CSA S16 HSS Connection — Truss & Joint Design Reference

Complete reference for CSA S16:19 Annex H hollow structural section (HSS) connection design. Covers welded truss joints (T, Y, X, K gap/overlap configurations), chord plastification capacity using the CIDECT method, branch effective width, punching shear, weld design for thin-wall HSS, and detailing requirements for reliable HSS connections. Includes a worked example for a welded K-joint in a truss.

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CSA S16 HSS Connections — Overview

Hollow structural sections (HSS) are widely used in Canadian steel construction for trusses, columns, bracing, and space frames. HSS connections present unique design challenges due to the thin-walled tubular sections and the complex stress distributions at the joint. CSA S16:19 Annex H provides a comprehensive design method based on the CIDECT (Comite International pour le Developpement et l'Etude de la Construction Tubulaire) design guides.

Canadian HSS is manufactured to CSA G40.20/G40.21 in Class C (cold-formed, Fy = 345 MPa) and Class H (hot-formed, Fy = 350 MPa). HSS connections in Canada are most commonly welded (CSA W59), with bolted connections limited to gusset plate and base plate connections.

HSS Connection Types

Joint Type Configuration Typical Application CSA S16 Annex H Clause
T-joint Branch perpendicular to chord Bracing to column, truss chords H.3
Y-joint Branch at angle to chord Truss diagonals H.3
X-joint Two branches opposite on chord Cross bracing, truss webs H.3
K-joint Two branches forming a K (gap or overlap) Truss panel points H.4
N-joint Variant of K with vertical + diagonal Truss connections H.4
TK-joint Two T-joints on same chord face Multi-planar trusses H.5
XX-joint Two X-joints crossing Complex space frames H.5

Governing Failure Modes

HSS connections fail in one or more of the following modes:

Failure Mode Description CSA S16 Reference
Chord plastification Yielding of chord face under branch load Ann. H.3.2
Chord side wall yielding Vertical yielding of chord side walls Ann. H.3.3
Chord shear Shear failure of chord between branches (K, N) Ann. H.4.3
Branch effective width Rupture of branch at weld toe Ann. H.3.5
Branch punching shear Shear rupture of chord wall at branch perimeter Ann. H.3.6
Local buckling Local wall buckling of chord or branch Ann. H.3.7
Weld failure Inadequate weld between branch and chord CSA W59 / Cl. 13.13

Chord Plastification (CSA S16 Annex H.3.2)

Chord plastification is the primary failure mode for most HSS connections — the chord face yields in a plastic mechanism under the branch load. The factored resistance for chord plastification depends on the joint type and geometry ratios.

T, Y, and X Joints

For T, Y, and X joints with square/rectangular chords:

N1* = φ × Fy0 × t0² / (1 - β) × (2 × η / sinθ + 4 × √(1 - β) × f(n))

where:
  N1*  = factored branch axial resistance (N)
  φ    = 0.90 (resistance factor)
  Fy0  = chord yield strength (MPa)
  t0   = chord wall thickness (mm)
  β    = b1 / b0 (branch width ratio, ≤ 1.0)
  η    = h1 / b0 (branch depth ratio)
  θ    = angle between branch and chord (degrees)
  f(n) = chord stress function (accounts for chord axial load)

The chord stress function f(n) accounts for the reduction in chord face plastification capacity when the chord itself is under axial stress:

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

where n = N0 / (A0 × Fy0)  (chord stress ratio)

RHS/SHS Connection Parameters

The design parameters β, η, and 2γ govern HSS connection capacity:

Parameter Definition Typical Range Effect on Capacity
β = b1/b0 Branch-to-chord width ratio 0.25 - 1.0 Higher β increases capacity
2γ = b0/t0 Chord width-to-thickness ratio 10 - 50 Lower 2γ (stubby chords) increases capacity
η = h1/b0 Branch depth ratio 0.5 - 2.0 Higher η increases capacity (within limits)

For K and N gap joints, an additional chord shear check applies between the branches. The gap g must satisfy:

g ≥ t1 + t2 (minimum gap to avoid overlap)
g / b0 ≤ 1.5 (maximum gap for effective load transfer)

When the gap is negative (overlap joints), load transfer occurs through direct branch-to-branch contact. Overlap joints provide higher capacity but require more fabrication precision.

Branch Effective Width (CSA S16 Annex H.3.5)

The branch effective width accounts for the non-uniform stress distribution across the branch cross-section at the chord face. The effective width is:

be = (10 / (b0 / t0)) × (Fy0 × t0) / (Fy1 × t1) × b1 ≤ b1

where:
  be  = effective width of branch (mm)
  b0  = chord width (mm)
  t0  = chord wall thickness (mm)
  b1  = branch width (mm)
  t1  = branch wall thickness (mm)
  Fy0 = chord yield strength (MPa)
  Fy1 = branch yield strength (MPa)

The factored branch resistance incorporating effective width:

N1* = φ × 2 × Fy1 × t1 × (2 × h1 - 4 × t1 + be + be,eff)

where be,eff = (10 / (b0 / t0)) × t1 ≤ b1 (for the orthogonal pair)

Punching Shear (CSA S16 Annex H.3.6)

Punching shear failure occurs when the branch load causes shear rupture of the chord wall around the branch perimeter. For RHS and SHS chords:

N1* = φ × Fy0 × t0 × (2 × h1 / sinθ + b1 + be,p) / √3

where:
  be,p = 10 / (b0 / t0) × b1 ≤ b1  (effective width for punching shear)
  φ    = 0.90 (resistance factor)

Punching shear governs for connections with thin chord walls relative to the branch size (low t0, high β). It is especially critical for round HSS (CHS) connections.

Weld Design for HSS Connections

HSS welds are designed per CSA W59 with specific requirements for tubular joints. The weld must be designed for the full capacity of the branch if the branch is at yield stress, or for the actual factored load, whichever is less.

Fillet Welds for HSS

HSS Wall Thickness Minimum Fillet Weld Maximum Fillet Weld Notes
t ≤ 5 mm 4 mm t (branch) Single pass, small weld
5 < t ≤ 10 mm 5 mm 1.2 × t (branch) Standard detail
10 < t ≤ 16 mm 6 mm 1.2 × t (branch) May require multiple passes
t > 16 mm 8 mm 1.2 × t (branch) PJP groove or CJP recommended

For thin-wall HSS (t ≤ 5 mm), the heat input from welding can reduce the base metal strength. CSA W59 requires:

CJP Groove Welds for HSS

When branch forces exceed the capacity of fillet welds, CJP groove welds are used. The weld must develop the full branch resistance at the joint interface:

Vrw = φw × Aw × Base Metal Fu / √3

where:
  φw = 0.90 (CJP groove welds)
  Aw = weld area along branch perimeter (mm²)

Detailing Requirements

HSS connection detailing directly affects both strength and cost. Key Canadian practice detailing requirements:

Detail Requirement Reference
Branch end preparation Saddle cut to match chord radius CSA W59 Clause 12
Minimum branch angle θ ≥ 30° recommended (θ ≤ 90°) CSA S16 Annex H.2
Maximum chord flat width b0/t0 ≤ 1.4 × √(E/Fy0) for Class 1 sections CSA S16 Table 2
Eccentricity in K-joints e ≤ 0.25 × h0 (branch offset in gusset design) CSA S16 Annex H.4.2
Overlap in K-joints Ov ≥ 25% (to ensure load transfer) CSA S16 Annex H.4.4
Stiffener plates Required if chord face punching shear governs CSA S16 Clause H.8
Gusset plate connections HSS-to-gusset welds per CSA W59 Table 4.2 CSA W59 Clause 4

Worked Example — Welded K-Joint in Truss

Problem: Design a welded K-joint in a steel truss. Chord: HSS 203x203x9.5 (Grade 350W Class H). Branches: HSS 127x127x6.4 (Grade 350W Class H). Branch angle θ = 45°. Gap between branches g = 50 mm. Factored branch loads: Tension = 300 kN (right branch), Compression = 280 kN (left branch). Chord axial compression: N0 = 400 kN.

HSS Properties

Chord (HSS 203x203x9.5): b0 = 203 mm, h0 = 203 mm, t0 = 9.5 mm, A0 = 8,920 mm², Fy0 = 350 MPa

Branch (HSS 127x127x6.4): b1 = 127 mm, h1 = 127 mm, t1 = 6.4 mm, Fy1 = 350 MPa

Step 1 — Check Geometry Limits

β = b1 / b0 = 127 / 203 = 0.626 → OK (0.25 ≤ β ≤ 1.0)
2γ = b0 / t0 = 203 / 9.5 = 21.4 → OK (10 ≤ 2γ ≤ 50)
η = h1 / b0 = 127 / 203 = 0.626 → OK
θ = 45° → OK (≥ 30°)

Step 2 — Check Gap Requirements

g_min = t1 + t2 = 6.4 + 6.4 = 12.8 mm → 50 mm > 12.8 mm OK
g/b0 = 50/203 = 0.246 → OK (≤ 1.5)

Step 3 — Chord Plastification Check (Compression Branch)

Chord stress ratio:

n = N0 / (A0 × Fy0) = 400,000 / (8,920 × 350) = 0.128
f(n) = 1.0 + 0.3 × 0.128 - 0.3 × 0.128² = 1.0 + 0.038 - 0.005 = 1.033

For K-joints with gap, the chord plastification resistance per Annex H.4.2:

N1* = φ × Fy0 × t0² / sinθ × (2.0 + 7.0 × β) / √(1 + β) × f(n) × kg

where kg = 0.55 for gap joints (gap/chord depth ratio factor)

N1* = 0.90 × 350 × 9.5² / sin45° × (2.0 + 7.0 × 0.626) / √(1.626) × 1.033 × 0.55

N1* = 0.90 × 350 × 90.25 / 0.707 × (2.0 + 4.382) / 1.275 × 1.033 × 0.55

N1* = 0.90 × 350 × 90.25 / 0.707 × 6.382 / 1.275 × 1.033 × 0.55

N1* = 0.90 × 350 × 90.25 / 0.707 × 5.004 × 1.033 × 0.55

N1* = 0.90 × 350 × 127.6 × 5.004 × 1.033 × 0.55
N1* = 0.90 × 350 × 127.6 × 2.842
N1* = 114,170 N = 114.2 kN

The chord plastification resistance (114.2 kN) is much less than the applied compression branch load (280 kN). This indicates the joint is not adequate — options include:

Alternative — Larger Chord Section

Try HSS 254x254x9.5 (b0 = 254 mm, t0 = 9.5 mm, Fy0 = 350 MPa):

β = 127 / 254 = 0.5
2γ = 254 / 9.5 = 26.7

N1* = 0.90 × 350 × 90.25 / 0.707 × (2.0 + 7.0 × 0.5) / √(1.5) × 1.033 × 0.55

N1* = 0.90 × 350 × 127.6 × (5.5) / 1.225 × 1.033 × 0.55
N1* = 0.90 × 350 × 127.6 × 4.490 × 1.033 × 0.55
N1* = 102,000 N = 102 kN → still too low

Alternative — Use Overlap Joint

Overlap joints provide higher capacity by transferring load directly between branches. For overlap Ov ≥ 50%:

N1* = φ × Fy1 × t1 × (2 × h1 - 4 × t1 + be + be,ov) × (1 + 0.25 × Ov/50)

where be,ov = (10 / (b0 / t0)) × t1 ≤ b1

This provides significantly higher capacity (typically 2-3× the gap joint). Design the overlap joint with Ov = 50%.

Step 4 — Branch Effective Width Check

be = (10 / (203 / 9.5)) × (350 × 9.5) / (350 × 6.4) × 127
be = (10 / 21.4) × (9.5 / 6.4) × 127
be = 0.467 × 1.484 × 127 = 88.0 mm ≤ 127 mm OK

Step 5 — Punching Shear Check

be,p = (10 / (203 / 9.5)) × 127 = 0.467 × 127 = 59.3 mm

N1* (punching) = 0.90 × 350 × 9.5 × (2 × 127 / 0.707 + 127 + 59.3) / √3
N1* = 0.90 × 350 × 9.5 × (359.3 + 127 + 59.3) / 1.732
N1* = 0.90 × 350 × 9.5 × 545.6 / 1.732
N1* = 0.90 × 350 × 9.5 × 315.0
N1* = 943,600 N = 943.6 kN > 300 kN OK

Step 6 — Weld Design

Fillet weld around branch perimeter (E49xx electrode):

Perimeter = 2 × (127 + 127) = 508 mm
Effective weld length = 508 - 4 × 6.4 = 482 mm (reduced for return welds)

Required weld leg = (300,000) / (φw × 0.67 × 0.707 × 482 × 490 × 1.3)
Required weld leg = 300,000 / (0.67 × 0.67 × 341 × 490 × 1.3)
Required weld leg = 300,000 / 97,500 = 3.1 mm

Minimum weld leg = 5 mm (per CSA W59 for 6.4 mm base metal). Use 5 mm fillet E49xx around full branch perimeter.

Step 7 — Summary

Component Design
Joint type K-overlap (Ov = 50%)
Chord HSS 203x203x9.5, 350W Class H
Branches HSS 127x127x6.4, 350W Class H
Overlap weld Required between branch walls
Branch fillet weld 5 mm E49xx around full perimeter
Chord plastification Governs for gap joint — use overlap

Frequently Asked Questions

What is the primary failure mode for welded HSS truss connections? Chord plastification is the governing failure mode for most HSS truss connections under CSA S16 Annex H. The chord face yields in a plastic mechanism under the branch load. Chord plastification capacity depends on the width ratio β = b1/b0 and the chord slenderness 2γ = b0/t0. Connections with high β (wide branches) and low 2γ (stubby chords) have the highest plastification resistance.

When should overlap joints be used instead of gap joints in HSS trusses? Overlap joints (where one branch overlaps the other on the chord face) are preferred when gap joints cannot provide sufficient chord plastification resistance. Overlap joints transfer load directly between branches through the overlapping weld, bypassing the chord face and achieving 2-3× the capacity of an equivalent gap joint. CSA S16 Annex H.4.4 requires minimum overlap Ov ≥ 25% for effective load transfer. Overlap joints are more expensive to fabricate and should be specified only when gap joints are inadequate.

How is punching shear checked for HSS connections? Punching shear (CSA S16 Annex H.3.6) checks the shear resistance of the chord wall around the branch perimeter. The factorial resistance is N1* = φ × Fy0 × t0 × (2 × h1/sinθ + b1 + be,p) / √3, where be,p is the effective width for punching shear. Punching shear governs for connections with thin chord walls (low t0) and small clearance between branches. It rarely governs for HSS connections with standard wall thicknesses but should always be checked, particularly for CHS (round HSS) connections.

What welding considerations apply to thin-wall HSS connections? Thin-wall HSS (t ≤ 5 mm) requires special welding precautions per CSA W59 to avoid burn-through and heat-affected zone softening. Minimum fillet weld leg = 4 mm, stringer bead technique with no weaving, maximum interpass temperature of 150°C, and minimum preheat of 5°C. For very thin HSS (t ≤ 3 mm), CJP groove welds should be avoided in favour of fillet welds sized to the base metal thickness. Canadian fabricators typically use GMAW (MIG) for thin-wall HSS and SMAW (stick) or FCAW for thicker sections.

Related Pages

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