Australian Beam Capacity Calculator — AS 4100

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Design per AS 4100:2020

This calculator checks steel beam flexural and shear capacity per AS 4100-2020 (Steel Structures Standard) for Australian UB and UC sections. Select "AS 4100" from the code selector at the top of the calculator to use Australian capacity factors and design rules.

Section Moment Capacity — Clause 5.2

For compact sections: φMsx = φ × Zex × fy (φ = 0.9)

The section moment capacity depends on the section's compactness classification per AS 4100 Table 5.1. Compact sections with λ ≤ λs can develop the full plastic moment. If the section is non-compact or slender, Zex is replaced by the effective section modulus Ze.

Member Moment Capacity — Clause 5.6

For laterally unrestrained beams: φMbx = φ × αm × αs × Zex × fy

• αm = moment modification factor (similar to AISC Cb)
• αs = slenderness reduction factor based on Mo (elastic lateral buckling moment)
• When the beam is fully laterally restrained, φMbx = φMsx

Shear Capacity — Clause 5.10

φVv = φ × 0.6 × fy × Aw (φ = 0.9)

For stiffened or slender webs, the shear buckling capacity Vb per Clause 5.11 may govern. Aw = d × tw for hot-rolled sections.

Worked Example

Problem: Check a 530UB92.4 beam, Grade 300, with full lateral restraint.

Span: 8.0 m | Load: w* = 45 kN/m (factored) | Steel: Grade 300 (fy = 300 MPa)

Solution:

1. Factored moment: M* = 45 × 8²/8 = 360 kNm
2. Section properties: Zex = 2480 × 10³ mm³ (from section table)
3. Section capacity: φMsx = 0.9 × 2480 × 10³ × 300 × 10⁻⁶ = 669.6 kNm
4. Check: 360 ≤ 669.6 → OK (usage: 54%)
5. Shear: V* = 45 × 8 / 2 = 180 kN; φVv = 0.9 × 0.6 × 300 × 9.9 × 528 × 10⁻³ = 847 kN → OK

Result: 530UB92.4 is adequate for both flexure and shear.

How to Use

1. Select "AS 4100" from the code selector
2. Choose an Australian UB or UC section from the database
3. Select Grade 300 or Grade 350 steel
4. Enter the unbraced length (0 for fully restrained)
5. Enter the applied factored moment and shear
6. Review φMs, φMb, and φVv results

Related Australian Resources

• Australian Beam Sizes (UB/UC/PFC)
• Australian Steel Grades
• AS 4100 Beam Design Example
• Australian Column Capacity Calculator
• All Australian References

FAQ

What is the difference between φMs and φMb? φMs is the section moment capacity assuming full lateral restraint (no LTB). φMb is the member moment capacity accounting for lateral-torsional buckling. For beams with continuous lateral restraint, φMb = φMs. For unrestrained beams, φMb ≤ φMs per AS 4100 Clause 5.6.

Can I check laterally unrestrained beams? Yes. Enter the segment length between points of lateral restraint, and the calculator computes φMb using the elastic lateral buckling moment Mo per AS 4100 Clause 5.6.1.2.

What steel grades are available for AS 4100? Grade 300 (fy = 300 MPa, fu = 440 MPa) for standard sections and Grade 350 (fy = 350 MPa, fu = 480 MPa) for higher-strength applications per AS/NZS 3679.1.