How does a steel moment connection transfer forces?

A moment connection transfers beam bending moment to the column as a force couple: tension in the top flange (P_tension = Mu/(d - tf)) and compression in the bottom flange. These flange forces are transferred through: (1) full-penetration groove welds (CJP) between beam flange and column flange, or (2) bolted flange plates or extended end plates. The beam web shear is transferred separately through a shear tab or web plate. The column must resist the panel zone shear (the shear in the column web between the beam flanges) and may require continuity plates (horizontal stiffeners aligned with beam flanges) and doubler plates (additional web plate) to prevent local yielding.

What is panel zone shear and how is it checked?

Panel zone is the column web region between beam flanges at a moment connection. The panel zone experiences high shear from the beam flange force couple. Per AISC 360-22 J10.6: nominal shear strength Rn = 0.6 x Fy x dc x tw for shear yielding (when frame stability is not affected). When Pu > 0.4 x Py (column carries substantial axial load): Rn = 0.6 x Fy x dc x tw x (1.4 - Pu/Py). phi = 0.90. If the panel zone shear demand exceeds capacity, a doubler plate is added to increase the effective web thickness. AISC 341 for seismic allows limited panel zone yielding to dissipate energy — a deliberate design choice for SMF.

When are continuity plates required in moment connections?

Per AISC 341-22 J7, continuity plates (horizontal stiffeners at beam flange level) are required when: (1) the column flange thickness is insufficient to resist the beam flange force in tension (flange local bending per AISC J10.1), (2) the column web local yielding or crippling strength is exceeded per AISC J10.2/J10.3, or (3) required by the prequalified connection design (e.g., RBS connections require continuity plates unless the column satisfies the strong-column/weak-beam check). Practical check: if beam flange width > column flange width, continuity plates are almost always required. Continuity plates must match the beam flange width and thickness, extend to match the column flange width, and have CJP welds to the column flanges and fillet welds to the column web.

What are bolted end-plate moment connections?

Bolted end-plate moment connections use a steel plate shop-welded to the beam end, then field-bolted to the column flange. Types: (1) Flush end plate (plate does not extend beyond beam depth) — limited moment capacity. (2) Extended end plate (plate extends above/below beam flanges with bolts outside flanges) — higher capacity, most common for moment frames. (3) Stiffened extended end plate (stiffeners between end plate and beam flanges) — highest capacity. Design per AISC DG 4 and DG 16 uses yield line theory for plate bending and prying action for bolt tension. End-plate connections avoid field welding, improving erection speed and quality — the connection is shop-welded and field-bolted.

How does seismic design change moment connections?

AISC 341-22 imposes stricter requirements for seismic moment connections in SMF (Special Moment Frame) and IMF (Intermediate Moment Frame): (1) Demand-critical welds — CJP groove welds at beam flanges require 100% UT inspection per J7 and CVN toughness qualification. (2) Protected zone — the beam plastic hinge region (typically 1/2 beam depth from column face) cannot have any attachments, tack welds, or deck connections. (3) Continuity plates — required unless specific column strength criteria are met per the prequalified connection. (4) Backing bars — bottom flange backing bar must be removed and the root back-gouged and reinforced with a fillet weld; top flange backing may remain. (5) RBS (Reduced Beam Section) — a common prequalified connection where the beam flange is reduced to force the plastic hinge away from the column.

Moment Connection Design — AISC Method & Examples

Moment connections transfer bending moment between beams and columns in steel moment frames. They are the backbone of lateral-force-resisting systems. This page covers the AISC 360 design procedure, panel zone requirements, prequalified connections, and worked examples.

How Moment Connections Work

A moment connection transfers beam flange forces into the column. The beam flanges carry the moment as a force couple:

Flange force: Ff = M / (d - tf)

where M = applied moment, d = beam depth, tf = beam flange thickness.

The connection must deliver this flange force from beam to column through welds, bolts, and plates. The column must resist the resulting forces through its panel zone, flanges, and web.

Connection Components

Component	Function	Design Check
Flange connection	Transfer Ff (tension and compression)	Weld, bolt, plate capacity
Web connection	Transfer beam shear	Bolt shear, bearing
Panel zone	Resist shear from force couple	Column web shear
Continuity plates	Stiffen column flanges against bending	Flange bending resistance
Doubler plate	Reinforce column web for panel zone shear	Web shear capacity
Column flange	Resist prying from tension bolts	Prying action check

Design Procedure — Flange Plate Moment Connection

Step 1: Determine Flange Force

Ff = Mu / (d - tf)

where Mu = factored moment at the connection, d = beam depth, tf = beam flange thickness.

For the compression flange, Ff is delivered as a bearing/compression force. For the tension flange, Ff is delivered through bolts or welds in tension.

Step 2: Size Flange Plates

Tension yielding: b × tp × Fy ≥ Ff Tension rupture: b × (tp - bolt holes) × Fu ≥ Ff / φ

where b = plate width, tp = plate thickness.

Step 3: Design Flange Bolts

Bolt shear: Number of bolts ≥ Ff / (φ × rn)

where rn = nominal bolt shear capacity per bolt (AISC Table 7-1).

Bolt bearing on plate: Check bearing and tearout per AISC Chapter J.

Step 4: Check Block Shear

Block shear failure is a combined tension and shear rupture through the bolt group:

φRn = φ × [0.6 × Fu × Anv + Fu × Ant] ≤ φ × [0.6 × Fy × Agv + Fu × Ant]

where Anv = net shear area, Ant = net tension area, Agv = gross shear area.

Step 5: Design Flange Welds

For shop-welded flange connections:

Weld capacity: φRw = φ × 0.6 × FEXX × 0.707 × a × Lw

where FEXX = weld electrode strength (typically 70 ksi), a = weld leg size, Lw = weld length.

CJP groove welds develop the full plate capacity. Fillet welds are sized for the flange force.

Step 6: Check Panel Zone Shear

The column web panel zone must resist the unbalanced moment from the beam flanges:

Panel zone shear force: Vpz = Ff,top - Ff,bottom (from opposite beam) + Vcol

Panel zone shear capacity (AISC Section J6.5):

Without doubler: φRn = φ × 0.6 × Fy,web × dc × tw × (1 + 3 × bfc × tfc² / dc × dc × tw)

where dc = column depth, tw = column web thickness, bfc = column flange width, tfc = column flange thickness.

If the capacity is insufficient, add a doubler plate.

Step 7: Check Continuity Plate Requirements

Continuity plates (transverse stiffeners) are required per AISC Section J6.5 when:

Ff > φ × Pwb (column web yielding)
Ff > φ × Pfb (column flange bending)

If needed, size the stiffener for the excess force:

A_st ≥ (Ff - φ × Pwb) / (φ × Fy,st)

Typical continuity plate: 3/8 × 4 minimum for moderate connections.

Prequalified Seismic Connections (AISC 358)

Seismic moment connections must be prequalified per AISC 358 or validated by testing. The most common prequalified connections:

Reduced Beam Section (RBS)

The RBS is the most widely used seismic moment connection. Flanges are cut back (trimmed) in a circular arc near the column to create a predictable plastic hinge location away from the weld.

Parameter	Requirement
Cut length	0.75 × bf to 0.85 × bf
Cut depth	0.20 × bf to 0.25 × bf (each side)
Distance from face	0.5 × bf to 0.625 × bf
Max beam depth	W36
Max beam weight	300 lb/ft
Flange thickness	≤ 1-3/4 in
Fy	≤ 55 ksi (A992)

The plastic section modulus at the reduced section:

Z_rbs = Z_x - 2 × c × tf × (d - tf)

where c = average cut depth, tf = flange thickness, d = beam depth.

Extended End Plate

Bolted unstiffened (BUEP) or stiffened (BSEP) end plate connections. The end plate extends beyond the beam flanges with bolts in the extended portion.

Parameter	BUEP	BSEP
Max beam depth	W24	W36
Bolts	4 per flange	6-8 per flange
End plate thickness	Per AISC 358	Per AISC 358
Prying action	Yes	Yes (reduced)

Worked Example — Flange Plate Connection

Given: W18×50 beam to W14×90 column, Mu = 250 kip-ft (LRFD), A992 steel.

Step 1: Flange Force W18×50: d = 18.0 in, tf = 0.57 in Ff = 250 × 12 / (18.0 - 0.57) = 3,000 / 17.43 = 172.1 kips

Step 2: Flange Plate Try PL 3/8 × 7 (A572 Gr 50):

Tension yielding: 0.375 × 7 × 50 = 131.3 kips < 172.1 → Too small
Try PL 1/2 × 7: 0.5 × 7 × 50 = 175 kips > 172.1 → OK

Step 3: Bolts Use 7/8 in A325-N bolts (φrn = 21.6 kips single shear): Number = 172.1 / 21.6 = 7.97 → Use 8 bolts (4 per side of beam flange)

Step 4: Panel Zone W14×90: dc = 14.0 in, tw = 0.44 in, bfc = 14.5 in, tfc = 0.71 in φRn = 0.9 × 0.6 × 50 × 14.0 × 0.44 × (1 + 3 × 14.5 × 0.71² / 14.0² × 0.44) = 166.3 × (1 + 0.159) = 192.8 kips > 172.1 → OK, no doubler needed

Step 5: Continuity Plate Check Column web yielding: φPwb = φ × Fy × tw × (tf_beam + 5k) where k = column k-distance. For W14×90: k = 1.31 in, tf_beam = 0.57 in φPwb = 0.9 × 50 × 0.44 × (0.57 + 5 × 1.31) = 0.9 × 50 × 0.44 × 7.12 = 141.2 kips < 172.1

Continuity plates required. Add 3/8 × 4 stiffeners each side.

Connection Stiffness Classification

Classification	Ratio θ_conn/θ_beam	Typical Application
Rigid (FR)	< 0.2	Moment frames
Semi-rigid	0.2 to 2.0	Partially restrained
Flexible (pin)	> 2.0	Simple shear

For most practical moment frames, connections are assumed rigid (FR) per AISC 360. Semi-rigid design requires published moment-rotation curves.

Frequently Asked Questions

When is a moment connection required? A moment connection is required when the frame uses moment-resisting connections for lateral stability (wind or seismic), when a beam cantilevers past a support, or when the beam continuity causes negative moment at the support.

What is the difference between FR and PR connections? FR (Fully Restrained) connections are rigid and transfer full moment. PR (Partially Restrained) connections allow some rotation and transfer partial moment. AISC 360 requires PR connections to have known moment-rotation characteristics. Most practical moment frames use FR connections.

What is a doubler plate? A doubler plate is a steel plate welded to the column web inside the panel zone to increase shear capacity. It is required when the applied panel zone shear exceeds the column web shear capacity. The doubler is typically plug-welded or fillet-welded to the column web.

Why is the RBS connection preferred for seismic design? The Reduced Beam Section moves the plastic hinge away from the column face, protecting the weld and connection. This improves ductility and prevents brittle fracture at the beam-to-column weld. The RBS connection has been extensively tested and is prequalified by AISC 358 for most seismic applications.

Welded Connections — Weld capacity calculator
Bolted Connections — Bolt capacity calculator
Connection Types Explained — All connection types
Base Plate Design — Base plate calculator
Beam Design Guide — Beam design overview

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.