Australian CHS/SHS Connection — Welded Tubular Joint Design Guide

Complete reference for AS 4100:2020 Clause 9.5 tubular welded connection design. Covers welded CHS (circular hollow section), SHS (square hollow section), and RHS (rectangular hollow section) truss joint design including chord plastification (face yielding), punching shear failure, branch effective width, chord side wall crushing, and chord shear. Includes joint capacity formulas for K-, T-, Y-, and X-joint configurations commonly used in Australian steel trusses.

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AS 4100 Tubular Connection Design — Overview

Hollow section connections are governed by AS 4100 Clause 9.5 which provides design capacities for welded connections between circular (CHS) and rectangular (RHS/SHS) hollow sections. Hollow sections in Australia are manufactured to AS/NZS 1163, with grades C350L0 and C450L0 being the most common structural grades.

Tubular connection design differs fundamentally from open section connection design because:

1. Chord wall flexibility — hollow section chords have thin walls that can undergo significant local deformation under branch loading, reducing joint capacity
2. No access for bolting — field connections must be welded, making CHS/SHS connections predominantly fabricated in the workshop
3. Complex stress interaction — chord stresses from global bending and axial forces interact with local branch forces, reducing joint capacity (the chord stress interaction factor)
4. Multiple failure modes — each joint configuration (T, Y, K, X) has its own set of potential failure modes

AS 4100 Clause 9.5 follows the internationally recognised CIDECT (Comite International pour le Developpement et l'Etude de la Construction Tubulaire) design methodology, which is also the basis for EN 1993-1-8 and ISO 14346. Australian practice uses the CIDECT Design Guides (No. 1 for CHS, No. 3 for RHS) supplemented by AS 4100 capacity factors.

AS/NZS 1163 Hollow Section Grades

C450L0 is the dominant grade for structural CHS/SHS/RHS in contemporary Australian construction, reflecting the shift toward higher-strength steel for weight and cost optimisation.

Joint Classification and Geometry Parameters

Joint Types

Geometry Parameters

The capacity of hollow section joints depends on dimensionless geometric parameters:

For RHS/SHS:

• Beta = b1/b0 or d1/b0 — branch width to chord width ratio
• 2-gamma = b0/t0 — chord width to thickness ratio
• Tau = t1/t0 — branch wall thickness to chord wall thickness ratio
• Eta = h1/b0 — branch depth to chord width ratio

For CHS:

• Beta = d1/d0 — branch diameter to chord diameter ratio
• 2-gamma = d0/t0 — chord diameter to thickness ratio
• Tau = t1/t0 — branch wall thickness ratio

RHS/SHS Joint Capacity Formulas (AS 4100 Clause 9.5.3)

AS 4100 provides design capacity formulas for RHS/SHS joints based on the CIDECT methodology. The available joint capacity is the minimum of all applicable failure modes.

T- and Y-Joint (RHS/SHS) — Chord Face Plastification

phi-N1 = phi × fy0 × t0² × (2 × eta / (1 - beta) + 4 / √(1 - beta)) × f(n) / sin(theta1)

where:
  phi      = 0.90 (capacity factor per Table 3.4)
  fy0      = yield strength of chord member (MPa)
  t0       = chord wall thickness (mm)
  eta      = h1 / b0 (branch depth / chord width)
  beta     = b1 / b0 (branch width / chord width)
  theta1   = branch angle from chord (90° for T-joint)
  f(n)     = chord stress interaction factor

The chord stress interaction factor f(n) accounts for the reduction in joint capacity when the chord is under compression from global loads:

f(n) = 1.0                         when n ≤ 0 (chord in tension)
f(n) = 1.0 + 0.3 × n - 0.3 × n²   when n < 0 (chord in compression)

where n = N0* / Ns0 (chord utilisation ratio, negative for compression)

For a chord at 60% compression utilisation (n = -0.6): f(n) = 1.0 + 0.3 × (-0.6) - 0.3 × (0.36) = 1.0 - 0.18 - 0.108 = 0.712

This represents a 29% reduction in joint capacity due to chord compression — a significant effect that must not be ignored in truss design.

T- and Y-Joint — Punching Shear

Punching shear failure occurs when the branch wall punches through the chord wall:

phi-N1 = phi × fy0 × t0 / √3 × (2 × h1 / sin(theta1) + 2 × b1 - 4 × t1)

The punching shear check applies when beta ≤ (1 - 1/gamma). For typical RHS joints with beta between 0.4 and 0.8, punching shear is rarely the governing limit state — chord face plastification or side wall crushing usually governs.

T- and Y-Joint — Chord Side Wall Crushing

For joints with beta close to 1.0 (branch nearly as wide as chord), the limit state transitions from chord face plastification to chord side wall crushing:

phi-N1 = phi × fy0 × t0 × (2 × h1 / sin(theta1) + 10 × t0) × f(n)

Side wall crushing governs when beta > 0.85. This is common in Vierendeel trusses where the branch is nearly full chord width.

CHS Joint Capacity Formulas (AS 4100 Clause 9.5.2)

T- and Y-Joint (CHS) — Chord Face Plastification

phi-N1 = phi × fy0 × t0² × (2.6 + 6.8 × beta²) × f(n') / sin(theta1)

where:
  phi      = 0.90
  beta     = d1 / d0
  f(n')    = chord stress interaction factor for CHS
           = 1.0 + 0.3 × n' - 0.3 × n'² for compression

For a typical CHS T-joint with beta = 0.5, chord C450L0 (fy0 = 450 MPa), chord 168.3 × 6.4 CHS (d0 = 168.3 mm, t0 = 6.4 mm), branch 88.9 × 5.0 CHS, chord in tension (f(n') = 1.0), theta = 90°:

phi-N1 = 0.90 × 450 × 6.4² × (2.6 + 6.8 × 0.528²) × 1.0 / sin(90°) = 0.90 × 450 × 40.96 × (2.6 + 6.8 × 0.279) × 1.0 = 0.90 × 450 × 40.96 × (2.6 + 1.90) = 0.90 × 450 × 40.96 × 4.50 = 0.90 × 450 × 184.3 = 0.90 × 82,935 = 74,642 N = 74.6 kN

CHS K-Joint — Chord Face Plastification

For K-joints with gap, the capacity is typically higher than T-joints because the two branch forces are approximately in equilibrium at the joint:

phi-N1 = phi × fy0 × t0² × (1.8 + 7.2 × beta²) × f(n') / sin(theta1) × f(g')

Where f(g') is a gap function that increases capacity as the gap between branches decreases. For standard K-joints with the recommended gap of t1 + t2 ≤ g ≤ 0.5 × b0, f(g') ≈ 1.0. The K-joint enhancement over T-joints is approximately 20-40%.

K-Joint Gap and Overlap Requirements (AS 4100 Clause 9.5)

Gapped K-Joints

For gapped K-joints, the gap g between the two branch members must satisfy:

t1 + t2 ≤ g ≤ 0.5 × b0 (for RHS)

The minimum gap ensures weld access between branches. The maximum gap ensures the gap function does not significantly reduce capacity. For RHS K-joints at the recommended gap, the efficiency is typically 70-90% of the branch member capacity.

Overlapped K-Joints

When the branches overlap (the gap becomes negative), the joint behaviour changes. For overlapped K-joints with overlap Ov = p / (t1 + t2) × 100%, the connected branch (through-branch) transfers force directly to the chord, while the overlapping branch transfers force through the connected branch:

• Small overlap (Ov < 25%): behaves like a gapped joint with reduced capacity
• Medium overlap (25% < Ov < 50%): partial load transfer through the connected branch
• Large overlap (Ov > 50%): full load transfer, approaching the efficiency of a gapped joint

Australian truss practice prefers gapped K-joints over overlapped joints because:

1. Gapped joints are easier to fabricate and inspect
2. The gap provides clear weld access for quality control
3. Gap size is directly measurable during inspection
4. Overlapped joints require precise branch end profiling

Failure Modes in Tubular Connections

The following failure modes must be checked for every tubular connection design:

Weld Design for Tubular Connections

AS 4100 Clause 9.7.5 specifies that welds in tubular connections must be designed as fillet welds with the throat thickness measured along the branch profile:

tt-req = N1* / (phi × 0.60 × fuw × Lw)

where:
  Lw = effective weld length along the branch perimeter
  N1* = design branch force

For RHS branches, the weld length is:

Lw = 2 × h1 / sin(theta1) + 2 × b1 (but not more than 2 × h1 / sin(theta1) + b1 × beta)

The effective weld length reduces when branch walls are thin (tau < 0.5), accounting for non-uniform load distribution around the branch perimeter. AS 4100 Clause 9.7.5.2 limits the weld throat to a minimum of the branch wall thickness for full strength design.

For typical CHS/SHS trusses in Australia (C450L0 hollow sections), E48XX electrodes are standard. Fillet weld leg lengths of 6-8 mm are typical for branch-to-chord welds in trusses using 89-114 mm CHS branches.

Practical Design Considerations

Chord Stress Ratio

The chord stress interaction factor f(n) is the most important secondary parameter affecting joint capacity. For trusses where chords are heavily utilised (> 60% in compression), joint capacity can be reduced by 25-40% compared to an unstressed chord. Design strategies to mitigate this include:

• Specifying thicker chord walls (reducing 2-gamma)
• Increasing chord section size (reducing n)
• Using C450L0 chord steel when C350 would suffice for member strength
• Checking joint capacity at the governing load combination (chord stress and joint load may not be at the same combination)

Recommended Geometric Limits

AS 4100 Clause 9.5.1 provides recommended geometric limits for joint validity:

Design Workflow

1. Determine joint geometry from truss layout (branch angles, dimensions, chord size)
2. Extract branch design forces from structural analysis (factored loads per AS 1170)
3. Select hollow section sizes (AS/NZS 1163 C350L0 or C450L0)
4. Calculate geometric parameters (beta, 2-gamma, tau, eta)
5. Check geometric limits (per AS 4100 Clause 9.5.1)
6. Calculate chord face plastification capacity for the governing joint type
7. Check all other applicable failure modes (punching shear, side wall, chord shear)
8. Apply chord stress interaction factor f(n) for the governing load case
9. Design weld between branch and chord
10. Verify all checks at each joint in the truss

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