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:

| Classification — Rotation Behavior — Moment Transferred — Frame Model | | ------------------- — ---------------------------- — -------------------------- — --------------------------------- | | Simple (pinned) — Free rotation, no moment — 0% of fixed-end moment — Pin support in analysis | | Semi-rigid (PR) — Partial rotational restraint — 20-80% of fixed-end moment — Spring element with M-theta curve | | Rigid (FR) — No relative rotation — ~100% of fixed-end moment — Fixed support in analysis |

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

| Connection Type — Typical Shear Capacity — Moment Capacity — Relative Cost — Erection Speed | | ------------------------ — ---------------------- — --------------- — ------------- — ------------------- | | Single plate (shear tab) — 30-150 kip — None (simple) — Low — Fast | | Double angle — 50-200 kip — None (simple) — Low-Medium — Moderate | | Extended end plate (4E) — Per beam reaction — 50-80% M_p — Medium — Fast (field bolted) | | Bolted flange plate — Per beam reaction — 80-100% M_p — Medium-High — Moderate | | Welded flange (WUF-W) — Per beam reaction — 100% M_p — High — Slow (field welded) | | Gusset plate (brace) — N/A — N/A (axial) — Medium — Moderate |

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.

AISC connection classification — detailed breakdown

AISC 360-22 Section B3.4 classifies connections into three categories based on their ability to transfer moment between connected members. This classification directly affects the structural analysis model and the design of both the connection and the connected members.

Type FR (Fully Restrained — Rigid)

Fully restrained connections are sufficiently stiff to maintain the angle between connected members essentially unchanged under loading. The connection must be designed for the full moment obtained from the structural analysis.

• Analysis model: Fixed support or rigid link between beam and column
• Moment transfer: 100% of the analysis moment at the joint
• Required design: Connection must develop the full factored moment from analysis; for seismic systems, must develop the expected strength of the beam (Ry x Fy x Zx)
• Typical details: Fully welded moment connections, bolted flange plate (BFP), extended end plates with stiffeners (4ES, 8ES)
• Frame behavior: Contributes to lateral stiffness and participates in the lateral-force-resisting system (LFRS)

Type PR (Partially Restrained — Semi-Rigid)

Partially restrained connections provide measurable but incomplete rotational restraint. The moment-rotation (M-theta) relationship must be known and incorporated into the structural analysis. AISC 360 Section B3.4b requires the designer to document the M-theta curve used in the analysis.

• Analysis model: Nonlinear rotational spring at the beam-column joint, with stiffness from published M-theta data or physical testing
• Moment transfer: Typically 20-80% of the fixed-end moment, depending on connection stiffness
• Required design: Connection designed for the moment obtained from the spring-inclusive analysis (not the full fixed-end moment)
• Typical details: Top-and-seat angles with web angles, partial-depth end plates, single-plate shear tabs with extended configuration, bolted flange plates with thin plates
• Frame behavior: Contributes to lateral stiffness but less than FR; drift calculations must include connection rotation

Simple (Pinned — Theoretical)

Simple connections are assumed to allow free rotation and transfer no moment. In practice, all connections have some rotational stiffness, but simple connections are designed and modeled as pins.

• Analysis model: Pin or roller support; beam analyzed as simply supported
• Moment transfer: 0% of the analysis moment (theoretical)
• Required design: Connection designed for vertical shear reaction plus a nominal moment (typically not less than the reaction x 3 in. for single-plate connections to account for eccentricity)
• Typical details: Single-plate (shear tab), double angle, single angle, seated connections
• Frame behavior: Does not contribute to lateral stiffness; lateral resistance provided by braced frames or shear walls

Shear connection types — detailed comparison

Shear (simple) connections are the most common connection type in steel buildings, representing 80-90% of all beam-to-column connections in a typical office building. The following table compares the primary shear connection types used in U.S. practice.

Shear connection selection guide

Use this decision flow to select the appropriate shear connection type:

1. Is the beam framing into the web of a column? Use single-plate (shear tab) or double angle. Single plate is preferred for economy; double angle is preferred when rotational flexibility is needed.
2. Is the beam framing to a column flange? Single plate (welded to column) is the default. For heavy reactions (> 100 kips), consider double angle or stiffened seated connections.
3. Is the beam framing to a beam web (beam-to-beam connection)? Single plate welded to the supporting beam web, or double angle. Ensure adequate clearance for bolt installation.
4. Is the beam sitting on top of the support? Seated connection (unstiffened for reactions under 60 kips, stiffened for heavier reactions). Common for roof beams and mezzanine framing.
5. Is the beam very shallow (under 8 in. depth)? Single angle or seated connection may be the only practical options due to limited web depth for bolt placement.

Moment connection types — detailed comparison

Moment connections are used in the lateral-force-resisting system and must be designed for combined shear, axial force, and moment. The following table covers the most common moment connection types in U.S. practice.

OMF = Ordinary Moment Frame, IMF = Intermediate Moment Frame, SMF = Special Moment Frame.

Connection cost comparison

Connection cost is a significant portion of the total structural steel erection budget. The following estimates are per connection for typical office building conditions (W18-W24 beams, W12-W14 columns) and include material, fabrication, and erection labor.

Rule of thumb: In a typical 5-story steel office building, moment connections represent 5-10% of all connections but 25-40% of the total connection budget. Specifying shear tabs wherever possible and reserving moment connections for the lateral system is the single most effective cost-saving measure in connection design.

Run this calculation

• Bolted Connections Calculator
• 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.

Design Resources

Calculator tools

• Bolted Connection Calculator
• Weld Capacity Calculator
• End Plate Moment Connection Calculator
• Fin Plate Shear Connection Calculator
• Gusset Plate Calculator

Design guides

• Bolted Connection Worked Example
• Bolted Connection Checklist
• Steel Connection Calculator Guide
• Weld Design Checklist
• EN 1993-1-8 Bolted Connection Worked Example

Reference pages

• Shear Tab