Australian HSS/CHS Connections — AS 4100 Design Guide

Complete reference for welded hollow structural section (HSS) connection design per AS 4100:2020 Clause 9.5. Covers circular (CHS), square (SHS), and rectangular (RHS) hollow section truss joints designed using the CIDECT-based capacity formulation. Includes chord face plastification, punching shear, chord side wall crushing, chord shear, and branch effective width checks for T-, Y-, X-, and K-joint configurations used in Australian steel fabrication.

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Hollow Section Grades and Standards

Hollow sections in Australian construction are manufactured to AS/NZS 1163:2016 (Cold-formed structural steel hollow sections). The standard covers electric-resistance welded (ERW) CHS, SHS, and RHS in the following grades:

C450L0 is the dominant structural grade in contemporary Australian fabrication. The "L0" designation indicates the minimum Charpy V-notch impact energy requirement of 27 J at 0 degrees C (AS/NZS 1163 Table 2.3). The "C" prefix denotes cold-formed (as opposed to hot-finished "H" sections — though hot-finished HF sections are rare in Australian practice).

Typical Hollow Section Sizes

Joint Types and Geometric Parameters — AS 4100 Clause 9.5

AS 4100 Clause 9.5 classifies welded HSS connections by joint configuration. Each configuration has distinct capacity functions governed by dimensionless geometric parameters.

Joint Classification

Geometric Parameters for Joint Capacity

For RHS/SHS joints, capacity depends on:

• Beta = b1 / b0 — branch width to chord width ratio (range 0.25 — 1.0)
• 2-gamma = b0 / t0 — chord width to thickness ratio (range 10 — 35)
• Tau = t1 / t0 — branch wall to chord wall thickness ratio (range 0.5 — 1.0)
• Eta = h1 / b0 — branch depth to chord width ratio (range 0.5 — 2.0)

For CHS joints:

• Beta = d1 / d0 — branch diameter to chord diameter ratio (range 0.2 — 1.0)
• 2-gamma = d0 / t0 — chord diameter to thickness ratio (range 10 — 50)
• Tau = t1 / t0 — branch wall thickness ratio (range 0.2 — 1.0)

Validity Limits (AS 4100 Clause 9.5.1)

CHS Joint Capacity Functions (AS 4100 Clause 9.5.2)

CHS T- and Y-Joint — Chord Face Plastification

phi-N1 = phi x fy0 x t0^2 x (2.6 + 6.8 x beta^2) x f(n') / sin(theta1)

Where:

• phi = 0.90 (capacity factor for parent metal, Table 3.4)
• fy0 = chord yield strength (MPa)
• t0 = chord wall thickness (mm)
• beta = d1 / d0
• theta1 = angle between branch and chord (90° for T-joint)
• f(n') = chord stress interaction factor:
  • f(n') = 1.0 for chord in tension (n' ≥ 0)
  • f(n') = 1.0 + 0.3 x n' - 0.3 x n'^2 for chord in compression (n' < 0)
  • n' = N0* / Ns0 (chord axial utilisation, negative for compression)

CHS K-Joint (Gap) — Chord Face Plastification

phi-N1 = phi x fy0 x t0^2 x (1.8 + 7.2 x beta^2) x f(n') / sin(theta1) x f(g')

The K-joint capacity formula includes a gap function f(g') that approaches 1.0 for standard gap dimensions. For K-joints with the recommended gap (t1 + t2 ≤ g ≤ d0/2), the joint efficiency is typically 70-90% of the branch member capacity.

CHS Punching Shear Check

phi-N1 = phi x fy0 x t0 / sqrt(3) x pi x d1 x (1 + sin(theta1)) / (2 x sin^2(theta1))

This check verifies that the branch force does not cause the chord wall to fail in shear around the branch perimeter. Punching shear governs when beta > 0.6 or chord wall thickness t0 is less than approximately 0.15 x d1.

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

RHS T- and Y-Joint — Chord Face Plastification

phi-N1 = phi x fy0 x t0^2 x (2 x eta / (1 - beta) + 4 / sqrt(1 - beta)) x f(n) / sin(theta1)

Where:

• eta = h1 / b0
• beta = b1 / b0
• f(n) = as defined for CHS above

RHS T- and Y-Joint — Punching Shear

phi-N1 = phi x fy0 x t0 / sqrt(3) x (2 x h1 / sin(theta1) + 2 x b1 — 4 x t1)

RHS T- and Y-Joint — Chord Side Wall Crushing (beta > 0.85)

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

Side wall crushing replaces chord face plastification when the branch is nearly as wide as the chord (beta > 0.85), because the load transfers directly into the chord side walls rather than through the chord face.

RHS K-Joint (Gap) — Chord Face Plastification

phi-N1 = phi x fy0 x t0^2 x (1.8 + 7.2 x beta^2) x f(n) / sin(theta1) x f(g')

The gap function f(g') = 1.0 at the recommended gap of 0.25 x b0, reducing by approximately 3% per 0.05 x b0 increase in gap beyond that.

Worked Example 1: CHS T-Joint

Problem: A CHS T-joint in a Warren truss has a C450L0 chord 168.3 x 6.4 CHS (d0 = 168.3 mm, t0 = 6.4 mm, fy0 = 450 MPa) and a branch 88.9 x 5.0 CHS (d1 = 88.9 mm, t1 = 5.0 mm). The chord is in tension (N0* = +250 kN, Ns0 = 1,500 kN, n' = +0.167). Determine the design capacity of the joint for chord face plastification.

Solution:

1. Beta = d1 / d0 = 88.9 / 168.3 = 0.528
2. Chord in tension: f(n') = 1.0
3. T-joint, theta1 = 90°, sin(90°) = 1.0

phi-N1 = 0.90 x 450 x 6.4^2 x (2.6 + 6.8 x 0.528^2) x 1.0 / 1.0 = 0.90 x 450 x 40.96 x (2.6 + 6.8 x 0.279) = 0.90 x 450 x 40.96 x (2.6 + 1.90) = 0.90 x 450 x 40.96 x 4.50 = 0.90 x 450 x 184.3 = 0.90 x 82,944 = 74,650 N = 74.6 kN

Check punching shear:

phi-N1 = 0.90 x 450 x 6.4 / 1.732 x pi x 88.9 x (1 + sin(90°)) / (2 x sin^2(90°)) = 0.90 x 450 x 3.695 x 3.142 x 88.9 x 2.0 / 2.0 = 0.90 x 450 x 3.695 x 279.3 = 0.90 x 464,216 = 417,794 N = 417.8 kN

Result: Chord face plastification governs at 74.6 kN. Punching shear (417.8 kN) does not control. The joint capacity is 74.6 kN. If the chord were in compression at 60% utilisation (n' = -0.6), f(n') = 0.712, reducing capacity to 74.6 x 0.712 = 53.1 kN — a 29% reduction.

Worked Example 2: SHS K-Joint

Problem: An SHS K-joint in a roof truss uses a C450L0 chord 150 x 150 x 6.0 SHS (b0 = 150 mm, t0 = 6.0 mm, fy0 = 450 MPa) with two C450L0 branches 100 x 100 x 5.0 SHS (b1 = 100 mm, h1 = 100 mm, t1 = 5.0 mm). The chord is in compression at 40% utilisation (n = -0.4). Branch angle theta = 45°. Determine joint capacity for chord face plastification.

Solution:

1. Beta = b1 / b0 = 100 / 150 = 0.667
2. Eta = h1 / b0 = 100 / 150 = 0.667
3. Chord in compression, n = -0.4: f(n) = 1.0 + 0.3 x (-0.4) - 0.3 x (0.16) = 1.0 - 0.12 - 0.048 = 0.832
4. Gap factor: assume standard gap g = 0.25 x b0 = 37.5 mm, f(g') ≈ 1.0

phi-N1 = 0.90 x 450 x 6.0^2 x (1.8 + 7.2 x 0.667^2) x 0.832 / sin(45°) x 1.0 = 0.90 x 450 x 36.0 x (1.8 + 7.2 x 0.445) x 0.832 / 0.707 = 0.90 x 450 x 36.0 x (1.8 + 3.20) x 0.832 / 0.707 = 0.90 x 450 x 36.0 x 5.0 x 0.832 / 0.707 = 0.90 x 450 x 36.0 x 5.885 = 0.90 x 450 x 211.9 = 0.90 x 95,355 = 85,820 N = 85.8 kN

Check validity limits:

• b0/t0 = 150/6.0 = 25 ≤ 35 ✓
• b1/t1 = 100/5.0 = 20 ≤ 35 ✓
• Beta = 0.667 ≥ 0.25 ✓
• Tau = 5.0/6.0 = 0.83 (within 0.5 — 1.0) ✓

Result: Joint capacity is 85.8 kN per branch. If chord were at 60% compression (n = -0.6), f(n) = 0.712 and capacity reduces to 85.8 x 0.712/0.832 = 73.4 kN.

Weld Design for HSS Connections

For welded HSS connections per AS 4100 Clause 9.7.5, the fillet weld must be designed to develop the branch member capacity. The required weld throat thickness is:

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

Where:

• phi = 0.80 (capacity factor for welds)
• fuw = weld electrode tensile strength (e.g., 490 MPa for E48XX)
• Lw = effective weld length around branch perimeter

For RHS branches, the effective weld length is:

Lw = 2 x h1 / sin(theta1) + 2 x b1 — with reductions for thin branches (tau < 0.5)

For CHS branches: Lw = pi x d1 / sin(theta1)

AS 4100 Clause 9.7.5.2 requires the weld throat to be at least the branch wall thickness for full-strength connections. In Australian truss fabrication, fillet welds of 6-8 mm leg length (4-6 mm throat) using E48XX electrodes are typical for 89-114 mm CHS or 100 mm SHS branches.

Design Resources

• Australian Steel Design Guide — AS 4100 overview
• Australian Steel Grades — C350L0 and C450L0 properties
• Australian Steel Properties — Section property tables
• Australian Weld Sizes — AS 1554 weld requirements
• AS 4100 Connection Design — Bolted and welded connections
• AS 4100 Beam Design — Beam section design
• Australian Bolt Capacity — Bolt shear and tension values
• All Australian References

Frequently Asked Questions

How does AS 4100 Clause 9.5 address HSS connection design? AS 4100 Clause 9.5 provides nominal capacity formulas for welded hollow section connections between CHS (Clause 9.5.2) and RHS/SHS members (Clause 9.5.3). The standard covers T-, Y-, X-, and K-joint configurations including both gap and overlap joints. Capacity functions address five failure modes: chord face plastification, punching shear, chord side wall crushing (beta > 0.85), chord shear (K-joints), and branch effective width (tau < 0.5). The capacity formulation follows the CIDECT methodology, consistent with EN 1993-1-8 and ISO 14346.

What are the validity limits for HSS joint design under AS 4100? AS 4100 Clause 9.5.1 specifies geometric validity limits: chord wall slenderness b0/t0 ≤ 35 (RHS) or d0/t0 ≤ 50 (CHS); branch slenderness b1/t1 ≤ 35 or d1/t1 ≤ 50; width ratio beta ≥ 0.25 (RHS) or 0.2 (CHS); depth ratio eta ≥ 0.5 (RHS); thickness ratio tau between 0.5-1.0 (RHS) or 0.2-1.0 (CHS). Designs falling outside these limits require specialist assessment or physical testing per AS 4100 Appendix B.

What is the chord stress interaction factor and why is it important? The chord stress interaction factor f(n) accounts for the reduction in joint capacity when the chord is under compression from global truss loads. For a chord at 60% compression utilisation (n = -0.6), f(n) = 0.712 — a 29% reduction in joint capacity. This effect is often overlooked in preliminary truss design, leading to underestimated joint capacities. The factor is 1.0 when the chord is in tension. Design strategies to mitigate the chord stress effect include specifying thicker chord walls, increasing chord size to reduce utilisation, and using C450L0 chord grades.

What are the typical HSS connection failure modes and how are they checked? The five failure modes for HSS connections are: (1) Chord face plastification — the chord face yields under branch loading, typically governing for beta < 0.85; (2) Punching shear — the branch punches through the chord wall, checked for beta ≤ (1 — 1/gamma); (3) Chord side wall crushing — governs for beta > 0.85 when the branch is nearly full chord width; (4) Chord shear — checked for K-joints where the chord force between branches causes shear failure; (5) Branch effective width — governs when the branch wall is thin (tau < 0.5), reducing the effective load transfer width. The joint design capacity is the minimum of all applicable failure modes.

How should welds be designed for HSS connections per AS 4100? AS 4100 Clause 9.7.5 specifies that HSS connection welds must be designed as fillet welds with throat thickness measured along the branch profile. The required throat is tt-req = N1* / (phi x 0.60 x fuw x Lw). The effective weld length Lw depends on the branch section profile and angle. AS 4100 Clause 9.7.5.2 limits the weld throat to a minimum of the branch wall thickness for full-strength design. For Australian C450L0 hollow sections, E48XX electrodes (fuw = 490 MPa) are standard, with 6-8 mm leg lengths typical for branch-to-chord connections.

Educational reference only. All design values must be verified against the current edition of AS 4100:2020 and AS/NZS 1163:2016. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.