AS 4100:2020: Steel Structures
AS 4100 is the Australian standard for the design of steel structures, published by Standards Australia. The current edition -- AS 4100:2020 -- applies to the design, fabrication, and erection of buildings, bridges, and other structures using hot-rolled or welded steel sections. In New Zealand, the companion standard NZS 3404 historically served a similar role, though AS/NZS alignment continues. This page covers the standard's scope, section organization, capacity reduction factors, key changes from the 1998 edition, and links to every calculator on this site that implements AS 4100 provisions.
Overview of AS 4100:2020
AS 4100 uses a limit state design philosophy, where the design capacity (phi _ R_n) of a member or connection must equal or exceed the design action effect (S_) derived from factored load combinations per AS/NZS 1170. The fundamental check is:
S <= phi * R_u*
where S* is the design action effect, phi is the capacity reduction factor, and R_u is the nominal capacity.
Scope and applicability
AS 4100 covers structural steelwork in buildings and structures where the steel complies with AS/NZS 3678 (plate), AS/NZS 3679.1 (hot-rolled sections), or AS/NZS 3679.2 (welded sections). Common grades include 250, 300, and 350 (with F_y in MPa directly matching the grade designation). The standard does not cover cold-formed steel members (governed by AS/NZS 4600), steel storage racking (AS 4084), or aluminium structures (AS/NZS 1664).
Loading standard
Design actions are determined per AS/NZS 1170 (Structural Design Actions):
- AS/NZS 1170.0 -- General principles
- AS/NZS 1170.1 -- Permanent, imposed, and other actions
- AS/NZS 1170.2 -- Wind actions
- AS/NZS 1170.3 -- Snow and ice actions
- AS 1170.4 -- Earthquake actions in Australia
Units
AS 4100 is fully metric. Forces in kN, moments in kN.m, stresses in MPa, dimensions in mm. All calculators on this site output in these units when AS 4100 is selected.
Key Sections
Section 1: Scope and General
Defines the scope, references normative documents, and establishes notation. AS 4100 uses lowercase symbols extensively: f_y for yield stress, f_u for tensile strength, phi for capacity factor.
Section 2: Materials
Specifies acceptable steel grades and mechanical properties. Key grades:
| Grade | f_y (MPa) | f_u (MPa) | Standard |
|---|---|---|---|
| 250 | 250 | 410 | AS/NZS 3678, 3679.1 |
| 300 | 300 | 440 | AS/NZS 3678, 3679.1 |
| 350 | 350 | 450 | AS/NZS 3678, 3679.1 |
| C350L0 | 350 | 430 | AS/NZS 1163 (HSS) |
| C450L0 | 450 | 500 | AS/NZS 1163 (HSS) |
Section 2 also covers bolt grades (4.6, 8.8, 10.9 per AS/NZS 1252) and weld consumables (matching electrodes per AS/NZS 1554).
Section 3: General Design Requirements
Introduces limit state design principles, load combinations, and capacity factors. Table 3.4 provides the capacity reduction factors (see detailed table below). This section also defines the strength limit state and the serviceability limit state (deflection, vibration).
Section 4: Methods of Structural Analysis
Covers elastic analysis, plastic analysis, and advanced analysis. For connection design, the distribution of forces among fasteners or weld elements depends on the analysis method and whether slip is permitted. Second-order effects (P-delta) must be accounted for in frame analysis.
Section 5: Members Subject to Bending
Governs beam design. Key provisions:
- Cl. 5.1 -- Section capacity in bending: M_s = f_y * Z_e, where Z_e is the effective section modulus accounting for local buckling (compact sections use the plastic modulus S, noncompact and slender sections use reduced values).
- Cl. 5.2 -- Section classification: Sections are classified by plate element slenderness (b/t or d/t ratios). Elements are compact, noncompact, or slender per Table 5.2.
- Cl. 5.6 -- Member capacity in bending: Mb = alpha_m * alphas * M_sx, where alpha_m is the moment modification factor (accounts for moment gradient -- equivalent to C_b in AISC) and alpha_s is the slenderness reduction factor (accounts for lateral-torsional buckling).
- Cl. 5.11 -- Shear capacity: Vv = 0.6 * fy * A_w for sections without shear buckling. For slender webs, a reduced shear buckling capacity applies.
Section 6: Members Subject to Axial Compression
Governs column design:
- Cl. 6.1 -- Section capacity: Ns = k_f * An * f_y, where k_f is the form factor accounting for local buckling of plate elements.
- Cl. 6.3 -- Member capacity: N_c = alpha_c * N_s, where alpha_c is the member slenderness reduction factor. Alpha_c depends on the modified member slenderness (lambda_n), which incorporates the effective length factor (k_e), member length, and radius of gyration. The column curve selection depends on section type and axis of buckling per Table 6.3.3.
- Cl. 6.6 -- Laced and battened compression members: Special provisions for built-up members.
Section 7: Members Subject to Axial Tension
Covers tensile yielding and tensile fracture:
- Cl. 7.1 -- Yield capacity: N_t = A_g * f_y (gross section yielding)
- Cl. 7.2 -- Fracture capacity: Nt = 0.85 * kt * A_n * f_u (net section rupture with correction factor k_t for connection eccentricity)
Section 8: Members Subject to Combined Actions
Interaction equations for combined axial and bending:
- Cl. 8.3 -- Section capacity: Uses the reduced section capacity approach, checking M*_x/M_sx + M*_y/M_sy <= 1.0 with reduced moments accounting for axial load.
- Cl. 8.4 -- Member capacity: Combined compression and bending interaction with member buckling, using the moment amplification approach.
Section 9: Connections
The connection design section, equivalent to AISC Chapter J:
- Cl. 9.2 -- Bolted connections: Bolt shear capacity (Vf = phi * 0.62 _ f_uf _ kr * (nn * Ac + n_x * A_o)), bolt tension capacity, combined shear-tension interaction, bearing capacity, and bolt group analysis.
- Cl. 9.3 -- Pin connections: Design of pin-connected members.
- Cl. 9.6 -- Welded connections: Fillet weld capacity using vw = 0.6 * fuw * t_t * k_r, where f_uw is the weld metal tensile strength and t_t is the design throat thickness. SP (structural purpose) and GP (general purpose) categories have different capacity factors.
- Cl. 9.1.9 -- Block shear (tear-out): A*nt * 0.6 _ f_u + A_gt * f_y for combined tension and shear failure.
Section 10: Brittle Fracture
Addresses notch-ductile steel requirements based on service temperature, plate thickness, and strain rate. Relevant for structures in cold climates or subject to dynamic loading.
Section 11: Fire
Provides methods for determining the fire resistance of steel members, including the limiting steel temperature approach and the strength-at-temperature method. Passive fire protection requirements are determined here.
Section 12: Earthquake
References AS 1170.4 for seismic actions and provides ductility classifications (limited ductile, moderately ductile, fully ductile) for steel structures. Beam-column connection and bracing detailing requirements depend on the ductility class.
Capacity Reduction Factors (Phi)
AS 4100 uses capacity reduction factors (phi) from Table 3.4. The following table shows the factors relevant to calculators on this site.
| Limit State | Phi | Clause |
|---|---|---|
| Members in bending (section and member capacity) | 0.90 | Table 3.4(1) |
| Members in compression (section and member capacity) | 0.90 | Table 3.4(1) |
| Members in tension (yield and fracture) | 0.90 | Table 3.4(1) |
| Members in shear | 0.90 | Table 3.4(1) |
| Combined actions (members) | 0.90 | Table 3.4(1) |
| Bolted connections (shear, bearing, tension) | 0.80 | Table 3.4(2) |
| Welded connections -- SP category | 0.80 | Table 3.4(3)(i) |
| Welded connections -- GP category | 0.60 | Table 3.4(3)(ii) |
| Bolt/nut stripping | 0.80 | Table 3.4(4) |
| Pins | 0.80 | Table 3.4(5) |
Note on SP vs GP welds: Structural Purpose (SP) welds are inspected to a higher standard per AS/NZS 1554.1 and receive phi = 0.80. General Purpose (GP) welds with reduced inspection receive phi = 0.60. For structural connections, SP welds are the norm.
Comparison with AISC: AS 4100 uses phi = 0.80 for connections vs. AISC's phi = 0.75 for bolts and welds. However, the nominal strength formulations differ, so direct factor-to-factor comparison is not meaningful -- you must compare the complete design capacity expression.
Key Differences from AS 4100:1998
AS 4100:2020 is a substantial revision after 22 years (with minor amendments in 2012). Key changes include:
Section classification and capacity
- Revised plate slenderness limits: Table 5.2 slenderness limits for plate elements have been updated, particularly for RHS/SHS sections and built-up sections with residual stress.
- Alpha_s formulation: The slenderness reduction factor for lateral-torsional buckling (alpha_s) received updated coefficients in the buckling curve formulation.
- Form factor k_f: Updated provisions for calculating the effective width of slender plate elements in compression.
Connections
- Bolt group analysis: Updated guidance on bolt group capacity with eccentric loading. The instantaneous center of rotation method is now better defined.
- Block shear: Refined block shear (tear-out) provisions in Cl. 9.1.9 with clearer definition of tension and shear failure planes.
- Weld metal strength: Alignment with current AS/NZS 1554 weld consumable strengths. The nominal weld metal tensile strength f_uw values were updated for current electrode classifications.
Fire and earthquake
- Fire design (Section 11): Significant expansion of the fire design section with updated material property reduction factors at elevated temperature, consistent with ISO 834 fire curve and Eurocode approaches.
- Earthquake provisions (Section 12): Coordinated with the 2018 revision of AS 1170.4, including updated ductility categories and connection detailing requirements.
General
- Advanced analysis (Section 4): Expanded provisions for geometric and material nonlinear analysis with imperfections (GMNIA), recognizing the increasing use of FEA in design practice.
- Notation harmonization: Partially aligned with ISO notation conventions, though the traditional AS 4100 symbol set is largely retained.
- Referenced standards: Updated to current editions of material standards (AS/NZS 3678, 3679), bolting standards (AS/NZS 1252.1), and welding standards (AS/NZS 1554).
Cross-References to Other Standards
| AS 4100 Concept | AISC 360 Equivalent | EN 1993 Equivalent | CSA S16 Equivalent |
|---|---|---|---|
| Capacity factor phi | Resistance factor phi | 1/gamma_M | Resistance factor phi |
| AS/NZS 1170 load combos | ASCE 7 load combos | EN 1990 load combos | NBCC load combos |
| Section 9 (Connections) | Chapter J (Connections) | EN 1993-1-8 | Clause 13 (Connections) |
| Section 5 (Bending) | Chapter F (Flexure) | EN 1993-1-1 Cl. 6.3.2 | Clause 13.5-13.6 |
| Section 6 (Compression) | Chapter E (Compression) | EN 1993-1-1 Cl. 6.3.1 | Clause 13.3 |
| alpha_m (moment mod. factor) | C_b (moment gradient) | C_1 (moment factor) | omega_2 (moment gradient) |
| alpha_s (slenderness factor) | F_cr / F_y (LTB curve) | chi_LT (LTB reduction) | (built into M_r formula) |
| k_e (effective length) | K (effective length) | L_cr / L (buckling length) | K (effective length) |
Available Calculators
Every calculator below implements AS 4100:2020 limit state provisions with full clause-by-clause derivation output. Select AS 4100 as the design code in the calculator interface.
Connection design
- Bolted Connection Calculator -- Bolt shear (Cl. 9.2.2), bearing (Cl. 9.2.2.4), block shear (Cl. 9.1.9), and bolt group analysis. Supports 4.6, 8.8, and 10.9 bolt grades per AS/NZS 1252.
- Welded Connection Calculator -- Fillet weld capacity per Cl. 9.6 for SP and GP categories, with directional loading effects.
- Base Plate & Anchors Calculator -- Concrete bearing per AS 3600, plate bending, and anchor bolt design per AS 4100 and AS 3600.
- Gusset Plate Calculator -- Whitmore section, block shear, and buckling for gusset plate design.
- Splice Connection Calculator -- Bolted and welded splice design per AS 4100.
Member design
- Beam Capacity Calculator -- Section capacity (Cl. 5.1), member capacity with alpha_m and alpha_s (Cl. 5.6), and shear (Cl. 5.11) for UB, UC, PFC, and welded sections.
- Column Capacity Calculator -- Section capacity with k_f (Cl. 6.1) and member capacity with alpha_c (Cl. 6.3) per AS 4100 column curves.
- Beam Deflection Calculator -- Serviceability checks per AS/NZS 1170.0 limits.
Utilities
- Load Combinations (AS 4100) -- ULS and SLS combinations per AS/NZS 1170.0.
- Steel Grades Reference -- f_y and f_u for AS/NZS 3678/3679 grades (250, 300, 350, 400, 450).
- Weld Electrode Reference -- Matching electrodes per AS/NZS 1554 classifications.
Frequently Asked Questions
What is the difference between SP and GP welds? SP (Structural Purpose) welds are fabricated and inspected to AS/NZS 1554.1 with a capacity factor of phi = 0.80. GP (General Purpose) welds have less stringent inspection requirements and use phi = 0.60. Use SP category for all primary structural connections.
Does AS 4100 use ASD or LRFD? AS 4100 uses limit state design exclusively (analogous to LRFD). There is no allowable stress alternative. Design actions (S*) from factored load combinations are compared against design capacities (phi * R_u).
How does alpha_m relate to C_b in AISC? Both are moment modification factors that account for non-uniform bending moment distribution. They serve the same purpose but use different formulations. alpha_m is calculated from the ratio of maximum moment to the bending moment distribution along the unbraced length, using AS 4100 Cl. 5.6.1.1 equations.
Which bolt grades are used in Australia? Standard Australian structural bolts are Grade 4.6 (snug-tight) and Grade 8.8 (fully tensioned for slip-critical and tension applications) per AS/NZS 1252. Grade 10.9 bolts are available for high-strength applications.
Related Pages
- Design Codes Overview
- AISC 360 Design Guide
- EN 1993 (Eurocode 3) Design Guide
- CSA S16 Design Guide
- Tools Directory
- Reference Tables
- AS 4100 Base Plate Worked Example
- AS 4100 Bolt Group Design
- AS 4100 Fillet Weld Design
- How to verify calculator results
- Disclaimer (educational use only)
Copyright and Standards Notice
This page is a high-level educational guide to help engineers navigate AS 4100 provisions and use our calculators effectively. It does not reproduce copyrighted code text, proprietary tables, or design examples from the published standard. For authoritative requirements, purchase the official AS 4100:2020 from Standards Australia.
Disclaimer
This page is provided for general technical information and educational use only. It does not constitute professional engineering advice or a substitute for review by a qualified structural engineer. All structural design depends on project-specific loads, combinations, stability requirements, detailing, fabrication tolerances, and the governing code edition. You are responsible for verifying inputs, validating results independently, and obtaining professional sign-off. The site operator provides this content "as is" without warranties of any kind.