AS 4100:2020

This page covers design checks to AS 4100:2020, Steel Structures for hot-rolled and welded I-sections, channels, tees, angles and hollow sections, with optional local bearing checks where the bearing details are supplied.

Open AS 4100:2020 Calculator

Standard Reference

Supported Geometry Families

Six geometry families are supported; catalogue variants such as UB/UC and RHS/SHS are grouped within their respective families.

• I-Shapes (UB, UC, WB, WC - rolled and welded)
• Channels (PFC)
• Tees (BT, CT catalogue sections; use the Sideways option to change section orientation in the FEA workflow)
• Rectangular Hollow Sections (SHS, RHS - hot-formed and cold-formed)
• Circular Hollow Sections (CHS)
• Angles (EA, UA - equal and unequal)

Checks Performed

The design-check table lists the check and clause behind each member result. Summary by clause group:

• Section classification (Table 5.2 / Cl 6.2.4): Bending classification and compression form-factor effects are selected from the active section state.
• Tension (Cl 7.2): Gross yielding and net-section fracture are checked using optional member An; blank An defaults to Ag because fastener-hole geometry is not inferred. Blank kt uses the Table 7.3.2 value for the section family. A valid tensile strength is required for fracture and dependent tension interactions.
• Compression (Cl 6.2 / Cl 6.3.3): Ordinary major/minor flexural buckling is checked. The additional flexural-torsional buckling (FTB) mode is numerical for fabricated unequal-flange I-sections, fabricated equal-flange channels, and minor-axis-braced hot-rolled channels; unequal-flange fabricated channels remain incomplete. AS 4100 design checks use E = 200,000 MPa and G = 80,000 MPa under Clause 2.2.4, independently of analysis stiffness inputs.
• Flexure (Cl 5.2 / Cl 5.6): Unequal-I section bending uses each flange's geometry. Supported sharp-cornered (no root fillet) segments restrained at both ends use Cl 5.6.1.2 monosymmetric LTB and applicable Cl 8.4 interactions. Reverse curvature uses the lower flange elastic buckling moment and a moment modification factor of 1. With continuous restraint, sagging uses section moment capacity and hogging is checked for member LTB; other configurations or missing properties explicitly omit member LTB while preserving section checks.
• Shear (Cl 5.11.3-Cl 5.11.5 / Cl 5.12.3): Shear, shear buckling, non-uniform shear, and shear-moment checks are reported for the governing local case.
• Bearing (Cl 5.13 / Cl 6.3.3): Optional local bearing checks are reported when R*, bearing length, distance from the member end, and restraint inputs are supplied; RHS/SHS bending-bearing interaction is included where applicable.
• Combined actions (Cl 4.4.2 / Cl 8.3 / Cl 8.4): Section and member axial-plus-bending checks use the applicable compression or tension provisions. Linear FEA and first-order standalone compression-bending checks apply Clause 4.4.2 member amplification; a factor above 1.4 requires an Appendix E second-order analysis. P-Delta FEA checks assess local member effects under Appendix E and retain the larger of the analysis and amplified member moments for section and member checks. The standalone External second-order moments option uses the entered moments directly. Verify that external analysis and frame sway separately. Invalid amplification prevents completion of the affected interaction.

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. Fabrication provenance also controls the member section constant (α_b) in Table 6.3.3: generic welded or unknown plate provenance uses the as-rolled welded value, while the more favourable flame-cut value is used only when Welded (Flame-Cut Plates) is selected explicitly.

Generic welded fabrication defaults conservatively to HW. LW requires verified compressive residual stresses below 40 MPa (Table 5.2 note). An incompatible fabrication/residual-stress pair returns INCOMPLETE with no capacity.

I-section, parallel-flange channel and tee plate widths follow the ASI/manufacturer convention without deducting root fillets. I/tee flange outstands use (B - tw)/2, channel outstands use B - tw, and I webs use D - Tt - Tb, including unequal flanges. Gross area, inertia and plastic moduli retain the physical fillets. Clauses 5.2.2 and 6.2.3 do not settle the deduction of rolled fillets from plate widths, so this is the adopted convention.

LTB Restraint Inputs

For I-shapes, channels, RHS, tees, and angles, new inputs default to Table 5.6.3 effective-length factors with top-flange gravity loading, FF end restraint (FU for cantilevers), loading within the segment, and no rotational restraint benefit. Check these assumptions against the actual member. With table factors disabled, enter the final lateral-buckling effective length; the load-height and restraint settings below then do not apply. For tees and angles, the Table 5.6.3(A) twist factor k_t takes d₁ = D − t_f (not the tension factor k_t of Clause 7.3).

• Load height: shear centre or top flange; sets k_l for gravity loading.
• Segment restraint: FF, FP, FL, PP, PL, LL, FU, or PU end-restraint arrangements.
• Load position: within segment or at segment end.
• Rotation restraint: neither end, one end, or both ends for the k_r reduction where Table 5.6.3(C) allows it.

For a cantilever (a segment unrestrained at one end), α_m is taken from Table 5.6.2 for the selected load case: end moment, tip point load, or uniformly distributed load. If no load case is selected, member LTB is reported incomplete; loading not covered by Table 5.6.2 requires a separate Clause 5.6.4 elastic buckling analysis. With Table 5.6.3 factors a cantilever uses the FU or PU arrangement with k_r = 1.0, and top-flange gravity loading gives k_l = 2.0 whether the load is within the segment or at its end. In the FEA Design tab the Cantilever control has three settings: Auto (default) follows the model and shows what it detected, for example "Auto (detected: cantilever, held at Node A)"; Span and Cantilever override the detection. CHS and SHS have no lateral-torsional buckling mode, so M_b = M_s (Clause 5.3.2.1) and the cantilever, L_e, α_m and Table 5.6.2/5.6.3 inputs do not apply to them.

Section Geometry

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

Material Properties

Member Lengths & Bracing

Continuous Restraint means the top flange is continuously restrained, for example by a concrete slab or welded metal deck. Sagging moment (negative M_x*) compresses the restrained top flange, so the segment is fully restrained and M_b = M_s. Hogging moment (positive M_x*) compresses the bottom flange, which is not braced by this setting (Clauses 5.3.2.2 and 5.5.2). The hogging member moment capacity is therefore checked over L_e with the α_m, load height, cantilever and Table 5.6.3 settings and no credit for the top-flange restraint, and is reported as a separate check. Clause 8.4 member checks use M_rx for sagging and the hogging φM_bx for hogging. Enter KLT = 0 if the bottom flange is also braced. The compression K_t is set independently of this setting. For single angles the setting applies to the whole member.

Clause 6.3.3 ordinary major/minor flexural buckling is checked for supported compression members. For a hot-rolled channel, confirming minor-principal-axis bracing activates the additional flexural-torsional mode. The AS 4100 design check evaluates the referenced AS/NZS 4600 equations locally, then applies the AS 4100 reduction of 0.85 and capacity factor of 0.90. The effective area A_e = k_fA_g is evaluated at f_y (N_c = A_ef_n per AS/NZS 4600 Clause 3.4.1), and N_c is capped at N_s. An unbraced hot-rolled channel is checked for ordinary flexural buckling only.

Design Actions

Bearing Inputs

AS 4100 Clause 5.13 bearing is optional because it depends on local support or patch-load details that are not determined by the member force envelope alone.

I-shapes and channels report web bearing yield and bearing buckling. RHS/SHS sections report flat-wall bearing yield and buckling, plus Cl 5.13.5 bending-bearing interaction where bending demand is present. The interaction uses the moment at the entered bearing position on the selected local section axis, not a member moment envelope. The Sideways option and global member orientation do not alter the selected physical face.

Capacity Reduction Factor

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

Limitations & Notes

• Minimum open-section thickness: AS 4100 Clause 1.1.2(a) excludes steel elements thinner than 3 mm, apart from the stated AS/NZS 1163 section and packer exceptions. I-shape, channel, tee, and angle members with a thinner web, flange, stem, or leg return OUT_OF_SCOPE rather than a design capacity, and the standalone manual-section form rejects those dimensions.
• Analysis and amplification basis: Linear FEA and first-order standalone compression-bending checks apply Clause 4.4.2.2 braced member amplification, with N_omb from the braced effective length factor (k_e no greater than 1.0, Clause 4.6.3). An entered k above 1.0 describes a sway member: Clause 4.4.2.3 sway amplification is not evaluated on a first-order analysis, and a Sway Amplification note qualifies the combined actions checks. A factor above 1.4 requires an Appendix E second-order analysis; on a linear run the compression-bending check is INCOMPLETE. P-Delta FEA checks assess local member effects under Appendix E and retain the larger of the analysis and amplified member moments for section and member checks. Where the member moment diagram is not available, the supplied moments are amplified conservatively; where amplification cannot be evaluated, the affected interaction check is not completed. The standalone calculator does not run a frame analysis; its External second-order moments option uses entered moments directly. Verify the external analysis and frame sway separately.
• Custom-section axes: Check that the major and minor axes, moments of inertia, and elastic and plastic section moduli match the section preview. Incorrect axis assignments affect bending, buckling and LTB resistance.
• Single-element Z_e: Effective section modulus uses the governing slender element for the reported stress state. Rare edge cases with multiple slender elements may need separate review.
• Web yield stress: Web shear (Cl 5.11.4) uses f_yw from the section strength record, or F_y where none is given (hollow sections, angles).
• Non-uniform shear distribution: Clause 5.11.3 is applied according to section type and shear direction.
• Cold-formed design thickness: The AS/NZS 1163 design-thickness convention is applied to slenderness calculations for cold-formed hollow sections, but not to gross area.
• Sideways members: Analysis and design use the physical section axes after the Sideways rotation, so read bending and shear actions against the displayed section orientation; no additional manual swap of major and minor actions is required.
• Flexural-torsional buckling: Bending LTB and compression FTB are separate checks. The additional Clause 6.3.3 compression FTB mode is evaluated for fabricated unequal-flange I-sections, fabricated equal-flange channels, and hot-rolled channels with confirmed minor-axis bracing. Unequal-flange fabricated channels remain incomplete because validated principal-axis and shear-centre properties are unavailable.
• Net sections and connection force distribution: Tension fracture under Cl 7.2 uses the entered A_n and k_t, with the defaults listed under Material Properties. The Cl 8.4.4.2 member tension check uses the governing Cl 7.2 resistance, including fracture. The Cl 8.3 section interaction also uses that resistance, since Cl 8.3.1 takes N_s in tension as the Cl 7.2 capacity. Fastener holes, staggered paths, and detailed Clause 7.3 connection layouts are not inferred.
• Minimum web thickness (Cl 5.9.3, 5.10.1): For I-sections and channels the unstiffened web must be at least (d₁/180)√(f_yw/250) thick. A thinner web fails the Web Thickness check; stiffened webs and the Clause 5.9.3 rational-analysis allowance are not evaluated.
• Bearing and stiffeners (Cl 5.13-5.16): Cl 5.13 bearing is checked only when bearing inputs are supplied; R*, bearing length, bearing position, loaded face, and web restraint are not inferred from reactions or point loads. Where bending is present, RHS/SHS bending-bearing interaction is reported incomplete until a valid bearing position is entered. Load-bearing stiffeners, end posts, and transverse or longitudinal web stiffeners remain separate checks.
• Bracing and restraint systems: Effective lengths, unbraced lengths, load height, cantilever load type, and continuous restraint are design inputs. The design check does not design brace members, restraint connections, diaphragms, anchorage, or the Clause 6.6 restraint forces for compression members.
• Built-up and compound members: Laced, battened, back-to-back, and multi-component tension or compression member detailing in Clauses 6.4-6.5 and 7.4 is outside the automatic member check.
• Connection-region checks: Section 9 bolt, weld, pin, ply bearing, block shear, prying, and minimum connection action checks are separate from the member-strength result.
• Channel LTB: Equal-flange channels use the Clause 5.6.1.1 M_o for equal-flange open sections (β_x = 0, Appendix H.4). AS 4100 gives no M_o for a channel with unequal flanges, so its member moment capacity is reported incomplete and needs a separate Clause 5.6.4 buckling analysis.
• α_m computation: For segments restrained at both ends, the automatic factor uses the quarter-point moment distribution. A cantilever takes α_m only from the selected Table 5.6.2 load case; the quarter-point formula and a manual factor alone do not complete cantilever LTB.
• Design stiffness: Resistance and stability checks use E = 200,000 MPa and G = 80,000 MPa under Clause 2.2.4. Material stiffness entered for structural analysis does not change these design values.
• Torsion (Bredt-Batho): Not an input in the standalone calculator. In the FEA tool, torsion is checked for closed hollow sections (RHS/SHS/CHS) only. The combined check adds peak transverse wall shear (both axes) and torsional shear along the member: a conservative single-cell, constant-thickness thin-wall envelope, not an AS 4100 equation. Corner and detail effects, restrained warping, and applied open-section torsion require separate verification.
• Serviceability and special design states: The analysis reports displacements, but the AS 4100 member check does not include serviceability limits, vibration, bolt slip, corrosion protection, brittle fracture, lamellar tearing, fatigue, fire, or earthquake detailing.

Verification

The public verification pages contain 38 published comparisons with an average numerical difference of 0.09%. Classification outcomes do not contribute to the numerical mean.