Load Combinations — AS 4100 Australian Standard

AS 4100 / AS/NZS 1170 strength, serviceability, and stability load combinations for Australian and New Zealand structural steel design. Educational use only.

This page documents the scope, inputs, outputs, and approach of the AS 4100 Load Combinations tool on steelcalculator.app. The interactive tool runs in your browser; this documentation ensures the page is useful even without JavaScript.

What this tool is for

What this tool is not for

Key concepts this page covers

Inputs and outputs

Typical inputs: characteristic permanent action G, characteristic imposed action Q, wind action W, earthquake action E, snow action S, and the importance level of the structure.

Typical outputs: all applicable ULS and SLS combinations with computed factored values, the governing combination and its factored demand, and the load case identifier.

Computation approach

The tool applies the load combination equations from AS/NZS 1170.0 Table 4.1 (strength) and Section 4.3 (serviceability). Each combination multiplies the characteristic actions by the appropriate partial factors and combination factors. The tool evaluates all applicable combinations and identifies the one producing the maximum demand for the selected effect (axial, shear, moment, etc.).

AS/NZS 1170.0 Strength (ULS) Combinations — Table 4.1

The seven standard strength combinations from AS/NZS 1170.0 Table 4.1:

1.  1.35G                    (permanent action only)
2.  1.2G + 1.5Q             (permanent + imposed)
3.  1.2G + 1.5Q + psi_c Q_c (permanent + imposed + companion imposed)
4.  1.2G + Wu + psi_c Q     (permanent + wind + companion imposed)
5.  1.2G + 1.5Q + Wu        (permanent + imposed + wind)
6.  0.9G + Wu               (permanent stabilising + wind destabilising)
7.  G + Q + Au              (serviceability + accidental, if applicable)

Where:

Combination factors (psi_c) per AS/NZS 1170.0 Table 4.1

Action Type psi_c (Combination) psi_l (Long-term) psi_s (Short-term)
General floor live load 0.40 0.20 0.70
Storage floor live load 0.60 0.50 0.90
Roof live load 0.40 0.00 0.70
Snow (non-mountainous) 0.30 0.00 0.50
Earthquake (static) 0.00 0.00 0.00
Wind 0.00 0.00 0.00

Wind and earthquake are either the principal action (full factor) or absent from the combination; they are never companion actions.

Serviceability (SLS) Combinations

AS/NZS 1170.0 Section 4 defines three SLS combinations:

Short-term:    G + psi_s Q    (e.g., 1.0G + 0.7Q for floor deflection)
Long-term:     G + psi_l Q    (e.g., 1.0G + 0.2Q for creep deflection)
Total:         G + Q          (unfactored, all variable actions)

For steel beams, the short-term combination typically governs live load deflection checks (L/360 for floors, L/240 for roofs). The long-term combination is used for estimating creep effects in composite beams with concrete slabs.

Worked Example — AS 4100 Load Combinations

Problem: A simply-supported steel floor beam (W310x45, Grade 300) spans 7 m and carries: dead load G = 8 kN/m (including self-weight), floor live load Q = 12 kN/m, and the building is in wind region B (non-cyclonic). Determine the governing ULS factored load for beam bending design.

Step 1 — Evaluate each combination

Combination 1: 1.35G = 1.35 × 8 = 10.8 kN/m
Combination 2: 1.2G + 1.5Q = 1.2 × 8 + 1.5 × 12 = 9.6 + 18.0 = 27.6 kN/m
Combination 4: 1.2G + Wu + 0.4Q = 1.2 × 8 + Wu + 0.4 × 12 = 9.6 + Wu + 4.8
               (wind may or may not govern for this floor beam)
Combination 6: 0.9G + Wu = 0.9 × 8 + Wu = 7.2 + Wu (uplift check)

Step 2 — Identify governing combination

Combination 2 governs for gravity: w* = 27.6 kN/m

M* = w*L²/8 = 27.6 × 7²/8 = 27.6 × 6.125 = 169.1 kN.m
V* = w*L/2 = 27.6 × 7/2 = 96.6 kN

Check against beam capacity per AS 4100:
W310x45: Zx = 625 × 10³ mm³, phi = 0.9
phiMs = 0.9 × 625 × 10³ × 300 / 10⁶ = 168.8 kN.m

M* = 169.1 kN.m ≈ phiMs = 168.8 kN.m → MARGINAL
Consider upgrading to W310x52 or checking if plastic analysis applies.

Step 3 — Serviceability check

Short-term SLS: G + 0.7Q = 8 + 0.7 × 12 = 16.4 kN/m
Long-term SLS: G + 0.2Q = 8 + 0.2 × 12 = 10.4 kN/m

Live load deflection (short-term):
Δ = 5 × 0.7Q × L⁴ / (384EI) = 5 × (0.7 × 12/1000) × 7000⁴ / (384 × 200000 × 99.5 × 10⁶)
Δ = 5 × 0.0084 × 2.401 × 10¹⁵ / 7.637 × 10¹² = 13.2 mm

L/360 = 7000/360 = 19.4 mm → 13.2 < 19.4 ✓

Comparison of International Load Combination Standards

Standard Primary Gravity Factor Live Load Factor Wind Factor Counteracting Dead Key Feature
AS/NZS 1170 (AU) 1.2G + 1.5Q 1.5 1.0 Wu 0.9G Companion psi factors
ASCE 7-16 (US) 1.2D + 1.6L 1.6 1.0W 0.9D Fixed load factors
EN 1990 (EU) 1.35G + 1.5Q 1.5 1.5W 1.0G (fav.) Twin equations 6.10a/b
NBCC (CA) 1.25D + 1.5L 1.5 1.4W 0.9D Principal/companion

Note: Wind factors are not directly comparable because wind speeds and pressure calculations differ between standards. AS/NZS 1170.2 applies a separate importance factor to the wind speed rather than the load factor.

Frequently Asked Questions

How do AS/NZS 1170 load combinations differ from ASCE 7? The general framework is similar (factored combinations at ULS, unfactored at SLS), but the partial factors and combination rules differ. AS/NZS 1170.0 uses psi factors (combination factor psi_c, long-term factor psi_l, short-term factor psi_s) to reduce imposed actions when combined with other variable actions, whereas ASCE 7 uses a single set of load factors for each combination. The companion action concept is equivalent but the numerical values are calibrated to different reliability targets.

What is the companion action combination factor psi_c? When multiple variable actions are present in a load combination, the dominant action is taken at its full characteristic value while companion actions are reduced by psi_c. For most floor live loads, psi_c = 0.4; for storage, psi_c = 0.6; for wind, psi_c = 0.0 (wind is either the dominant action or absent in the combination). This reflects the low probability of two variable actions reaching their peak values simultaneously.

When do I use 0.9G instead of 1.2G? The 0.9G factor applies in combinations where permanent action is stabilising and the variable action (such as wind uplift or overturning) is destabilising. For example, the combination 0.9G + W_u checks whether wind uplift exceeds the stabilising effect of self-weight. Using 0.9G with destabilising actions is essential for checking uplift, sliding, and overturning stability.

What is the difference between short-term and long-term SLS combinations? The short-term combination uses psi_s (typically 0.7 for floor live load) to represent the peak load that occurs over a short period, used for checking deflection limits like L/360. The long-term combination uses psi_l (typically 0.2 for floor live load) to represent the sustained load that causes creep and long-term deflection. Both must be checked separately because different limit states apply.

How does the importance level affect load combinations in AS/NZS 1170? AS/NZS 1170.0 defines importance levels 1 through 4 that affect the annual probability of exceedance for wind, snow, and earthquake actions. Importance level 1 (farm buildings) uses lower wind speeds, while importance level 4 (post-disaster facilities) uses higher wind speeds and higher seismic hazard. The load factors in Table 4.1 remain the same, but the characteristic actions W, S, and E are scaled by the importance level through the annual probability of exceedance.

Why is the dead load factor 1.35 in combination 1 but 1.2 in combination 2? Combination 1 (1.35G) covers the case where only permanent action is present. The higher factor reflects that the uncertainty in dead load is not offset by the statistical independence of variable actions. When a variable action is present (combination 2), the probability of both actions simultaneously exceeding their characteristic values is lower, so the dead load factor can be reduced to 1.2 while the live load factor is 1.5.

What is the difference between AS 4100 and NZS 3404 for steel design? AS 4100 is the Australian steel structures standard, while NZS 3404 is the New Zealand equivalent. Both share the same load combination framework from AS/NZS 1170.0, but NZS 3404 has additional seismic design provisions reflecting New Zealand's higher seismic hazard, including capacity design requirements, member ductility checks, and specific connection design rules for seismic-resisting systems. For non-seismic design, the two standards produce nearly identical results.

What is the capacity reduction factor phi in AS 4100? AS 4100 uses capacity reduction factors (phi) to reduce the nominal section capacity to the design capacity. For bending: phi = 0.9. For compression: phi = 0.9. For shear: phi = 0.9. For tension: phi = 0.9 (yielding) or 0.9 (fracture, with additional factor). These are applied after computing the nominal capacity using the full steel yield stress. The phi factor accounts for variability in material properties, fabrication tolerances, and the accuracy of the design model.

How do crane loads enter AS/NZS 1170 combinations? Crane loads are treated as a special imposed action in AS/NZS 1170.0. Vertical wheel loads include an impact factor (typically 1.25 for cab-operated cranes, 1.10 for pendant-operated). Horizontal lateral forces from crane trolley movement (typically 20% of lifted load plus trolley weight) and longitudinal braking forces are applied as separate load cases. Fatigue loading from crane cycles is checked separately per AS 4100 Section 11 with load factors of 1.0.

What is the difference between a structural analysis model and load combinations? A structural analysis model computes member forces (moments, shears, axial loads) for a given set of loads. Load combinations tell you which loads to apply simultaneously and with what factors. The analysis model is run once for each load combination, producing a different set of member forces each time. The designer then checks each member against the most critical set of forces from any combination. Software automates this by running all combinations and extracting the envelope of maximum forces.

What is the significance level in AS/NZS 1170.0? AS/NZS 1170.0 defines four importance levels that correspond to annual probabilities of exceedance for wind, snow, and earthquake. Level 1 (low consequence, annual probability 1:100) applies to farm buildings. Level 2 (ordinary, 1:250-1:500) applies to standard buildings. Level 3 (major, 1:1000) applies to schools and public assembly. Level 4 (post-disaster, 1:1500-1:2500) applies to hospitals and emergency services. Higher importance levels result in higher design loads because the acceptable probability of failure is lower.

What is the difference between strength and stability limit states in AS 4100? Strength (section capacity) checks that the member can resist the factored loads without yielding or crushing. Stability (member capacity) checks that the member will not buckle under the factored loads. For a steel beam, the strength check compares Mu to phiMs (plastic moment capacity), while the stability check compares Mu to phiMb (buckling capacity, which is lower). AS 4100 Clause 5.1 requires checking both: section capacity (nominal moment) and member capacity (moment with lateral buckling reduction).

What is the notional horizontal load in AS 4100? AS 4100 Clause 3.2.4 requires a notional horizontal load of 0.002 times the total vertical load applied at each floor level, to account for out-of-plumbness and construction tolerances. This load is applied simultaneously with the factored gravity loads and acts in the direction that produces the most adverse effect. For a building with 10,000 kN total gravity load at a floor, the notional horizontal load is 20 kN at that level. The notional load need not be applied simultaneously with wind or earthquake loads, but it must be considered when lateral loads are small or absent.

What are the deflection limits for steel beams per AS 4100? AS 4100 does not prescribe mandatory deflection limits; these are governed by AS/NZS 1170.0 and the project specification. Standard industry practice for steel beams: floor beams L/360 for live load and L/240 for total load, roof beams L/240 for total load, and crane runway beams L/500 to L/1000 depending on the crane class. For composite beams supporting brittle finishes (masonry walls, terrazzo), tighter limits of L/500 to L/600 may be specified. The designer must verify that the calculated deflection does not damage supported construction or impair the building function.

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Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

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