When are continuity plates required in AS 4100 moment connections?

Continuity plates are required per AS 4100 Clause 9.5 when the calculated column flange bending capacity is insufficient to resist the beam flange force. If Tf exceeds phi x fyc x bfc-squared x tfc divided by (3.5 x s), continuity plates matching the beam flange thickness are required. Alternatively, doubler plates may be used on the column web.

What is panel zone shear and how is it checked in AS 4100?

Panel zone shear is the shear force resisted by the column web within the joint region. AS 4100 Clause 5.10 checks panel zone shear capacity as phi-Vv = phi x 0.60 x fy x Aw. Unlike AISC practice, AS 4100 does not permit panel zone yielding as a ductility mechanism in seismic design — the panel zone must remain elastic under ULS loads.

How is prying action handled in Australian end plate design?

Prying action per AS 4100 Clause 9.3.3.1 amplifies bolt tension through flexural deformation of the end plate. The AS 4100 T-stub model calculates the prying ratio Q based on plate geometry and bolt position. Prying can be reduced by increasing end plate thickness, reducing bolt gauge, or adding stiffeners between bolt rows. Australian practice uses 20-25 mm end plates to limit prying to under 20 percent.

AS 4100 Moment Connection — End Plate & Flange Connection Design

Q: What is the difference between flush and extended end plate moment connections in AS 4100?

A flush end plate in Australian practice provides limited rotational capacity suitable for moments up to approximately 50 percent 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 the dominant moment connection type in Australian steel construction for buildings over 4 storeys.

Complete reference for AS 4100:2020 moment-resisting steel connections including bolted end plate (flush and extended), flange welded connections with CJP groove welds, stiffener requirements (Clause 9.5), prying action (Clause 9.3.3.1), and bolt group design for moment transfer. Includes a worked example for a bolted extended end-plate moment connection to a 310UC137 column.

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AS 4100 Moment Connections — Overview

Moment connections transfer bending moment and associated shear between connected members. In Australian practice, moment connections are classified by their rotational stiffness and strength following AS 4100 Clause 9.1.3. The National Construction Code (NCC 2022) references AS 4100 as the primary standard for steel design, and moment connections must satisfy the design requirements of Clause 9 for connections and Clause 5 for member design.

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

Connection Type	Stiffness	AS 4100 Classification	Typical Use
Bolted end plate (flush)	Semi-rigid	PR or rigid	Low-to-medium moment frames
Bolted end plate (extended)	Rigid	Full-restraint	Medium-to-high moment frames, seismic
Welded flange (CJP)	Rigid	Full-restraint	Heavy moment frames, industrial, bridges
Flange plate (bolted)	Semi-rigid	PR or rigid	Field-bolted moment splices
Welded web + bolted flange	Semi-rigid	PR	Composite construction, simple connections
Fin plate with stiffeners	Semi-rigid	PR	Light moment frames, low-rise

AS 4100 Clause 9.1.3.1 requires that connections be classified as either full-restraint (FR) or partial-restraint (PR). For FR connections, the connection must have adequate strength and stiffness to develop the full plastic moment of the connected member. PR connections must account for the rotational stiffness in the structural analysis.

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 at the tension flange
Bolt rows: typically 2 rows at tension flange, 2 at compression flange
End plate thickness: 12 mm to 25 mm typical in Australian practice
Limitation: does not develop full beam plastic moment in heavier sections (above 410UB series)
Fit for purpose: suitable for simple moment frames and low-to-medium rise buildings

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 + 2 rows between flanges
End plate thickness: 16 mm to 35 mm typical
Application: moment-resisting frames in high seismic zones (per AS 1170.4)
Detailing: requires stiffeners between bolt rows for thick plates (> 25 mm)

Australian practice follows the AISC End Plate Design methodology adapted for AS 4100 capacity factors and Grade 300 steel. Extended end plates are the dominant moment connection type in Australian steel construction for buildings over 4 storeys.

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 Australian practice):

Tension flange force: Tf = Mf* / (d - tf)
Bolt tension per row at tension flange: Nt = Tf / n

where:
  Mf* = design bending moment (N·mm)
  d   = beam depth (mm)
  tf  = beam flange thickness (mm)
  n   = number of bolt rows at tension flange

The neutral axis is taken at the compression flange centreline, which is conservative because it ignores any compression contribution from the web below the neutral axis. The actual neutral axis is approximately at the mid-depth of the beam, which would reduce the tension flange force by about 15-25% depending on the beam depth.

Bolt Tension Design Check (AS 4100 Clause 9.3.3)

Each bolt row is checked against:

Ntf* ≤ phi × 0.80 × fuf × Ac

The bolt force Ntf* must include prying action effects (see below). For extended end plates with bolts outside the tension flange, the outside bolt row carries the largest tension force because of the longer lever arm to the compression flange.

Prying Action in End Plate Design (AS 4100 Clause 9.3.3.1)

Prying action significantly affects bolt tension in end plate connections. AS 4100 uses a T-stub model where the end plate at the tension flange is treated as an inverted T-section loaded by bolts on both sides.

The prying ratio Q is calculated from:

Q = (b/a) × √((Nt-a²)/(Nt-b²) - 1)

where:
  a   = distance from bolt centreline to plate edge
  b   = distance from bolt centreline to flange face
  Nt = applied tension per bolt

When Q > 0 and the plate is sufficiently thick, prying action amplifies the bolt force. When Q is large (> 0.3), the designer should either:

Increase the end plate thickness (reduces deformation and prying)
Reduce the bolt gauge g (brings bolts closer to the flange face)
Add stiffeners between the bolt rows (effective for thick plates > 25 mm)
Increase bolt diameter to provide additional tension capacity

For Australian practice, end plates of 20-25 mm thickness typically control prying to less than 20% amplification for Grade 8.8 M20 bolts. Thinner plates (16 mm) can experience prying amplification of 30-50%, significantly reducing the available moment capacity.

Stiffener Requirements (AS 4100 Clause 9.5)

AS 4100 Clause 9.5 requires stiffeners or continuity plates when the calculated column flange bending capacity is insufficient to resist the beam flange force:

Continuity Plates (Column Flange Stiffeners)

Required when:

Tf > phi × fyc × bfc² × tfc / (3.5 × s)

where:
  Tf  = beam flange force from moment (N)
  fyc = yield strength of column (MPa)
  bfc = column flange width (mm)
  tfc = column flange thickness (mm)
  s   = stiffener spacing = beam flange thickness + 2 × fillet (mm)

If Tf exceeds the column flange bending capacity, continuity plates of minimum thickness matching the beam flange thickness are required. Alternatively, doubler plates may be used on the column web to increase the resistance.

Panel Zone Shear (Column Web at Joint)

The column web panel zone in the joint region must be checked for shear:

Vpz* = Tf - Vcol

where:
  Vcol = column shear at the joint (from frame analysis)

Panel zone shear resistance per AS 4100 Clause 5.10:

phi-Vv = phi × 0.60 × fy × Aw

where:
  phi  = 0.90 (member capacity factor)
  fy   = yield stress of column web (MPa)
  Aw   = column web area = dc × twc (mm²)
  dc   = column web depth (mm)
  twc  = column web thickness (mm)

When Vpz* exceeds phi-Vv, a doubler plate must be welded to the web. In contrast to AISC practice, AS 4100 does not permit panel zone yielding as a ductility mechanism in seismic design — the panel zone must remain elastic under ULS loads.

Welded Flange Moment Connections (AS 4100 Clause 9.7.2)

For heavy moment connections, the beam flanges are connected to the column flange using CJP (complete joint penetration) groove welds:

Flange welds: CJP groove welds with backing bars, designed to develop the full flange strength per AS 4100 Clause 9.7.2.1
Web weld: Fillet or PJP weld designed for shear transfer only
Backing bars: May be left in place or removed for critical fatigue applications

CJP Groove Weld Design

CJP groove welds are designed as equal to the base metal strength:

phi-Nw = phi × fyf × bf × tf

where:
  phi  = 0.90 (for groove welds)
  fyf  = yield strength of beam flange (MPa)
  bf   = beam flange width (mm)
  tf   = beam flange thickness (mm)

This is identical in principle to the AISC "matched strength" requirement — a CJP weld with matching electrode (e.g., E48XX for Grade 300 steel) is considered to develop the full flange capacity without further calculation.

Worked Example — Bolted End Plate Moment Connection

Problem: Design a bolted extended end plate moment connection between a 530UB92.4 beam and a 310UC137 column. Beam moment demand Mf* = 500 kN·m at the column face.

Design Parameters:

Beam: 530UB92.4 (d = 528 mm, bf = 209 mm, tf = 15.6 mm, tw = 10.2 mm, fy = 300 MPa)
Column: 310UC137 (d = 321 mm, bf = 309 mm, tf = 21.2 mm, tw = 13.8 mm, fy = 300 MPa)
End plate: Grade 300 steel, 25 mm thick
Bolts: M24 Grade 8.8 (fuf = 830 MPa, Ac = 353 mm²)
Electrode: E48XX (fuw = 480 MPa)

Step 1 — Flange Force: Tf = Mf* / (d - tf) = 500 × 10⁶ / (528 - 15.6) = 500 × 10⁶ / 512.4 = 975,800 N = 976 kN

Step 2 — Bolt Configuration: Extended end plate with 4 rows of M24 bolts:

2 rows above tension flange (outside rows)
2 rows between beam flanges (inside rows)
Bolt gauge: 120 mm (horizontal spacing)
Vertical pitch: 70 mm

Assume outside row carries 55% of flange force, inside row carries 45%: Outside bolt tension: Nt = 0.55 × 976 / 4 = 134.2 kN per bolt (each side, 2 bolts per row)

Check with prying (assume Q = 1.15 for 25 mm plate): Ntf* = 1.15 × 134.2 = 154.3 kN per bolt

Step 3 — Bolt Tension Capacity: phi-Ntf = 0.80 × 0.80 × 830 × 353 / 1000 = 187.5 kN per bolt 154.3 kN ≤ 187.5 kN → OK (82% utilisation)

Step 4 — Check Continuity Plates: Beam flange force Tf = 976 kN Column flange bending capacity: phi × fyc × bfc² × tfc / (3.5 × s) = 0.90 × 300 × 309² × 21.2 / (3.5 × (15.6 + 2 × 15)) = 0.90 × 300 × 95,481 × 21.2 / (3.5 × 45.6) = 0.90 × 300 × 95,481 × 21.2 / 159.6 = 0.90 × 300 × 12,681 = 3,423,870 N = 3,424 kN

976 kN ≤ 3,424 kN → Continuity plates not required by strength. However, provided as standard practice for deep beams (530UB).

Step 5 — Panel Zone Shear Check: Column shear (from frame analysis): Vcol ≈ 150 kN Vpz* = Tf - Vcol = 976 - 150 = 826 kN

Web area: Aw = 321 × 13.8 = 4,430 mm² Panel zone shear capacity: phi-Vv = 0.90 × 0.60 × 300 × 4,430 / 1000 = 0.90 × 0.60 × 300 × 4,430 / 1000 = 717 kN

826 kN > 717 kN → Doubler plate required. Required doubler: Add 12 mm plate matching column Grade 300. Total web area with doubler = 13.8 + 12.0 = 25.8 mm → 321 × 25.8 = 8,282 mm² phi-Vv = 0.90 × 0.60 × 300 × 8,282 / 1000 = 1,341 kN > 826 kN → OK

Step 6 — Weld Design: Flange-to-end plate weld: Full-length CJP groove weld with E48XX electrode. Web-to-end plate weld: 8 mm fillet weld, both sides. Shear per web weld: Vf* = 200 kN (from frame analysis) Design capacity per mm: phi × 0.60 × fuw × tt = 0.80 × 0.60 × 480 × 0.707 × 8 / 1000 = 1.30 kN/mm Total web weld length: 2 × (528 - 2 × 15.6) = 2 × 496.8 = 994 mm Capacity: 1.30 × 994 = 1,292 kN > 200 kN → OK

Result: Connection design is acceptable. Extended end plate 25 mm thick, 4 rows × 2 M24 Grade 8.8 bolts, 12 mm doubler plate on column web. Flange CJP weld, 8 mm web fillet welds.

Educational reference only. Verify against AS 4100 and relevant standards. Results are PRELIMINARY — NOT FOR CONSTRUCTION.