Canadian HSS Connection Design — CSA S16 Welded and Bolted Connections
Complete reference for hollow structural section (HSS) connection design per CSA S16-19 for Canadian steel construction. Covers welded truss connections (T, Y, K-joints), bolted connections with through-bolts and slotted gussets, chord face plastification, punching shear, and a worked example for an HSS Warren truss connection.
Quick access: HSS section properties | Brace connection | Welded connection calculator
CSA S16 HSS Connection Types
HSS connections in Canadian practice fall into two categories:
| Connection Type | Description | Design Method |
|---|---|---|
| Welded truss connections | Direct welding of branch to chord | Chord face plastification, punching shear (CIDECT/CISC) |
| Bolted connections | Gusset plate through slot or bolted to face | Shear lag, net section (Clause 13.2) |
| Moment connections | HSS beam to HSS column with stiffeners | Panel zone shear, flange bending |
| Splices | HSS to HSS with cover plates or internal stiffeners | Net section, bolt shear |
| Base plate connections | HSS column to base plate | Concrete bearing, plate bending |
Welded HSS Truss Connections
Per CSA S16 and the CISC Handbook (CIDECT method):
Chord Face Plastification
For T and Y joints with square/rectangular HSS:
N1* = (Q_u × Q_f × f_y0 × t_0^2) / (sin(theta_1) × (1 - beta))
Where:
- Q_u = function factor depending on joint type
- Q_f = chord stress function
- f_y0 = chord yield strength
- t_0 = chord wall thickness
- theta_1 = branch angle (30-90°)
- beta = b_1/b_0 or d_1/b_0 (width ratio)
Section Requirements
Per CSA S16 Table 1, HSS sections used in welded connections must meet width-thickness limits:
| Limit State | Round HSS | Rectangular HSS |
|---|---|---|
| Class 1 (seismic) | D/t ≤ 13,000/Fy | b/t ≤ 145/sqrt(Fy) |
| Class 2 | D/t ≤ 13,000/Fy | b/t ≤ 170/sqrt(Fy) |
| Class 3 | D/t ≤ 30,000/Fy | b/t ≤ 200/sqrt(Fy) |
For 350W: Class 1 rectangular limit = 145/sqrt(350) = 7.75; round HSS limit = 13000/350 = 37.1.
K-Joint Design
For K-joints with a gap between branches:
| Parameter | Square/Rectangular HSS | Round HSS |
|---|---|---|
| Gap g ≥ t_1 + t_2 | Yes | Yes |
| N1* (branch capacity) | Q_u × Q_f × f_y0 × t_0^2 / sin(theta) | Same formula |
| Chord shear check | Required when e > 0.25 × h_0 | Required |
| Punching shear (branch) | f_y1 × t_1 / sin(theta) ≤ Q_f × f_y0 × t_0 | Same |
Bolted HSS Connections
Gusset Plate Through-Slot Connection
The most efficient bolted HSS connection: a gusset plate passes through a slot cut in the HSS and is welded to both sides.
| Design Check | Formula | Limit |
|---|---|---|
| HSS net section | Tr = phi_u × A_ne × Fu | A_ne = A_gross - A_slot |
| Slot weld | Vr = 0.67 × phi_w × Xu × 0.707 × D × 2 sides | Full length of slot |
| Gusset plate yield | Tr = phi × A_g × Fy | Gross section |
| Gusset plate block shear | Tr_block per Clause 13.2 | Net section |
Bolted Connection to HSS Face
When bolts pass through the HSS face (hollow section), the face must be checked for:
- Bolt bearing: Br = 3.0 × phi_br × t_hss × d_hole × Fu_hss
- HSS face bending: Must be checked if bolt tension is present (similar to end plate prying)
For thin-walled HSS (t < 6 mm), face bending governs for bolts in tension. Through-bolts (bolts passing completely through both walls) are recommended for tension connections.
Worked Example — HSS Warren Truss Connection
Given: HSS 152×152×6.4 chord (350W) and HSS 102×102×6.4 branch (350W) in a Warren truss. Branch angle = 45°. Factored branch force = 250 kN (compression). Gap between branches = 30 mm.
Step 1 — Check Section Limits: Chord b/t = 152/6.4 = 23.75 > 7.75 but ≤ 200/sqrt(350) = 10.69... wait. 152/6.4 = 23.75 for individual walls. For HSS, b/t uses flat width = 152 - 4×6.4 = 127 mm. b/t = 127/6.4 = 19.8. Class 3 limit = 200/sqrt(350) = 10.69. 19.8 > 10.69 → Class 4 slender. Use effective width per Clause 13.5.3.
Step 2 — Chord Face Plastification (per CIDECT): beta = b_1/b_0 = 102/152 = 0.67 Q_u = 4.9 for T-joint (no gap) Q_f = 1.0 (assume low chord stress) f_y0 = 350 MPa t_0 = 6.4 mm
N1* = (4.9 × 1.0 × 350 × 6.4^2) / (sin(45°) × (1 - 0.67)) N1* = (4.9 × 350 × 40.96) / (0.707 × 0.33) N1* = 70,246 / 0.233 = 301,485 N = 301 kN
Step 3 — Check: N1* = 301 kN ≥ 250 kN. OK. Connection capacity is adequate.
Step 4 — Weld Design: Branch-to-chord weld: 6 mm fillet around branch perimeter. Perimeter = 2 × (102 + 102) = 408 mm Vr per mm for 6 mm fillet, E48XX = 0.915 kN/mm Total Vr = 408 × 0.915 = 373 kN ≥ 250 kN. OK.
Result: HSS 152×152×6.4 chord with HSS 102×102×6.4 branch welded at 45°, 6 mm fillet welds around perimeter.
Design Considerations
| Consideration | Recommendation | Reference |
|---|---|---|
| Minimum wall thickness | 4.8 mm for welded connections | CSA W59 Clause 5.1 |
| Maximum beta ratio | beta = b_1/b_0 ≤ 0.85 | CIDECT |
| Minimum gap (K-joints) | g ≥ t_1 + t_2 | CIDECT |
| Eccentricity (K-joints) | e ≤ 0.25 × h_0 | CIDECT |
| Punching shear | f_y1 × t_1 ≤ Q_f × f_y0 × t_0 | Clause 13.11 |
Design Resources
- Canadian HSS Section Properties
- CSA S16 Brace Connection Design
- Canadian Weld Capacity
- CSA S16 Gusset Plate Design
- Canadian HSS Column Design
- All Canadian References
Frequently Asked Questions
How is chord face plastification calculated for HSS welded connections? Chord face plastification is the limit state where the chord face yields under the branch load. Per the CIDECT method (adopted by CISC Handbook): N1* = (Q_u × Q_f × f_y0 × t_0^2) / (sin(theta_1) × (1 - beta)). Q_u depends on the joint type (T = 4.9, Y = 4.9, K with gap = function of gap ratio). Q_f accounts for chord stress (reducing capacity when chord is in compression). This is the governing limit state for most HSS truss connections.
What is the minimum HSS wall thickness for welded connections per CSA W59? Per CSA W59-18, the minimum wall thickness for welded HSS connections is 4.8 mm. For thinner walls (t < 4.8 mm), the heat from welding may cause burn-through or excessive distortion. For branch connections in trusses, 6.4 mm is the typical minimum for reliable welding. For bolted connections, there is no minimum wall thickness from CSA W59, but the HSS face bending capacity must be checked for tension bolts.
When should a through-bolt connection be used instead of a welded gusset for HSS? Through-bolts (bolts passing through both walls of the HSS via a through-plate or slotted gusset) are used when field bolting is preferred over field welding. Through-bolt connections provide reliable tension capacity without relying on the HSS face bending resistance. Welded gussets are more efficient for shop fabrication but require field welding for erection. Through-bolt connections are common in bracing where the brace is field-bolted to welded gusset plates.
Can HSS sections be directly bolted to each other (face-to-face)? Face-to-face bolting of HSS sections is not recommended for primary connections because the hollow section provides limited bearing resistance for bolts in tension (the face bends). For shear connections between HSS members, through-bolts with a spacer tube inside the HSS can work. For moment connections between HSS beam and column, a welded connection with diaphragm plates or internal stiffeners is preferred. The CISC Handbook provides details for these connections.
Connection Design Methods
Eccentric Load on Bolt Groups
When a bolt group is subject to combined shear and moment, the instantaneous center of rotation (ICR) method provides the most accurate analysis. The critical bolt has the maximum resultant force from:
- Direct shear component: P/n (equal distribution assumed for serviceability)
- Moment component: M × r / Σr² (elastic vector method for preliminary design)
For ultimate design, the ICR method accounts for nonlinear bolt deformation using: Rn = Rult(1 - e⁻¹⁰Δ)⁰·⁵⁵ (per AISC Manual)
Block Shear
Block shear is a limit state combining tension rupture on one plane and shear rupture or yielding on a perpendicular plane. The controlling resistance is:
AISC: Rn = min(0.60FuAnv + UbsFuAnt, 0.60FyAgv + UbsFuAnt)
Where Ant = net tension area, Anv = net shear area, Agv = gross shear area, and Ubs = 1.0 for uniform tension stress.
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Frequently Asked Questions
What is the recommended design procedure for this structural element?
The standard design procedure follows: (1) establish design criteria including applicable code, material grade, and loading; (2) determine loads and applicable load combinations; (3) analyze the structure for internal forces; (4) check member strength for all applicable limit states; (5) verify serviceability requirements; and (6) detail connections. Computer analysis is recommended for complex structures, but hand calculations should be used for verification of critical elements.
How do different design codes compare for this calculation?
AISC 360 (US), EN 1993 (Eurocode), AS 4100 (Australia), and CSA S16 (Canada) follow similar limit states design philosophy but differ in specific resistance factors, slenderness limits, and partial safety factors. Generally, EN 1993 uses partial factors on both load and resistance sides (γM0 = 1.0, γM1 = 1.0, γM2 = 1.25), while AISC 360 uses a single resistance factor (φ). Engineers should verify which code is adopted in their jurisdiction.
Educational reference only. HSS connection design per CSA S16-19 and CIDECT/CISC Handbook. Verify section classification and chord face plastification for the specific joint geometry. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.