----------- | ----------------------------------------- | -------------------------- | | 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.

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.

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

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.

Try it now: Check your connection design with our free Bolted Connection calculator →

Related Pages

• 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.

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