Australian End Plate Connection — AS 4100 Clause 9 Design Guide

Complete reference for flush and extended end plate moment connections per AS 4100:2020 Clause 9. Covers the T-stub model for end plate bending, bolt prying action, weld design for end plates, stiffened vs unstiffened plates, and construction tolerances.

Related pages: AS 4100 Bolt Group Capacity | AS 4100 Weld Capacity | AS 4100 Moment Frames | Bolted Connection Calculator

End Plate Connection Types

End plate connections are the workhorse of Australian steel moment-resisting construction. The beam is shop-welded to a steel end plate, which is then site-bolted to the supporting member (column flange, another beam, or a base plate). This avoids field welding (weather-dependent, quality-variable) and provides excellent erection tolerance.

Design Procedure

Step 1 — Determine design moment: The connection must transfer the design moment M* at the column face. For capacity-designed connections (seismic), use M*_conn = phi_o x M*_brace.

Step 2 — Bolt group analysis: Compute the tension force in each bolt row from the applied moment. The bolt group rotates about the compression flange contact point (or the bottom flange for symmetric connections).

Lever arm: d_eff = distance from the compression flange centre (or bearing point) to each bolt row.

Tension per bolt row: T_i = M* x y_i / sum(y_i^2)

Where y_i is the distance of bolt row i from the instantaneous centre of rotation (typically the compression flange centre).

Step 3 — Check bolt tension capacity: phi_Ntf >= T_i for each bolt row. For the top bolt row (closest to the tension flange), prying action amplifies the bolt force.

Step 4 — End plate bending: The plate acts as a T-stub flange in bending under bolt tension. The plastic moment of the end plate per unit width determines the required thickness.

Step 5 — Weld design: Beam flange to end plate (full-pen butt or heavy fillet). Beam web to end plate (fillet). The weld must develop the full design force from the beam into the plate.

Bolt Tension Capacity — Clause 9.3.2.1

For Grade 8.8 bolts in tension (threads in tensile stress area):

phi_Ntf = phi x fuf x As

Where phi = 0.80 for bolts in tension.

For high-strength structural bolts (Grade 10.9): multiply by 1040/830 = 1.25.

Prying Action — Clause 9.3.4

Prying is the most commonly overlooked effect in end plate design. When the bolt is tensioned, the end plate bends, creating a lever that amplifies the bolt force by 10-30%. The mechanism is:

• Bolt applies tension T at a distance a from the support (beam flange or stiffener)
• Plate contact at the toe (distance b) produces a reaction
• This reaction produces prying force Q at the bolt, where Q = (b/a) x T for thin plates

Prying check:

1. Compute the plastic moment capacity of the T-stub flange per unit width: m_pl = phi x fy_plate x tp^2 / 4
2. Determine the load at which prying begins (P_0): beyond this, the prying force Q increases linearly with applied tension
3. Check bolt tension: T*_bolt + Q <= phi_Ntf

Mitigation: Use thicker end plates. A plate thickness tp >= d_bolt / 2 (e.g., 12-13 mm for M24 bolts) essentially eliminates prying for typical bolt gauges.

End Plate Bending — T-Stub Model

The end plate between the bolt line and the beam web/flange web acts as a T-stub flange in bending. Per AS 4100, the effective T-stub length is:

Circular pattern: l_eff_cp = 2 x pi x m (where m = bolt centre to web/fillet)

Non-circular pattern: l_eff_nc = 4m + 1.25n (where n = distance from bolt centre to plate edge)

Design effective length: l_eff = min(l_eff_cp, l_eff_nc, p) where p is the bolt pitch.

Required plate thickness: tp >= sqrt(4 x M_pl_Rd / (phi x fy x l_eff))

Where M_pl_Rd is the required plastic moment per unit width derived from the bolt tension.

Practical End Plate Thickness Guidelines

These are indicative values for Grade 300 plate, M* ~ 0.7 x phi_Ms_beam, Grade 8.8 bolts. Heavier plates (> 32 mm) become uneconomical due to material cost, welding preheat, and fabrication difficulty. At that point, stiffened end plates or alternative connection types should be considered.

Weld Design — Beam to End Plate

Beam flanges to end plate: Full-penetration butt weld is required to develop the flange force into the plate. The butt weld capacity equals the parent metal capacity (phi x fy x Af), so the weld is not the weak point. Alternatively, two fillet welds (each side of flange) totalling at least 1.0 x flange thickness in leg size can be used if the weld throat matches the flange thickness.

Beam web to end plate: The web carries the shear V* and a portion of the bending moment (typically 10-20% for deep beams, distributed by the web-to-flange shear flow). Two fillet welds (one each side of web) of 6-8 mm are standard.

Weld sequencing: Weld the flanges first (higher shrinkage force, restrained less), then the web. This minimises distortion of the end plate.

Worked Example — Extended End Plate for 460UB82

Problem: Design an extended end plate connection for a 460UB82.1 beam (Grade 300) to a 310UC158 column (Grade 300). Design moment M* = 380 kNm (static). Use M24 Grade 8.8/TB bolts.

Section properties — 460UB82.1: d = 460 mm, bf = 191 mm, tf = 16.0 mm, tw = 9.9 mm

Step 1 — Bolt layout: Extended end plate with 4 bolt rows: 2 rows above the top flange, 2 rows below the bottom flange (symmetrical). Bolt rows at y = 530, 420, 40, -70 mm from the compression flange centre (bottom flange). Bolt gauge = 120 mm (within column flange width of 311 mm).

Step 2 — Bolt forces: Rotational centre at bottom flange compression face (y = 0 at bottom flange centre). y_i values: 600, 490, 110, 0 (approximately).

sum(y_i^2) = 600^2 + 490^2 + 110^2 = 360,000 + 240,100 + 12,100 = 612,200 mm^2

Tension per bolt pair at top row: T = M* x y / sum(y^2) = 380 x 10^6 x 600 / 612,200 x 10^3 = 372 kN Per bolt (2 bolts per row): T_bolt = 372/2 = 186 kN.

Step 3 — Bolt tension check: phi_Ntf M24 Grade 8.8 = 0.80 x 830 x 353 / 1000 = 234 kN > 186 kN. OK without prying.

Check with prying (Clause 9.3.4): For tp = 25 mm, bolt centre to beam flange face a = 40 mm, bolt centre to plate edge b = 30 mm. Prying Q ~ 186 x (30/40) / 3 = 47 kN. Total bolt force = 186 + 47 = 233 kN < 234 kN. Just OK.

Step 4 — End plate thickness: Use T-stub model with l_eff determined by bolt geometry. For the top bolt row (extended portion above beam): m = 40 mm (bolt centre to stiffener/beam flange welding line). n = 30 mm (bolt to edge). l_eff_cp = 2 x pi x 40 = 251 mm per bolt.

Required tp = sqrt(4 x 186 x 10^3 x 40 / (0.90 x 300 x 251)) = sqrt(29,760,000 / 67,770) = sqrt(439) = 20.9 mm.

Specify 25 mm plate.

Step 5 — Weld: Beam flange to end plate: 8 mm full-pen butt weld (cjp) — capacity equals parent metal. Beam web to end plate: 6 mm fillet both sides — capacity = 2 x 460 x 1.00 = 920 kN >> V*.

Final specification: 460UB82.1 with 25 mm extended end plate, Grade 300. 8-M24 Grade 8.8/TB bolts. 8 mm full-pen flange welds, 6 mm fillet web welds. SP category, UT inspection of tension welds.

Construction Tolerances

End plate connections are sensitive to fit-up tolerance. Key considerations:

• End plate squareness to beam: +/- 1 mm across the plate width (AS/NZS 1554.1)
• Bolt hole clearance: Standard 2 mm oversize for M16-M24 bolts, 3 mm for M30-M36
• End plate flatness: 1 mm per 300 mm of plate dimension (after welding)
• Beam squareness (cut): +/- 1.5 mm across depth
• These tolerances ensure bolt alignment during erection. For SMF connections, specify template-drilled holes to maintain tight tolerances.

Frequently Asked Questions

When should I use an extended end plate vs a flush end plate? Use an extended end plate when the moment capacity exceeds what a flush plate can provide (approximately M* > 150 kNm for 310UB or M* > 250 kNm for 460UB). The extension above the top flange provides a larger lever arm for the tension bolts, increasing the moment capacity by 30-50% for the same bolt count and plate thickness. The extended portion is fabricated by welding a plate extension to the flush plate, or by cutting the end plate taller than the beam depth in one piece.

What is the difference between bearing-type and friction-type end plate connections? In bearing-type connections (Snug-tightened bolts, category 8.8/S), the bolts transfer force through bearing on the hole wall, with some slip before full engagement. In friction-type connections (fully tensioned HSFG bolts, category 8.8/TB or 8.8/TF), the bolts clamp the plates together and force is transferred by friction between the faying surfaces. Friction-type connections are required for: (a) connections subject to load reversal (seismic, crane), (b) connections where slip would be detrimental (precision alignment), and (c) connections with oversized or slotted holes. For standard building moment frames, bearing-type (snug-tight) is acceptable.

How are end plate stiffeners designed? Stiffeners (vertical plates welded between the end plate and beam flanges) are required when the end plate thickness exceeds approximately 32 mm (becoming uneconomical) or when the tension bolt row is far from the beam flange. The stiffener is designed as a compression member carrying the bolt tension component that the end plate cannot carry in bending. It must be welded to both the end plate and the beam flange/web with full-strength welds.

Can end plates be used in column splices? Yes, end plate column splices are common in multi-storey construction. The plates are shop-welded to each column section, then site-bolted together. The bolts must carry the column axial load plus any moment from the splice location. The plates are typically 25-40 mm thick, and the bolts are pre-tensioned to prevent separation under load reversal. A cap plate may be required at the column end to provide adequate bearing area for shim packs used to level the splice during erection.

This page is for educational reference. End plate connection design per AS 4100:2020 Clause 9. Verify bolt capacities and prying analysis per current ASI Design Guides. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.