AS 4100 Load Combinations — AS 1170.0 Ultimate & Serviceability Limit States Guide
Complete reference for load combinations in Australian steel design per AS 1170.0:2002 (Structural Design Actions — General Principles), referenced by AS 4100:2020. Ultimate limit state (strength) combinations with load factors, serviceability limit state combinations, stability combinations, importance factors, and a worked example for a steel portal frame building.
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AS 1170.0 Load Combination Framework
AS 1170.0:2002 establishes the basis for structural design actions in Australia. It defines load combinations for both ultimate limit state (ULS — strength and stability) and serviceability limit state (SLS — deflection, vibration, durability). AS 4100:2020 references AS 1170.0 for all load combination requirements.
Unlike ASCE 7 (which uses multiple load combination equations organised by load type) or NBCC 2020 (which uses a principal/companion load framework), AS 1170.0 uses a compact set of combination rules organised by limit state:
Load Types and Symbols
| Symbol | Load Type | Source Standard | Notes |
|---|---|---|---|
| G | Dead load (permanent) | AS 1170.1 | Self-weight of structure, finishes, services |
| Q | Live load (imposed) | AS 1170.1 | Floor loads, partition loads, roof maintenance |
| W_u | Ultimate wind load | AS 1170.2 | Design wind action at ULS |
| W_s | Serviceability wind load | AS 1170.2 | Wind action for SLS (typically lower return period) |
| E_u | Ultimate earthquake load | AS 1170.4 | Seismic action at ULS |
| E_s | Serviceability earthquake load | AS 1170.4 | Seismic action for SLS |
| S | Snow load | AS 1170.3 | Snow on roof (alpine regions) |
| T | Thermal action | AS 1170.5 | Temperature effects |
| A | Liquid pressure | AS 1170.1 | For tanks, reservoirs |
| H | Soil pressure | AS 1170.1 | Retaining walls, foundations |
| F | Settlement, creep, shrinkage | AS 1170.1 | Time-dependent deformation |
Combination Factors
AS 1170.0 uses combination factors (ψ) to account for the reduced probability of multiple loads reaching their maximum simultaneously:
| Load | ψ_s (serviceability) | ψ_l (long-term) | ψ_c (combination — ULS) |
|---|---|---|---|
| Live load — residential | 0.7 | 0.4 | 0.4 |
| Live load — office/general | 0.7 | 0.4 | 0.4 |
| Live load — storage | 0.7 | 0.6 | 0.6 |
| Live load — roof maintenance | 0.7 | 0.0 | 0.0 |
| Live load — parking | 0.7 | 0.4 | 0.4 |
| Live load — assembly areas | 0.7 | 0.4 | 0.4 |
| Wind load | 1.0 | 0.0 | 0.0 |
| Snow load (non-alpine) | 0.0 | 0.0 | 0.0 |
| Snow load (alpine) | 0.7 | 0.0 | 0.0 |
Ultimate Limit State (Strength) Combinations
AS 1170.0 Clause 4.2 specifies the ULS load combinations for strength design. All steel members designed to AS 4100:2020 must be checked against the governing ULS combinations.
Dead + Live Load (No Wind or Earthquake)
| Combination | Equation | Use Case |
|---|---|---|
| 1 | 1.35 × G | Dead load only (minimum) |
| 2 | 1.2 × G + 1.5 × Q | Dead + live (most common) |
| 3 | 1.2 × G + ψ_l × Q + S | Dead + live + snow (alpine regions) |
Combination 2 (1.2G + 1.5Q) is the most frequently governing combination for steel beams and columns in non-cyclonic regions. The dead load factor of 1.2 and live load factor of 1.5 are comparable to AISC 360 LRFD (1.2D + 1.6L) and EN 1990 (1.35G + 1.5Q).
Dead + Live + Wind Load
| Combination | Equation | Use Case |
|---|---|---|
| 4a | 1.2 × G + ψ_l × Q + W_u | Wind as primary action |
| 4b | 1.2 × G + 1.5 × Q + ψ_c × W_u | Live as primary action, wind as companion |
| 5a | 0.9 × G + W_u | Wind uplift (stabilising dead load minimised) |
For steel portal frames in cyclonic regions (Regions C and D per AS 1170.2), combinations 4a and 5a often govern the rafter and column design. The 0.9G factor reflects the reduced probability of full dead load being present during a design wind event.
Dead + Live + Earthquake Load
| Combination | Equation | Use Case |
|---|---|---|
| 6a | G + ψ_l × Q + E_u | Earthquake as primary action |
| 6b | G + ψ_l × Q + ψ_s × E_u | Service-level check (rarely governs) |
Note: The earthquake load factor is 1.0 (not 1.2) for E_u in AS 1170.0. This is because the earthquake action already includes a structural response factor that accounts for ductility and overstrength. AISC 360 similarly uses 1.0E for LRFD seismic combinations.
Dead + Live + Snow (Alpine Regions)
| Combination | Equation | Use Case |
|---|---|---|
| 7a | 1.2 × G + ψ_l × Q + S | Snow primary |
| 7b | 1.2 × G + 1.5 × S + ψ_c × Q | Drift or high snow region |
Snow loads per AS 1170.3 govern only in alpine regions of New South Wales (Snowy Mountains), Victoria (Alpine National Park), and Tasmania (Central Highlands). For most of Australia, snow load combinations are not required.
Stability (Overturning, Sliding, Uplift)
| Combination | Equation | Use Case |
|---|---|---|
| 8a | 0.9 × G + W_u | Overturning with minimum dead load |
| 8b | 0.9 × G + ψ_l × Q + W_u | Sliding check |
| 8c | 1.2 × G + 1.5 × Q | Overturning with destabilising live load |
Stability combinations use reduced dead load factors (0.9) because the dead load provides the restoring force. This is consistent with international practice — AISC 360 uses 0.9D for stability checks.
Serviceability Limit State Combinations
AS 1170.0 Clause 4.3 specifies SLS combinations for deflection, vibration, and durability checks under AS 4100:
| Combination | Equation | Application |
|---|---|---|
| Short-term | G + ψ_s × Q | Immediate deflection under live load |
| Long-term | G + ψ_l × Q | Long-term creep deflection |
| Wind service | G + ψ_s × Q + W_s | Wind-induced lateral drift |
| Earthquake service | G + ψ_s × Q + E_s | Seismic serviceability |
AS 4100 Deflection Limits for Steel Members
AS 4100 Table B1 provides recommended deflection limits:
| Member Type | Δ (live load) | Δ (total load) |
|---|---|---|
| Roof beams | Span/250 | Span/150 |
| Floor beams | Span/300 | Span/200 |
| Gantry girders (manual crane) | Span/400 | Span/300 |
| Gantry girders (electrically operated) | Span/600 | Span/400 |
| Purlins and girts | Span/150 | Span/100 |
| Canopy beams | Span/200 | Span/150 |
Wind Drift Limits
For steel building frames subject to wind load:
| Building Type | Interstorey Drift | Total Drift |
|---|---|---|
| Elastic drift (wind serviceability) | H/300 | H/500 |
| Elastic drift (wind, brittle cladding) | H/500 | H/1000 |
These limits are recommendations, not mandatory requirements. The design engineer may adopt more or less stringent limits based on cladding type, occupant comfort requirements, and client specifications.
Worked Example: Steel Portal Frame Load Combinations
Problem: A steel portal frame building in Melbourne (non-cyclonic Region A) with:
- Dead load G = 5 kN/m on rafter (steel frame + cladding)
- Live load Q = 3 kN/m on rafter (roof maintenance — ψ_l = 0.0)
- Ultimate wind load W_u = 8 kN/m uplift on rafter, 4 kN/m lateral on column
- Wind serviceability W_s = 5 kN/m (10-year return period)
- Frame span = 24 m, column height = 6 m, bay spacing = 6 m
Step 1 — Determine governing ULS combinations for rafter design:
Combination 2 (dead + live — downward): 1.2 × 5 + 1.5 × 3 = 6.0 + 4.5 = 10.5 kN/m downward
Combination 4a (wind primary — uplift): 1.2 × 5 + 0.0 × 3 + 1.0 × (-8) = 6.0 + 0 - 8.0 = -2.0 kN/m uplift
Combination 5a (wind uplift, reduced dead): 0.9 × 5 + 1.0 × (-8) = 4.5 - 8.0 = -3.5 kN/m uplift ← GOVERNING UPLIFT
The rafter must resist 10.5 kN/m downward (governs section capacity in positive bending) and 3.5 kN/m uplift (governs rafter bottom flange restraint and connection design at the ridge and eaves).
Step 2 — Determine governing ULS combinations for column design:
Combination 4a (dead + wind — lateral): 1.2 × 5 + 0.0 × 3 + 1.0 × (-4) lateral = 6.0 kN vertical + 4.0 kN lateral
The column must resist combined compression and bending from the lateral wind load. The maximum moment at the column base from 4.0 kN/m lateral over 6 m height:
M_max = w × L² / 8 = 4.0 × 6² / 8 = 18.0 kN·m (if pinned base) or w × L² / 12 = 12.0 kN·m (if fixed base)
Step 3 — Serviceability check (deflection):
Short-term combination for live load deflection: G + ψ_s × Q = 5 + 0.7 × 3 = 7.1 kN/m
Wind serviceability for lateral drift: G + ψ_s × Q + W_s = 5 + 0.7 × 3 + 5 = 12.1 kN/m (total vertical) + 5 kN/m (lateral)
The elastic lateral drift must not exceed H/300 = 6000/300 = 20 mm for the column top under the wind serviceability combination.
Importance Factors
AS 1170.0 defines importance levels based on building consequences of failure, matching NCC 2022 Table B1.2a:
| Importance Level | Building Type | I_1 (ULS Strength) | I_2 (ULS Stability) | I_3 (SLS) |
|---|---|---|---|---|
| 1 | Low consequence (storage sheds, farm buildings) | 0.87 | 0.77 | 0.77 |
| 2 | Normal (offices, residential, industrial) | 1.00 | 1.00 | 1.00 |
| 3 | High consequence (hospitals, emergency services) | 1.15 | 1.15 | 1.15 |
| 4 | Exceptional (post-disaster facilities, dams) | 1.30 | 1.30 | 1.30 |
Importance factors adjust the nominal loads (particularly wind and earthquake) based on the consequence of failure. Most steel building designs fall under Importance Level 2. For hospitals and emergency services, the 15% increase in design loads significantly affects member sizing and connection design.
Combined Actions with AS 4100
When applying AS 1170.0 load combinations in AS 4100 design:
- Bending and shear: Use the factored actions (M*, V*) from governing ULS combinations in AS 4100 Clause 5 for beam design
- Compression: Use N* from ULS combinations in AS 4100 Clause 6 for column design
- Combined actions: Apply beam-column interaction (Clause 8) using M* and N* from the same ULS combination
- Connections: Design bolts and welds for the maximum forces from any ULS combination (AS 4100 Clause 9)
- Serviceability: Check deflections per AS 4100 Appendix B using SLS combinations
Comparison with Other Codes
| Aspect | AS 1170.0 + AS 4100 | AISC 360 + ASCE 7 | EN 1993 + EN 1990 |
|---|---|---|---|
| Dead load factor | 1.2 | 1.2 (LRFD) | 1.35 |
| Live load factor | 1.5 | 1.6 (LRFD) | 1.5 |
| Wind load factor | 1.0 (ultimate with ψ_l) | 1.0 (LRFD) | 1.5 |
| Earthquake factor | 1.0 | 1.0 (LRFD) | 1.0 |
| Uplift dead factor | 0.9 | 0.9 | 1.0 (favourable) or 0.9 |
| Serviceability | ψ_s factors | Deflection not in code | Quasi-permanent combination |
The Australian system uses wind and earthquake factors of 1.0 at ULS (the nominal return period action already incorporates the probability factor), whereas EN 1990 applies a 1.5 factor to wind. Despite these differences, the resulting design actions for typical buildings are comparable — the Australian 1-in-1000-year wind speed is higher than the European 1-in-50-year value, compensating for the lower load factor.
Frequently Asked Questions
What is the most common load combination for steel beam design in Australia?
The most common governing combination is 1.2G + 1.5Q (dead + live load, AS 1170.0 Combination 2). For roof beams in non-cyclonic regions, this typically governs the positive bending moment design. For portal frame rafters, the uplift combination 0.9G + W_u often governs the bottom flange bracing and connection design. For columns in open buildings, the wind-governed combination 1.2G + ψ_l × Q + W_u typically produces the maximum axial-moment interaction.
How does AS 1170.0 handle wind load combinations differently from ASCE 7?
AS 1170.0 treats wind as a primary action with a combination factor of 1.0 (no additional load factor beyond the nominal wind action at the specified return period), but requires companion live load reduction (ψ_l = 0.4 for most occupancies). ASCE 7 LRFD applies a 1.0 wind factor similarly but uses a 0.5 companion live load factor. The key difference is that AS 1170.2 defines the ultimate wind speed at a 1:1000 return period (compared to ASCE 7's 1:700 for risk category II), so the Australian nominal wind load at ULS is inherently higher than the ASCE 7 value.
When is a 0.9G factor used in AS 4100 design?
The 0.9G factor is used in stability and uplift combinations (AS 1170.0 Combinations 5a and 8a) where the dead load provides the restoring force. This accounts for the statistical probability that the permanent load may be less than the nominal value at the time of the extreme event. Typical applications: wind uplift on portal frame rafters, overturning of cantilever columns, sliding of base plates on foundations, and anchorage design for tension in column hold-down bolts.
Related Pages
- AS 4100 Steel Design Overview — Australia — Full AS 4100 design standard reference
- AS 4100 Column Buckling Guide — Compression member design per AS 4100
- Australian Steel Grades — AS/NZS 3678 & 3679.1 — Material properties reference
- AS 4100 Base Plate Design Guide — Column base plate design per AS 4100
- Beam Capacity Calculator — Free multi-code beam calculator
- Column Capacity Calculator — Free multi-code column calculator
- Section Properties — UB, UC, PFC — Australian section tables
Educational reference only. Load combination rules per AS 1170.0:2002 Clause 4 and AS 4100:2020 Clause 3. Verify applicable importance factors and combination rules for your specific project jurisdiction. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.