Australian Brace Connection — AS 4100 Design Guide

Quick access: [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]

Reference for brace connection design per AS 4100:2020 for concentrically braced frames. Covers gusset plate, block shear, Whitmore section, and gusset-to-beam/column interface weld design.

Connection Types

Design Checks — AS 4100

1. Brace net section:

• φNt = 0.90 × 0.85 × fu × An (fracture at bolt holes)
• φNt = 0.90 × fy × Ag (gross section yield)

2. Gusset plate checks:

• Tension: Whitmore section (as per gusset plate design)
• Block shear: φVb = φ × (0.5×fu×Ant + 0.6×fy×Agv)
• Buckling: Unbraced gusset edge L/r ≤ 50

3. Bolts:

• Shear: φVfn per bolt
• Bearing: φVb per Clause 9.3.2.2
• Combined (if tension present)

4. Gusset-to-beam/column weld:

• Fillet weld along gusset perimeter
• Subjected to N* and eccentric moment M* = N* × e

Worked Example

Problem: 200UC52 brace (N*=600 kN tension). Bolted gusset: 12 mm plate, Grade 300. 6-M20 8.8/S bolts. Check connection.

Solution:

1. Brace net section (2 bolt holes): An = Ag - 2×dh×tf = 6,670 - 2×22×12.7 = 6,111 mm²
2. Fracture: φNt = 0.90×0.85×440×6,111 = 2,056 kN > 600 OK
3. Yield: φNt = 0.90×300×6,670 = 1,801 kN > 600 OK
4. Block shear: Ant=630 mm², Agv=2,880 mm². φVb = 0.90(0.5×440×630 + 0.6×300×2,880) = 125+466 = 591 kN < 600 NG
5. Increase plate: 16 mm plate or add bolt row to increase Agv.

• With 16 mm plate: φVb = 788 kN > 600 OK

Design Resources

• [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]

FAQ

What governs brace connection capacity? Block shear is often critical for brace gusset connections. Bolt bearing and Whitmore section tension also common limit states.

How is gusset-to-beam weld designed? Fillet weld along gusset perimeter designed for N* plus the eccentric moment from the work point offset.

Minimum gusset plate thickness for brace connections? 12 mm minimum for buildings, 16 mm for seismic frames. Governed by block shear and bolt bearing.

Educational Use Only — This reference is for educational and preliminary design purposes only. All structural designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) in accordance with AS 4100:2020 and all applicable Australian Standards. Results are not for construction.

Design Applications

Common Design Scenarios

This reference covers structural design scenarios commonly encountered in Australian steel design practice:

• Strength verification: Check member or connection capacity against factored loads per the applicable design code
• Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
• Code compliance: Ensure design meets all provisions of the governing standard
• Connection detailing: Verify weld sizes, bolt quantities, and edge distances

Related Design Considerations

• System behavior: consider the interaction between members and connections
• Load paths: verify that forces can be transferred through the structure to the foundations
• Constructability: check that the design can be fabricated and erected practically
• Cost optimization: evaluate alternative sections or connection types for economy

Worked Example

Problem: Verify a Grade 300 member for the following conditions:

Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category

Design Check:

1. Determine governing load combination (LRFD or ASD per applicable code)
2. Calculate maximum internal forces (moment, shear, axial)
3. Compute nominal capacity per code provisions
4. Apply resistance/safety factors
5. Verify interaction if combined forces exist

Result: Use the results from the Steel Calculator tool to verify design adequacy.

Brace Connection Design per AS 4100

Brace connections transfer seismic or wind forces between bracing members and the structural frame.

Key design considerations:

1. Capacity design: The connection must be designed for the brace axial capacity (1.2RyFyAg) per AS 4100 Clause 14.3 — not the design action. This ensures the connection remains elastic during a seismic event.

2. Gusset plate: Per Clause 5.14.3, the gusset free edge buckling must be checked for the full compressive force. Corner clips (1-2 bolt diameter radius) are recommended to reduce weld stress concentration.

3. Eccentricity: In-plane eccentricity between the brace line of action and the gusset centroid creates secondary moments. This must be explicitly accounted for in the gusset plate design per AS 4100 Clause 5.14.

4. Slenderness limits: The gusset plate out-of-plane KL/r should not exceed 200 for compression braces. For plates shorter than 300 mm free edge length, a 12 mm minimum thickness is recommended.

Frequently Asked Questions

What Australian Standard governs structural steel design?

AS 4100-2020 (Steel Structures) is the primary standard for structural steel design in Australia. It covers all aspects of design including member capacity, connections, serviceability, and fire resistance. The standard uses a limit states design philosophy with resistance factors (φ) applied to nominal capacities. Companion standards include AS/NZS 3679.1 for hot-rolled sections, AS/NZS 1554 for welding, and AS/NZS 4600 for cold-formed steel.

What are the common steel grades used in Australian construction?

The most common steel grades for Australian construction are Grade 300 and Grade 350 per AS/NZS 3679.1. Grade 300 (minimum yield 300 MPa for sections > 12 mm thick) is the standard for general structural applications. Grade 350 (minimum yield 340 MPa for sections > 12 mm) is used where higher strength reduces weight. Grade 400 and Grade 450 are available for specialized applications requiring higher strength-to-weight ratios.

How does AS 4100 compare to AISC 360?

Both AS 4100 and AISC 360 use limit states design (LRFD) principles. Key differences include: AS 4100 uses a single "capacity factor" φ approach rather than separate φ for different failure modes; AS 4100 specifies distinct buckling curves for hot-rolled and welded sections; the moment capacity formula in AS 4100 uses αm factor directly rather than Cb; and AS 4100 has more detailed provisions for slender sections and combined actions. Despite philosophical differences, both codes produce similar results for typical members.