HSS Connection Design — AISC Chapter K Guide

Hollow Structural Section (HSS) connections present unique design challenges because the hollow walls can deform locally under concentrated loads. AISC 360 Chapter K provides specific provisions for truss connections, moment connections, and other HSS joints. This page covers the connection types, limit states, and design procedure.

HSS Connection Types

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.

Truss Connections (Branch-to-Chord)

| Type | Configuration | Load Pattern | | ---

• | --------------------------------- | ---------------------- | | T | One branch perpendicular to chord | Branch axial force | | Y | One branch at angle to chord | Branch axial force | | X | Branch through chord (both sides) | Cross-chord force | | K | Two branches, gap between them | Balanced branch forces | | N | Two branches, overlap | Higher capacity than K |

Moment Connections

Key Parameters

Limit States for HSS Truss Connections

AISC Chapter K requires checking multiple limit states. The lowest capacity governs.

Limit States for Square/Rectangular HSS

Chord Wall Plastification (β ≤ 0.85)

This is typically the governing limit state for gap connections with β < 0.85.

For T and Y connections: φPn = φ × Fy × t² × [1.5 / (1 - 0.81β)] × Qf

For K (gap) connections: φPn = φ × Fy × t² × [1.5 / (1 - 0.81β)] × Qf × (1 + gap correction)

where Qf = chord stress interaction factor, t = chord wall thickness.

Shear Yielding (Sidewall, β ≥ 0.85)

When the branch width approaches the chord width, the chord sidewalls resist the load:

φPn = φ × 0.6 × Fy × 2 × t × (branch height / sin θ)

Local Yielding of Branch

φPn = φ × Fy,branch × tbranch × (2 × ηbranch)

where ηbranch = effective branch contact length.

Chord Stress Function (Qf)

The chord stress function accounts for the presence of axial and bending stresses in the chord at the connection:

Qf = 1.0 (when chord is unstressed at the connection)

For axial stress: Qf = 1.0 - U × (P / Py)

where U = chord utilization factor (varies by connection type), P = chord axial force, Py = chord yield capacity.

For combined axial + bending: Qf = 1.0 - U × (P / Py + M / Mp)

Typical U values:

K Connection Gap Requirements

When the gap is too small to fit both branch welds, an overlapping (N) connection is used instead.

Overlapping (N) Connections

In overlapping connections, one branch sits on top of the other at the chord face. The overlap ratio:

Ov = (q / p) × 100%

where q = overlap length measured along the chord, p = branch footprint length.

Higher overlap ratios increase connection capacity because load is shared between branches. Minimum overlap is typically 25%.

HSS-to-HSS Moment Connections

Flange Plate Connection

Top and bottom flange plates transfer moment to the HSS column:

Flange force: Ff = M / (d_beam - tf)

Check: Plate tension/compression capacity, plate-to-column weld, local chord wall yielding.

Through-Plate Connection

A plate passes through a slot in the HSS column and is welded to both sides. This is the most efficient moment connection for HSS columns because it engages both walls of the HSS.

Design: Size the plate for the full flange force. Check plate tension, compression, and local buckling. Weld the plate to both HSS walls with CJP or fillet welds.

Effective Width for Rectangular HSS

Not all of the branch cross section is effective at the connection. The effective width accounts for uneven stress distribution:

be = bbranch × (effective width factor)

For rectangular branches on rectangular chords:

be = bbranch × [1.0 - 0.82 × (1 - β)^(0.6)]

For square branches, the effective width is more uniform but still requires checking per AISC Table K2.2.

Worked Example — K Connection

Given: Square HSS truss with HSS8x8x3/8 chord and HSS4x4x1/4 branches at 45°. Gap = 2 in. Branch force = 40 kips (LRFD). A500 Gr B (Fy = 46 ksi).

Parameters:

• β = 4/8 = 0.50
• γ = 8/(2×0.375) = 10.67
• θ = 45°
• t = 0.375 in

Chord wall plastification (governs for β = 0.50): φPn = 0.90 × 46 × 0.375² × [1.5 / (1 - 0.81 × 0.50)] × 1.0 (assuming Qf = 1.0) = 0.90 × 46 × 0.1406 × [1.5 / 0.595] × 1.0 = 5.82 × 2.521 = 14.7 kips

14.7 kips < 40 kips → Connection fails. Options:

1. Increase chord thickness to HSS8x8x1/2 (t = 0.465 in)
2. Use an overlapping connection
3. Add stiffening plates

Retry with HSS8x8x1/2: φPn = 0.90 × 46 × 0.465² × 2.521 × 1.0 = 0.90 × 46 × 0.216 × 2.521 = 22.6 kips

Still insufficient. Need stiffening plates or redesign truss geometry.

Reinforcement Methods

When the connection capacity is insufficient:

Frequently Asked Questions

Why are HSS connections different from W-shape connections? HSS walls are thin relative to the member size. Concentrated loads from branches can cause local wall bending, plastification, and punching shear that do not occur in wide-flange sections with thick webs and flanges. Chapter K addresses these unique limit states.

What is β in HSS connection design? β (beta) is the ratio of branch width to chord width. It is the most important parameter in HSS connection capacity. Connections with β close to 1.0 (branch nearly as wide as chord) have much higher capacity than those with β = 0.3-0.5.

When do I need a through plate? Through plates are needed when the HSS wall plastification limit state governs and increasing the chord thickness is not practical. They are common in heavy truss connections and moment connections to HSS columns.

Can I weld directly to an HSS wall? Yes, but the connection capacity is limited by the thin HSS wall. For small loads, direct fillet welds may be sufficient. For larger loads, you must check all Chapter K limit states and may need reinforcement.

Try it now: Check your connection design with our free Bolted Connection calculator →

Related Pages

• Section Properties — HSS section data
• HSS Weight — HSS weight chart
• Welded Connections — Weld capacity calculator
• Connection Types Explained — All connection types
• Truss Design — Truss analysis

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.

Design Resources

Calculator tools

• Bolted Connection Calculator
• Weld Capacity Calculator
• End Plate Moment Connection Calculator
• Fin Plate Shear Connection Calculator
• Gusset Plate Calculator

Design guides

• Bolted Connection Worked Example
• Bolted Connection Checklist
• Steel Connection Calculator Guide
• Weld Design Checklist
• EN 1993-1-8 Bolted Connection Worked Example

Reference pages

• Shear Tab