AS 4100:2020

The AS 4100 engine implements AS 4100:2020 - Steel Structures, the Australian standard for the design of steel structures. The engine supports all common fabrication types and residual stress categories.

Open AS 4100:2020 Calculator

Standard Reference

Supported Section Types

• I-Shapes (UB, UC - rolled and welded)
• Channels (PFC)
• Tees (BT, CT - beam tees and column tees cut from UB/UC parents)
• Rectangular Hollow Sections (SHS, RHS - hot-formed and cold-formed)
• Circular Hollow Sections (CHS)
• Angles (EA, UA - equal and unequal)

Checks Performed

Section Classification (Table 5.2)

Classification is action-specific - different slenderness limits apply for members in compression versus flexure. Elements are classified using plate element slenderness (λ_e) against plasticity (λ_ep) and yield (λ_ey) limits.

Residual Stress Categories

The engine accounts for residual stresses via fabrication-dependent plate element yield slenderness limits:

Effective Section Properties (Clause 5.2 & 6.2)

For noncompact and slender sections, the engine computes effective section modulus Z_e(Clause 5.2, for bending) and form factor K_f (Clause 6.2.4, for compression) using the governing plate element slenderness approach.

Tension (Clause 7)

• φN_t: Gross section yielding (φ × A_g × f_y)
• φN_t: Net section fracture (φ × 0.85 × k_t × A_n × f_u)

Compression (Clause 6)

• Section capacity: φN_s = φ × K_f × A × f_y
• Member capacity: φN_c = φ × K_f × A × f_cr(λ) using member section constant α_b per Table 6.3.3
• Flexural-torsional buckling for monosymmetric sections (tees, channels)

Flexure (Clause 5)

• Section moment capacity: φM_s = φ × Z_e × f_y
• Member moment capacity (LTB): φM_b computed using α_m (moment modification factor), k_l (load height factor), and the elastic buckling moment M_o
• Load height: k_l = 1.4 for top-flange loading (destabilising), 1.0 for shear centre, 2.0 for cantilevers
• α_m: Auto-computed from the moment distribution along the member. Can be overridden in the calculator.
• Monosymmetry constant β_x included for channels and tees

Shear (Clause 5.11)

• Strong-axis shear: web area for I-shapes and channels, two webs for RHS, full area with 0.36f_y coefficient for CHS (Clause 5.11.4)
• Weak-axis shear: flanges for I-shapes and channels, perpendicular walls for RHS
• Design thickness (0.93× nominal) applied for cold-formed sections per AS/NZS 1163

Combined Actions (Clause 8)

• Compression + Bending (8.3.3/8.3.4): General linear interaction is checked for all sections. For compact I-shapes and RHS, the alternative power-law form is also evaluated and the governing section ratio is taken conservatively.
• Tension + Bending (8.3.2/8.3.4): General linear interaction is checked for all sections. For compact I-shapes and RHS, the alternative power-law form is also evaluated and the governing section ratio is taken conservatively.
• Shear-Bending (5.12.3): Reduced shear capacity when M* > 0.75φM_s

Calculator Inputs

The standalone AS4100 calculator accepts the following inputs. All values are in metric units.

Fabrication & Residual Stress

Two dropdowns control how residual stresses affect section classification and buckling:

The residual stress category determines the plate element yield slenderness limits (λ_ey) in Table 5.2 and the member section constant (α_b) in Table 6.3.3.

Load Height

For I-shapes, channels, and tees, a dropdown selects the load application point:

• Shear Centre (k_l = 1.0) - default, load applied at the shear centre
• Top Flange (k_l = 1.4) - load applied at top flange (destabilising for LTB)

When the Cantilever toggle is enabled with top flange loading, k_l is automatically increased to 2.0 per Clause 5.6.3(2).

Section Geometry

Select a shape group, then enter dimensions or pick from the built-in section library (Australian sections).

Material Properties

Member Lengths & Bracing

A Continuously Restrained checkbox sets L_b = 0, bypassing the LTB check.

Design Actions

Capacity Reduction Factor

A single capacity reduction factor of φ = 0.9 is used across all checks (tension, compression, bending, and shear) per AS 4100:2020 Table 3.4.

Limitations & Notes

• Single-element Z_e: The effective section modulus calculation uses a governing-slenderness approach rather than simultaneously reducing all slender elements. This is conservative and accurate for standard rolled sections; rare edge cases with both slender flanges and webs may see minor differences.
• Non-uniform shear distribution: The f*vm/f*va ratio in Clause 5.11.3 is chosen per section type and shear direction: 1.5 for rectangular flanges (I, tee, RHS), 2.0 for channel flange shear (asymmetric shear flow), Q·D/I elastic-flow ratio for I and channel webs, 1.0 (no reduction) for angle legs. AS4100 is silent on whether 5.11.3 applies to angles; the angle-leg choice follows common industry benchmark practice.
• Cold-formed design thickness: 0.93× nominal thickness per AS/NZS 1163 is applied to slenderness calculations (b/t ratios) for cold-formed hollow sections, but not to gross area.
• Sideways members: For members marked sideways=true, AS4100 major/minor design actions and capacities are reported after the section orientation mapping is applied. These design-axis results may not line up one-for-one with raw local My/Mz result labels or third-party report row headings for mono-symmetric sections (tees, channels, angles), even when the governing capacities match.
• Flexural-torsional buckling: Monosymmetry constant β_x is included in the LTB elastic buckling moment M_o for tees and channels. A standalone FTB member compression check is not performed. Channels and angles used as compression members should verify FTB separately per Clause 6.3.3.
• Bearing capacity (Cl 5.13): Web bearing yield (R_by) and buckling (R_bb) at supports and point loads are not checked. Verify bearing capacity separately for concentrated loads and reactions.
• α_m computation: Auto-computed from the quarter-point moment distribution. For specific loading patterns, the manual override is available.
• Torsion (Bredt-Batho): Not an input in the standalone calculator. In the FEA tool, torsion is checked for closed hollow sections (RHS/CHS) only. Warping torsion for open sections (I-beams, channels, angles, tees) is not performed.

Verification

The engine is benchmarked against 58 independent test cases from the Steel Structures Sample Worked Problems, the Design Manual by Kirke and Al-Jamel, and STAAD.Pro verification cases, with an average difference of 0.11%.