CSA S16 Moment Connection — End Plate & Flange Connection Design

Complete reference for CSA S16:19 moment-resisting connections including bolted end plate (flush and extended), welded flange connections with CJP groove welds, continuity plates, panel zone shear, and prying action per CSA S16 Clause 23.6. Includes a worked example for a bolted extended end-plate moment connection.

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CSA S16 Moment Connections — Overview

Moment connections transfer bending moment (and associated shear) between connected members. In Canadian practice, moment connections are classified as either rigid (full-restraint) or partially-restrained (PR). CSA S16 Clause 23 classifies connections by their rotational stiffness and strength.

The two most common moment connection types in Canadian steel construction are:

Bolted End Plate Moment Connections

Flush End Plate

A flush end plate has the plate flush with the beam top and bottom flanges. It is typically used for connections where the beam depth does not exceed approximately 600 mm:

• Rotational capacity: limited by end plate yielding
• Bolt rows: typically 2 rows at tension flange, 2 at compression flange
• End plate thickness: 12 mm to 25 mm typical
• Limitation: does not develop full beam plastic moment in heavier sections

Extended End Plate

An extended end plate projects beyond the beam tension flange, allowing additional bolt rows above the flange:

• Rotational capacity: can develop the full plastic moment of the beam
• Bolt rows: 2 rows outside the tension flange (outside + inside) + optional compression rows
• End plate thickness: 16 mm to 35 mm typical
• Application: seismic moment frames (SMF), rigid frames where full moment transfer is required

Moment Connection Design — Bolt Group in Tension

The bolt group in a moment end plate connection is subjected primarily to tension from the applied moment, with the neutral axis located at the compression flange (conservative assumption per CSA S16):

Tension force in bolt group = Mf / (d - tf)

where:
  Mf = factored moment (N·mm)
  d  = beam depth (mm)
  tf = beam flange thickness (mm)

The lever arm (d - tf) is the distance between the centre of the compression flange and the centre of the tension flange bolt group. This simplified approach conservatively places the neutral axis at the compression flange centreline.

Bolt Tension Check

Each tension bolt is checked per CSA S16 Clause 22.9:

Trb = φb × 0.75 × Ab × Fu

where:
  φb = 0.75 (bolts in tension)
  Ab = nominal bolt area (mm²)
  Fu = bolt tensile strength (MPa)

The total tension force is distributed among the bolt rows in proportion to their distance from the neutral axis. The outer bolt row carries the largest tension force and typically governs the design.

Prying Action (CSA S16 Clause 23.6)

Prying action in moment end plate connections amplifies the bolt tension beyond the direct applied load. The prying force Q develops from flexural deformation of the end plate as it bears against the bolt head or nut.

CSA S16 Clause 23.6 provides the design procedure based on the T-stub analogy:

The required end plate thickness to limit prying effects:
t_min = √(4 × T × b / (φ × Fy × (1 + δ × α)))

where:
  T  = applied tension per bolt (N)
  b  = distance from bolt centreline to flange face (mm)
  φ  = 0.90 (base metal flexure)
  Fy = end plate yield strength (MPa)
  δ  = ratio of net area at bolt line to gross area at flange face
  α  = ratio of distance from bolt line to flange face to edge distance

For practical design, prying action can be minimised by:

• Increasing end plate thickness until t ≥ t_min
• Reducing the bolt gauge distance (moving bolts closer to the web)
• Using stiffeners or continuity plates
• Specifying thicker end plates (Canadian practice typically uses 20-25 mm minimum for extended end plates)

Prying Factor q

The prying factor q is defined as the ratio of amplified bolt force to applied bolt force:

q = (T + Q) / T

where:
  T = applied tension per bolt
  Q = prying force

A prying factor of q = 1.2 to 1.4 is typical for well-designed end plate connections. Values above q = 1.6 indicate the end plate is too thin, requiring stiffening or an increased plate thickness.

Welded Flange Moment Connections

Welded flange connections use CJP groove welds to connect the beam flanges directly to the column flange. The web is typically bolted to a shear tab. This is the most rigid moment connection type.

Flange Weld Design (CSA S16 Clause 13.13.2)

CJP groove welds develop the full base metal strength:

Flange force at connection:
Ff = Mf / (d - tf) ± Vf × e / (d - tf)

CJP weld tension resistance:
Trw = φw × bf × tf × Fy

where:
  φw = 0.90 (CJP groove welds)
  bf = beam flange width (mm)
  tf = beam flange thickness (mm)
  Fy = beam yield strength (MPa)

Column Flange Continuity Plates

When the beam tension flange force exceeds the column flange's ability to resist local bending, continuity plates are required per CSA S16 Clause 23.5:

Column flange bending resistance:
Mr_cf = φ × tcf² × Fyc × (bf - k1) / 4

where:
  tcf = column flange thickness (mm)
  Fyc = column yield strength (MPa)
  k1  = distance from column web centreline to flange toe of fillet (mm)

Continuity plates are required when Mr_cf < Ff × (bf - k1)/2. In Canadian practice, most moment connections to W310 and smaller columns require continuity plates.

Continuity plate requirements (per CSA S16 Clause 23.5.3):

• Thickness = beam flange thickness or thicker
• Width = column flange width minus 2 × weld access holes
• Weld: CJP groove weld to column flange, fillet weld to column web
• Material: same grade as column or higher

Panel Zone Shear (CSA S16 Clause 23.4)

The panel zone is the region of the column web bounded by the beam flanges (or continuity plates) on the vertical sides and the column flanges on the horizontal sides. The panel zone must resist the shear force:

Vpz = (Mf1 / (d1 - tf1) + Mf2 / (d2 - tf2) - Vc) / db

where:
  Mf1, Mf2 = factored moments at top and bottom connections
  d1, d2   = beam depths (mm)
  Vc       = column shear above the joint (N)
  db       = column depth between flange centrelines (mm)

Panel zone shear resistance per CSA S16 Clause 23.4.2:

Vrz = φ × 0.60 × Fyc × dw × tw × (1 + (3 × bcf × tcf²) / (dw × db × tw))

where:
  dw  = column web depth (mm)
  tw  = column web thickness (mm)
  bcf = column flange width (mm)
  tcf = column flange thickness (mm)
  db  = distance between continuity plates (mm)

If Vrz < Vpz, a doubler plate is welded to the column web to increase the panel zone thickness. Doubler plates are common in Canadian seismic moment frames where the column web alone cannot resist the large joint shear forces.

Worked Example — Bolted Extended End Plate Moment Connection

Problem: Design a bolted extended end plate moment connection for a W460x74 beam (Grade 350W) to a W310x97 column (Grade 350W). Factored moment Mf = 300 kN·m, factored shear Vf = 150 kN. Connection to be pre-qualified for seismic moment frame (SMF) use.

Beam Properties

W460x74: d = 460 mm, bf = 191 mm, tf = 14.5 mm, tw = 9.0 mm, Zx = 1,460 × 10³ mm³ Column: W310x97: d = 310 mm, bf = 305 mm, tf = 15.4 mm, tw = 9.9 mm

Step 1 — Determine Bolt Tension Force

Lever arm = d - tf = 460 - 14.5 = 445.5 mm

Flange force Ff = Mf / (d - tf) = 300 × 10⁶ / 445.5 = 673,400 N = 673.4 kN

Step 2 — Select Bolt Configuration

Extended end plate with 8 × M24 A325M bolts:

• 4 bolts outside tension flange (2 rows × 2 bolts)
• 4 bolts inside tension flange (2 rows × 2 bolts)
• Bolt gauge: 120 mm (transverse spacing)

Bolt tension resistance per bolt (M24 A325M, Ab = 353 mm², Fu = 830 MPa):

Trb = 0.75 × 0.75 × 353 × 830 = 164,774 N = 164.8 kN

Step 3 — Distribute Flange Force to Bolt Rows

Using the neutral axis at compression flange centreline:

Row 1 (outer, 50 mm beyond flange): distance = 460 + 50 = 510 mm
Row 2 (inner, 30 mm inside flange): distance = 460 - 30 = 430 mm

Total moment of bolt group about compression flange centroid:
Σri² = 510² + 430² = 260,100 + 184,900 = 445,000 mm²

Force in Row 1 = Ff × r1² / Σri² × 2 bolts = 673.4 × 260,100 / 445,000 = 393.5 kN
Force per bolt in Row 1 = 393.5 / 2 = 196.8 kN

Trb = 164.8 kN < Tf = 196.8 kN → NOT OK

Row 1 bolts are overstressed. Options:

1. Increase bolts to M30 (Trb = 261.9 kN > 196.8 kN OK)
2. Add additional bolt row outside flange
3. Increase end plate thickness to reduce prying

Try M30 A325M bolts (Ab = 561 mm², Fu = 830 MPa):

Trb = 0.75 × 0.75 × 561 × 830 = 261,942 N = 261.9 kN > 196.8 kN OK

Step 4 — Check Prying Action

End plate dimensions: Width = 200 mm (across column flange), Thickness = 25 mm (350W)

Bolt gauge distance from web face: b = 60 mm Edge distance: a = 40 mm

Using CSA S16 Clause 23.6 T-stub model:

b/a = 60/40 = 1.5
δ = (b + a) / b = (60 + 40) / 60 = 1.67

Required end plate thickness:
t_min = √(4 × T × b / (φ × Fy))
t_min = √(4 × 196,800 × 60 / (0.90 × 350))
t_min = √(47,232,000 / 315)
t_min = √150,000 = 387 mm → governs

This indicates the 25 mm end plate is inadequate for the prying forces. Increase end plate to 35 mm:

t_provided = 35 mm > t_min = 387 mm? → NO

The issue is that b = 60 mm (bolt gauge) is too large. Reduce bolt gauge to 40 mm (b = 40 mm):

t_min = √(4 × 196,800 × 40 / (0.90 × 350))
t_min = √(31,488,000 / 315)
t_min = √100,000 = 316 mm → still governs

Since prying forces dominate, consider using tension-only bolts (no prying assumption by providing column stiffener between bolt lines) or transition to a welded flange connection.

Alternative — Welded Flange Connection

Use CJP groove welds for flanges and bolted shear tab for web:

Flange weld capacity (CJP, φw = 0.90):
Trw = 0.90 × 191 × 14.5 × 350 = 873,292 N = 873.3 kN > 673.4 kN OK

Shear tab: design for Vf = 150 kN with standard bolts (see connection design guide)

Step 5 — Continuity Plate Check

Check if column flange requires continuity plates:

Column flange bending resistance:
Mr_cf = 0.90 × 15.4² × 350 × (305 - 15)/4
Mr_cf = 0.90 × 237.2 × 350 × 290 / 4
Mr_cf = 5,417,025 N·mm = 5.42 kN·m

Required moment:
Ff × (bf - k1)/2 = 673.4 × (305 - 25) / 2 / 1000 = 94.3 kN·m

94.3 kN·m >> 5.42 kN·m → Continuity plates REQUIRED

Provide continuity plates: 15 mm thick (same as beam flange), 350W, CJP welded to column flanges, fillet to column web.

Step 6 — Panel Zone Shear Check

Vpz = 673.4 kN (from flange force, assuming equal moment above and below)

Panel zone shear resistance:
Vrz = 0.90 × 0.60 × 350 × 280 × 9.9 × (1 + (3 × 305 × 15.4²) / (280 × 280 × 9.9))
Vrz = 0.90 × 600 × 280 × 9.9 × (1 + (216,000) / (776,160))
Vrz = 0.90 × 600 × 280 × 9.9 × 1.28
Vrz = 1,915,750 N = 1,916 kN > 673.4 kN OK

Panel zone is adequate without doubler plate.

Step 7 — Summary

Seismic Moment Connections

For seismic moment frames (SMF, IMF per NBCC 2020 seismic provisions), CSA S16 Clause 27 imposes additional requirements:

• Protected zone: The region within one beam depth of the column face where no welds or bolt holes are permitted (except shear tab connections)
• Beam-to-column weld: Pre-qualified per CSA W59 for seismic
• Panel zone: Must yield before the protected zone reaches its ultimate capacity (capacity design)
• Continuity plates: Required for all SMF connections regardless of calculated adequacy

Pre-qualified seismic moment connections in Canadian practice include:

• Welded unreinforced flange (WUF-B per CISC Seismic Guide)
• Reduced beam section (RBS, radius cut)
• Bolted extended end plate (BEEP) per AISC 358 (adopted through CISC for Canadian practice)

Frequently Asked Questions

What is the difference between flush and extended end plate moment connections? A flush end plate (plate flush with beam flanges) provides limited rotational capacity and is suitable for moments up to approximately 50% of the beam plastic moment. An extended end plate projects beyond the beam tension flange, allowing additional bolt rows that can develop the full beam plastic moment. Extended end plates are required for seismic moment frames and high moment applications. CSA S16 Clause 23 classifies flush end plates as PR (partially restrained) unless specifically proportioned for full moment capacity.

When are continuity plates required in CSA S16 moment connections? Continuity plates are required when the column flange is not sufficiently stiff to transfer the beam flange force without excessive local deformation. Per CSA S16 Clause 23.5, they are required when the calculated column flange bending resistance is less than the beam flange force. In practice, continuity plates are required in virtually all moment connections to W310 or smaller columns, and in many connections to larger columns. For seismic moment frames, continuity plates are mandatory regardless of calculated adequacy.

What is panel zone shear and how is it checked? Panel zone shear (CSA S16 Clause 23.4) is the shear force resisted by the column web within the joint region defined by the beam flanges (or continuity plates) and the column flanges. If the calculated panel zone shear exceeds the factored resistance Vrz, a doubler plate must be welded to the column web. Panel zone yielding can be desirable in seismic design as an energy dissipation mechanism, but excessive shear deformation compromises the connection performance.

How is prying action handled in Canadian end plate design? Prying action (CSA S16 Clause 23.6) amplifies bolt tension through flexural deformation of the end plate. The CSA S16 T-stub analogy calculates the required plate thickness to limit prying effects. Prying action can be reduced by increasing end plate thickness, decreasing bolt gauge, providing stiffeners behind the tension flange, or using column continuity plates between bolt lines. In Canadian practice, prying often governs end plate design and leads to thicker plates than equivalent US designs.

Related Pages

• Canada CSA S16 Guide — Full CSA S16:19 steel design reference
• Canadian Steel Beam Sizes — W, WWF, HSS sections per CISC
• CSA S16 Connection Design — Bolted and welded connections
• CSA S16 Beam Design — Flexure, LTB, shear checks
• End Plate Connection Design — End plate reference
• Shear Tab Connection — Single plate shear connection guide
• Column Splice — Column moment splice design
• Bolted Connection Checklist — Quality control for connections

This page is for educational reference. CSA S16:19 moment connection design must comply with the current edition of CSA S16, CSA W59, and NBCC 2020. All results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification by a licensed Professional Engineer (P.Eng.).