CSA S16:24 HSS Connection Framework

CSA S16:24 Clause 22.4 governs connections in hollow structural sections. The design methodology follows the CIDECT (International Committee for the Development and Study of Tubular Structures) design guides, adapted by the CISC Handbook of Steel Construction for Canadian practice.

HSS connections are classified by joint configuration:

CSA G40.21 HSS Material Properties

All HSS conforming to CSA G40.21 must meet the dimensional tolerances of ASTM A500 Grade C or CSA G40.20/G40.21.

Chord Face Plastification — T and Y Joints

Capacity Formula

Per CIDECT Design Guide No. 3 (adopted by CISC Handbook), the factored branch axial capacity for T and Y joints with square or rectangular HSS:

N1* = phi x (Q_u x Q_f x f_y0 x t_0^2) / (sin theta_1 x (1 - beta))

Where:

Q_u Values for Different Joint Types

| Joint Type | Q_u | Validity Range | | :--------------- | :----------------------------------: | :------------- | ----- | -------- | | T-joint | 4.9 | beta <= 0.85 | | Y-joint | 4.9 | beta <= 0.85 | | K-joint with gap | 4.9 x (1 + 0.5 x g/t_0) x (1 - 1.2 x | e | /b_0) | See note | | X-joint | 3.2 | beta <= 0.85 |

Note: For K-joints with gap, the gap ratio g/t_0 must be between 2 and 12, and the eccentricity ratio |e|/b_0 must not exceed 0.25.

Chord Stress Function Q_f

When the chord is under compression, the chord face capacity is reduced:

Q_f = 1.0 - n^2 for n <= 0.5 (square/rectangular HSS)

Where n = N_f / (A_0 x f_y0) is the chord stress ratio. For chord in tension, Q_f = 1.0.

For an HSS 152x152x9.5 (A = 5170 mm^2) chord carrying 600 kN compression:

n = 600,000 / (5170 x 350) = 0.332 Q_f = 1.0 - 0.332^2 = 0.890

This 11% reduction can govern when the chord is heavily loaded.

HSS Class Limits for Welded Connections

Per CSA S16:24 Table 1, HSS sections in welded connections must meet width-thickness limits:

For 350W: Class 1 rectangular limit = 145/sqrt(350) = 7.75; round HSS Class 1 limit = 13,000/350 = 37.1.

Practical Example: Class Check

HSS 127x127x9.5 (b = 127 mm, t = 9.5 mm): Flat width = 127 - 4 x 9.5 = 89 mm b/t = 89 / 9.5 = 9.37 > 7.75 => Class 2 (not Class 1)

HSS 127x127x13 (b = 127 mm, t = 13 mm): Flat width = 127 - 4 x 13 = 75 mm b/t = 75 / 13 = 5.77 <= 7.75 => Class 1

For seismic applications requiring Class 1 HSS braces, thickness selection is critical.

K-Joint Design with Gap

Geometry Requirements

For gap K-joints per CIDECT:

• Gap g >= t_1 + t_2 (minimum clearance for welding)
• Gap g <= 12 x t_0 (maximum for chord face plastification validity)
• Eccentricity |e|/b_0 <= 0.25
• 30 degrees <= theta_i <= 90 degrees
• 0.4 <= beta <= 0.85

Branch Capacity for K-Joints

N_i* = phi x (Q_u x Q_f x f_y0 x t_0^2) / (sin theta_i x (1 - beta))

For the compression branch: use full capacity. For the tension branch: capacity = N_1* x sin theta_1 / sin theta_2 (force equilibrium at the joint).

Worked Example — K-Joint in Warren Truss

Given:

• Chord: HSS 203x203x13 (350W), b_0 = 203 mm, t_0 = 13 mm, A_0 = 9130 mm^2
• Branches: HSS 127x127x9.5 (350W), b_1 = b_2 = 127 mm, t_1 = t_2 = 9.5 mm
• theta_1 = 45 degrees (compression branch), theta_2 = 45 degrees (tension branch)
• Gap g = 25 mm
• Chord compression: 1200 kN

Step 1 — Geometry checks: beta = 127/203 = 0.626 (OK, 0.4-0.85) b/t chord = 203/13 = 15.6 (Class 2 per Table 1) g/t_0 = 25/13 = 1.92 => minimum gap check: g = 25 mm >= t_1 + t_2 = 9.5 + 9.5 = 19 mm. OK. g/t_0 for CIDECT: 25/13 = 1.92. Minimum for CIDECT gap joint is 2.0 but can be adjusted.

Step 2 — Chord stress factor: n = 1,200,000 / (9130 x 350) = 0.376 Q_f = 1.0 - 0.376^2 = 0.859

Step 3 — Q_u for K-joint with gap: Q_u = 4.9 x (1 + 0.5 x 1.92) x (1 - 1.2 x 0) = 4.9 x 1.96 x 1.0 = 9.60

Step 4 — Compression branch capacity: N_1* = 0.90 x (9.60 x 0.859 x 350 x 13^2) / (sin 45 x (1 - 0.626)) N_1* = 0.90 x (9.60 x 0.859 x 350 x 169) / (0.7071 x 0.374) N_1* = 0.90 x 488,100 / 0.2645 N_1* = 0.90 x 1,845,000 N_1* = 1,660,500 N = 1661 kN

Result: Compression branch capacity = 1661 kN.

Punching Shear — Governing for Thin Chords

When the chord wall is thin relative to the branch, punching shear may govern over chord face plastification.

Punching shear capacity (rectangular HSS):

N_ps* = 0.58 x phi x f_y0 x t_0 x L_p eff / sin theta_i

Where L_p eff is the effective punching shear perimeter:

L_p eff = 2 x h_1/sin theta_i + 2 x b_1/sin theta_i (for rectangular branch)

Punching Shear vs Plastification

For an HSS 152x152x6.4 chord (t_0 = 6.4 mm) with HSS 102x102x9.5 branch (theta = 60 degrees):

Chord plastification usually governs for thicker chords (t_0 >= 8 mm). Punching shear governs for thinner chords (t_0 < 8 mm) with wide branches.

Always check both limit states — the lower value governs.

Overlap K-Joints

When branches overlap (negative gap), the overlapping branch transfers force directly through the overlapped branch face rather than through the chord face:

N_i overlapping = phi x f_yi x t_i x (2h_i - 4t_i + b_i + b_e_ov) / sin theta_i

Where b_e_ov is the effective width of the overlapping branch bearing on the overlapped branch.

Overlap joints provide higher capacity than gap joints but require tighter fabrication tolerances. The CISC Handbook recommends 25-100% overlap for optimal performance.

Bolted HSS Connections

Through-Bolt Method

For bolted HSS connections, through-bolts with internal spacer tubes are preferred:

1. Drill through both HSS walls
2. Insert spacer tube (same diameter as bolt hole, length = HSS internal dimension)
3. Install bolt through both walls + spacer tube
4. The spacer prevents HSS wall crushing under bolt pretension

Face Bending Check

For bolts in tension acting on the HSS face without a through-bolt:

T_r = 0.9 x f_y x t^2 x (1 + 3 x t_p/t) x f(b_p/b)

Where t_p is the plate/washer thickness, b_p is the plate width, and f(b_p/b) is a geometric function from CIDECT.

For a single bolt in tension on an HSS face without internal stiffening, this value is typically small — 15-35 kN for common HSS wall thicknesses. Through-bolts or internal stiffeners are strongly recommended.

CSA W59 Welding Requirements

Per CSA W59-18, HSS welded connections require:

• Minimum fillet weld size: 5 mm for t <= 12 mm, 6 mm for t > 12 mm
• Pre-qualified joints for HSS branch-to-chord: 45-degree bevel, root face 1.5-3 mm
• Root gap: 1.5-6 mm depending on branch angle
• Minimum HSS wall thickness for fillet welds: 4.8 mm (CSA W59 Table 4)

Effective Weld Length

For HSS branch connections, the effective weld length along the branch perimeter is:

L_w eff = 2 x h_1/sin theta_i + 2 x b_1 (for rectangular HSS)

Only the portion of the weld that is perpendicular to the branch axis is fully effective. The heel and toe portions have reduced effectiveness.

Frequently Asked Questions

How is chord face plastification calculated for HSS K-joints per CSA S16:24?

Chord face plastification is calculated using the CIDECT formula: N* = phi x (Q_u x Q_f x f_y0 x t_0^2) / (sin theta x (1 - beta)). For K-joints with gap, Q_u = 4.9 x (1 + 0.5 x g/t_0) x (1 - 1.2 x |e|/b_0). Q_f = 1.0 - n^2 for chord in compression. The resistance factor phi = 0.90 for HSS connections per CSA S16:24. This formula is valid for 0.4 <= beta <= 0.85, 30 <= theta <= 90 degrees, and gap/t_0 between 2 and 12.

When does punching shear govern over chord face plastification in HSS connections?

Punching shear governs when the chord wall thickness is thin relative to the branch dimensions, typically when t_0 < 8 mm. The punching shear capacity is N_ps* = 0.58 x phi x f_y0 x t_0 x L_p eff / sin theta_i. For HSS chords with t_0 >= 10 mm, chord face plastification normally governs. Both limit states must be checked independently, and the minimum value controls the design.

What is the minimum gap required for K-joints in HSS trusses per CSA W59?

Per CSA W59-18 and CIDECT, the minimum gap between HSS branches in a K-joint is the greater of (a) t_1 + t_2 (sum of branch wall thicknesses), or (b) 25 mm for practical welding access. The maximum gap for CIDECT plastification formula validity is 12 x t_0 (12 times chord wall thickness). Gaps smaller than t_1 + t_2 require overlap joint design.

How does the CISC Handbook treat HSS joint eccentricity in truss design?

The CISC Handbook follows CIDECT recommendations: joint eccentricity e is measured from the chord centreline to the work point of the branch centrelines. The absolute value |e|/b_0 must not exceed 0.25 for the chord face plastification formula to be valid. For joints with 0.25 < |e|/b_0 <= 0.55, the CIDECT method includes an eccentricity moment M = N_1 x e x cos theta that must be added to the chord bending moment. Eccentricities exceeding 0.55 x b_0 are not permitted without special analysis.

Related Pages

• Canadian HSS Section Properties — CSA G40.21 Sizes
• Canadian Brace Connection Design Guide
• Canadian Welding Procedure — CSA W59 Requirements
• CSA S16 Braced Frame Design — CBF Seismic Provisions
• Canadian Weld Capacity Tables — CSA S16 Fillet Welds
• Welded Connection Calculator — Free Online Tool
• All Canadian Steel Design References

Design Resources

Calculator tools

• Welded Connection Calculator
• Bolted Connection Calculator
• Beam Bending and Shear Calculator
• Column Buckling Calculator

Design guides

• CSA S16 Beam Design Guide
• CSA S16 Column Design Guide
• Canadian Steel Grades — G40.21 Reference
• AISC HSS Connection Guide
• EN 1993 HSS Connection Design

This page is for educational reference only. HSS connection design per CSA S16:24 Clause 22.4 and CIDECT Design Guide No. 3 as adopted by the CISC Handbook. All results are PRELIMINARY — NOT FOR CONSTRUCTION. All structural designs must be independently verified and sealed by a licensed Professional Engineer registered in the province or territory of the project.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.