF-W) — beam flanges welded directly to the column flange with CJP groove welds. Prequalified per AISC 358 Chapter 8.
• Reduced beam section (RBS) — circular flange cuts force the plastic hinge into the reduced section, protecting the column connection. Prequalified per AISC 358 Chapter 5 and the most commonly used seismic moment connection.

Worked example — shear tab capacity check

Given: A single-plate shear tab connection: 1/4" x 3" x 8.5" A36 plate (Fy = 36 ksi, Fu = 58 ksi), shop-welded to a W14x61 column flange with 3/16" fillet welds, field-bolted to a W18x50 beam web with 3 bolts (3/4" A325-N, single shear), bolt spacing s = 3 in, edge distances Lev = 1.25 in, Leh = 1.5 in.

Step 1 — Bolt shear (AISC 360 Section J3.6): phi*Rn per bolt = 0.75 * 54 _ 0.4418 = 17.9 kips. Group capacity = 3 _ 17.9 = 53.7 kips.

Step 2 — Bearing on plate (Section J3.10): Edge bolt: Lc = 1.25 - 0.8125/2 = 0.844 in. phiRn = 0.75 * 1.2 _ 0.844 _ 0.25 * 58 = 11.0 kips (tearout governs this bolt). Interior bolts: Lc = 3.0 - 0.8125 = 2.188 in. phiRn = 0.75 _ 1.2 _ 2.188 _ 0.25 _ 58 = 28.6 kips, but capped at 0.75 _ 2.4 _ 0.75 _ 0.25 _ 58 = 19.6 kips (bearing governs). Total bearing: 11.0 + 2 * 19.6 = 50.2 kips.

Step 3 — Weld capacity: 3/16" fillet weld, both sides of plate: effective throat = 0.707 _ 3/16 = 0.133 in. phiRn per inch = 0.75 _ 0.60 _ 70 _ 0.133 = 4.18 kip/in. Two welds at 8.5 in: phiRn = 2 _ 4.18 _ 8.5 = 71.1 kips.

Step 4 — Block shear on plate (Section J4.3): Agv = 0.25 * (2*3 + 1.25) = 1.81 in^2. Anv = 1.81 - 2.5 _ 0.8125 _ 0.25 = 1.30 in^2. Ant = 0.25 _ (1.5 - 0.8125/2) = 0.273 in^2. phiRn = 0.75 _ (0.6 _ 58 _ 1.30 + 58 _ 0.273) = 0.75 _ (45.2 + 15.8) = 45.8 kips.

Governing capacity: Block shear = 45.8 kips (governs over bolt shear, bearing, and weld).

Connection capacity comparison

Multi-code comparison

AISC 360-22 (USA): Connection design per Chapter J. Bolt shear per Section J3.6 (phi = 0.75). Bearing per Section J3.10 (phi = 0.75). Weld strength per Section J2.4. Block shear per Section J4.3. Prequalified seismic connections per AISC 358. FR/PR classification per Section B3.4 (qualitative).

AS 4100-2020 (Australia): Connection design per Section 9. Bolt shear per Clause 9.2.2.1 (phi = 0.8). Bearing per Clause 9.2.2.4. Weld strength per Section 9.6 (phi = 0.8 for SP category, 0.6 for GP category). Block shear per Clause 9.1.10. AS 4100 classifies connections as rigid, semi-rigid, or pinned per Clause 4.2, but does not provide explicit numerical stiffness boundaries -- engineering judgment is used.

EN 1993-1-8 (Europe): Connection classification per Clause 5.2 with explicit numerical stiffness boundaries (S_j,ini vs. E*I_b/L_b). Bolt shear per Clause 3.6.1 (gamma_M2 = 1.25). Bearing per Clause 3.6.1 (gamma_M2 = 1.25). Weld strength per Section 4 (gamma_M2 = 1.25). Block tearing per Clause 3.10.2. Component method (Clause 6.2) allows analytical determination of connection moment-rotation behavior from individual bolt row, T-stub, and panel zone contributions.

CSA S16-19 (Canada): Connection design per Clause 13. Bolt shear per Clause 13.12.1.2 (phi_b = 0.80). Bearing per Clause 13.12.1.4. Weld strength per Clause 13.13.2 (phi_w = 0.67). Block shear per Clause 13.11. CSA classifies connections as simple, rigid, or semi-rigid per Clause 7.2.3. Moment connections for seismic applications per Clause 27.2 must satisfy interstory drift angle requirements similar to AISC 341/358.

Common mistakes

1. Treating semi-rigid connections as pinned. Partially restrained connections (e.g., top-and-seat angles with web angles) transfer 20-50% of the fixed-end moment. Modeling them as pins overestimates beam mid-span moment and underestimates column moment, potentially leading to unconservative column design.

2. Using moment connections where shear tabs suffice. Specifying moment connections for gravity beams adds unnecessary fabrication cost ($3,000-$8,000 per connection vs. $300-$800 for a shear tab). Reserve moment connections for the lateral-force-resisting frame.

3. Ignoring erection stability. During erection, before the deck and lateral bracing are placed, connections must provide temporary stability. At least two bolts per connection must be installed before releasing the crane load per OSHA 29 CFR 1926.756.

4. Not checking column panel zone for moment connections. Moment connections impose high shear on the column web panel. Panel zone yielding can occur even when the beam and connection are adequate. Check per AISC 360 Section J10.6 and add doubler plates as needed.

5. Neglecting prying action on end-plate connections. Bolts in tension on end-plate and T-stub connections experience prying forces from plate bending. The actual bolt tension can be 20-40% higher than the applied flange force. AISC Manual Part 9 provides the prying action analysis method.

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• Welded Connections Calculator
• Gusset Plate Calculator

Related references

• Bolt Capacity Table
• Fillet Weld Size Chart
• Bolt Pattern
• Connection Checks
• End Plate Connection
• Coped Beam Design
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.