Australian Gusset Plate — AS 4100 Whitmore Section Design
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Reference for gusset plate design per AS 4100:2020 and AISC Steel Construction Manual principles. Covers Whitmore effective width, block shear, gusset buckling, and connection design to braces.
Whitmore Effective Width Method
The Whitmore section defines the effective width for tension resistance at 30° spread from the first bolt row:
Leff = 2 × tan(30°) × Lg + Lb
Where:
- Lg = distance from first to last bolt row
- Lb = bolt group width
- Leff = Whitmore effective width
Tension check: φNt = φ × fy × Leff × tpg ≥ N* Where tpg = gusset plate thickness
Design Checks Summary
| Check | Formula | Clause |
|---|---|---|
| Tension yield | φNt = φ × fy × Ag | 9.2.1 |
| Tension fracture | φNt = φ × 0.85 × fu × An | 9.2.2 |
| Block shear | φVb = φ × (0.5fuAnt + 0.6fyAgv) | 9.2.4 |
| Buckling (compression brace) | φNc = φ × αc × Ns | 6.3 |
| Weld (gusset to beam/column) | φvw = φ × 0.6 × fuw × tt | 9.7 |
| Bolt bearing/tearout | φVb = φ × 3.2 × df × tp × fup | 9.3.2.2 |
Typical Gusset Plate Geometry
| Brace Size | Plate Thickness | Bolt Rows | Plate Size (mm) |
|---|---|---|---|
| 150UC | 12 mm | 2 | 250×250 |
| 200UC | 16 mm | 3 | 300×350 |
| 250UC | 20 mm | 3 | 350×400 |
| 310UC | 25 mm | 4 | 400×500 |
| CHS 114.3 | 12 mm | 2 | 250×300 |
| CHS 168.3 | 16 mm | 3 | 300×400 |
Worked Example
Problem: Gusset plate for 200UC brace, N*=800 kN tension. 3 bolt rows (Lg=200 mm), bolt group width Lb=100 mm. fy=300 MPa, tpg=16 mm.
Solution:
- Whitmore width: Leff = 2×tan30°×200 + 100 = 2×0.577×200 + 100 = 331 mm
- Tension yield: φNt = 0.90 × 300 × 331 × 16 = 1,430 kN > 800 OK
- Block shear check through critical path
- Gusset buckling (if brace is compression): Check slenderness L/r ≤ 50 per AISC recommendation for gusset design.
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 is the Whitmore section? The effective width of the gusset plate resisting tension, determined by a 30° spread angle from the first to last bolt row. Critical for tension capacity.
How is gusset buckling checked? Using the unbraced length of the gusset edge (typically L/r ≤ 50 recommended). Conservatively check as a compression member per AS 4100 Clause 6.3.
What is the minimum gusset plate thickness? 10 mm minimum, 12-16 mm typical for buildings. Governed by bolt bearing, tension capacity, and buckling resistance.
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:
- Determine governing load combination (LRFD or ASD per applicable code)
- Calculate maximum internal forces (moment, shear, axial)
- Compute nominal capacity per code provisions
- Apply resistance/safety factors
- Verify interaction if combined forces exist
Result: Use the results from the Steel Calculator tool to verify design adequacy.
Gusset Plate Design per AS 4100
Gusset plates connect bracing members to the primary structure and must be designed for the full brace force including overstrength:
Design checks per AS 4100:
- Tension capacity (Whitmore section method): φNt = φ × fy × Ag (φ = 0.9)
- Compression buckling: Check the gusset free edge slenderness ratio Kb/t against AS 4100 Clause 5.14.3
- Block shear: Per AS 4100 Clause 9.2 — usually governs for heavily loaded braces
- Weld capacity: Return welds at the brace-to-gusset interface
Whitmore Section Method
The Whitmore effective width for tension capacity is measured at 30° from the first and last bolt rows (or weld start/end), projected to the bolt centroid. The effective area Aw = (2 × tan(30°) × Lb + w) × tp, where Lb is the bolt group length and w is the connection width at the first bolt. AS 4100 does not specifically name the Whitmore method but the principle is embedded in Clause 9.2 for tension section capacity.
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.