Australian Snow Load — AS 1170.3:2003 Snow & Ice Actions for Steel Design

Complete reference for snow load calculation on steel structures in Australian alpine regions per AS 1170.3:2003 (Structural Design Actions — Snow and Ice Actions). Ground snow loads for the Snowy Mountains (NSW), Victorian Alps, and Tasmanian highlands. Roof snow load shape coefficients, drift loads at steps and parapets, snow load combinations with AS 4100 steel design, and a worked example for a steel portal frame in Charlotte Pass.

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Australian Snow Load Context

Snow loads in Australia are limited to alpine regions — primarily the Snowy Mountains of New South Wales, the Victorian Alps (Mount Hotham, Falls Creek, Mount Buller), and the Tasmanian Central Highlands. Unlike Canada, Scandinavia, or the European Alps where snow loads are a primary design consideration for most buildings, snow loads in Australia govern only for:

• Buildings above approximately 1,200 m elevation in NSW and Victoria
• Ski resort infrastructure (lodges, ski lifts, equipment buildings)
• Alpine communication towers and weather stations
• Tasmanian highland structures (e.g., Central Plateau, Mount Wellington)

For the vast majority of Australian steel buildings in coastal cities and lowland areas, snow loads are not considered in design — the live load and wind load combinations govern.

Ground Snow Loads (S_s) — Design Values

AS 1170.3 defines the ground snow load S_s (kPa) as the 1-in-50-year snow load on the ground. The ground snow load varies with elevation and location:

S_r (rain-on-snow) is the additional load from rain falling on an existing snowpack. It is applied separately in the load combination.

For non-alpine locations (elevation < 1,000 m), S_s can be taken as zero unless local experience indicates otherwise.

Roof Snow Load Calculation

AS 1170.3 Clause 4 specifies the roof snow load:

S = S_s × C_shape × C_e × C_t + S_r × C_rain

Where:

• S_s = ground snow load (from AS 1170.3 Appendix A or site-specific study)
• C_shape = roof shape coefficient (accounts for roof slope, drift, accumulation)
• C_e = exposure coefficient (accounts for wind scour)
• C_t = thermal coefficient (accounts for snow melt from building heat loss)
• S_r = rain-on-snow load (if applicable)
• C_rain = rain-on-snow coefficient (typically 0.5-1.0)

Roof Shape Coefficient (C_shape)

For pitched roofs (steel portal frame buildings):

For an alpine steel portal frame with 15° roof pitch: C_shape = 0.8 (no reduction).

For a 30° pitch: C_shape = 0.8 × (60-30)/45 = 0.8 × 30/45 = 0.53.

Exposure Coefficient (C_e)

Most alpine building sites in Australia are classified as "normal" exposure (C_e = 1.0). Buildings in sheltered valleys subject to heavy accumulation may use C_e = 1.2.

Thermal Coefficient (C_t)

For typical alpine steel buildings (ski lodge, equipment storage): C_t = 1.0-1.2.

Snow Drift Loads

AS 1170.3 requires consideration of drifted snow at:

1. Steps and changes in roof height — Accumulation on the lower roof adjacent to a higher roof
2. Parapets and roof obstructions — Accumulation upwind of parapets
3. Valley roofs — Accumulation in roof valleys
4. Gable roofs — Unbalanced snow distribution

Drift at Steps

For a steel building with a step (e.g., a lower roof adjacent to a taller main building):

Drift height: h_d = 0.2 × (γ × S_s × C_e / p_0)^0.5 + 0.5 × (γ × S_u / p_0)

Where γ = snow density (approximately 2.0-3.0 kN/m³ for dry alpine snow in Australia), and S_u is the snow load on the upper roof.

The drift load is applied as a triangular distribution over a width of 4 × h_d (or the roof length, whichever is less). The peak drift load at the step is:

p_dr = γ × h_d (limited by the step height)

Unbalanced Snow — Gable and Pitched Roofs

For steel portal frame buildings in alpine regions, AS 1170.3 requires an unbalanced snow load case:

• Windward side: 0.5 × 0.8 × S_s × C_e × C_t = 0.4 × S_s × C_e × C_t
• Leeward side: 1.5 × 0.8 × S_s × C_e × C_t = 1.2 × S_s × C_e × C_t

The unbalanced case accounts for snow redistribution by wind — snow scours from the windward roof and accumulates on the leeward roof. This can produce a load differential of 3:1 across the ridge, which is critical for rafter design, particularly at the ridge connection and eaves.

Snow Load Combinations with AS 4100

AS 1170.0 Clause 4.2 provides the snow load combinations for ULS design in alpine regions:

Live load combination factors for snow: ψ_l = 0.4 (residential/office), ψ_l = 0.6 (storage).

AS 4100 Steel Design Checks for Snow

When snow loads govern in alpine regions, AS 4100 steel members must be checked for:

1. Rafter design (positive bending): 1.2G + ψ_l × Q + S_s (uniform) and 1.2G + 1.5 × S_d (drift/unbalanced) — governs rafter section capacity in sagging moment

2. Rafter design (uplift): 0.9G + W_u + 0.7 × S (rarely governs in alpine areas where dead + snow dominates)

3. Column design: Combined axial from dead + snow + minimum live — governs column section capacity

4. Deflection (SLS): G + ψ_s × Q + S (short-term) using ψ_s = 0.7 for roof live load — snow load on flat roofs (C_shape = 0.8) can produce significant long-term creep deflection in steel beams

5. Base plate and anchor bolts: Uplift rarely governs in alpine buildings because dead + snow provides net compression — but the drift load pattern creates eccentric compression on the lower side of the frame

Worked Example: Steel Portal Frame in Charlotte Pass

Problem: Calculate the snow load on a steel portal frame building at Charlotte Pass, NSW (elevation 1,755 m).

Building data:

• Location: Charlotte Pass, NSW — S_s = 7.1 kPa, S_r = 0.5 kPa
• Building: 18 m span × 30 m length × 5 m eaves height
• Roof pitch: 20° (moderate pitch)
• Exposure: Normal (C_e = 1.0) — surrounded by alpine vegetation
• Building type: Ski lodge — heated (C_t = 1.0)
• Frame spacing: 5 m centres
• Roof cladding: Steel, no snow retention (assume potential sliding on pitch > 20°)

Step 1 — Uniform roof snow load:

C_shape = 0.8 (20° pitch — no reduction, as α < 30° for C_shape = 0.8)

S = 7.1 × 0.8 × 1.0 × 1.0 + 0.5 × 1.0 = 5.68 + 0.5 = 6.18 kPa

Step 2 — Unbalanced snow case (wind redistributed):

Windward side: 0.4 × 7.1 × 1.0 × 1.0 = 2.84 kPa Leeward side: 1.2 × 7.1 × 1.0 × 1.0 = 8.52 kPa

Unbalanced ratio: 8.52/2.84 = 3.0. The leeward side of the frame carries 3 times the snow load of the windward side. This governs the rafter design at the ridge and eaves connection.

Step 3 — ULS load combination for rafter (uniform):

Dead load G = 0.5 kPa (steel frame + cladding + services) Live load Q = 0.25 kPa (roof maintenance — ψ_l = 0.0 per AS 1170.0 for roof)

1.2G + ψ_l × Q + S = 1.2 × 0.5 + 0.0 × 0.25 + 6.18 = 6.78 kPa

Distributed load on rafter = 6.78 × 5 m = 33.9 kN/m → This is a very heavy load, typical of alpine construction. A deep UB or welded plate girder would be required.

Step 4 — ULS load combination for rafter (unbalanced):

1.2G + 1.5 × S_d (drift case, no live load)

Windward: 1.2 × 0.5 + 1.5 × 2.84 = 0.6 + 4.26 = 4.86 kPa → 24.3 kN/m on rafter Leeward: 1.2 × 0.5 + 1.5 × 8.52 = 0.6 + 12.78 = 13.38 kPa → 66.9 kN/m on rafter

The leeward side of the frame carries nearly 67 kN/m — this is comparable to a multi-storey building load. The rafter section would need to be significantly heavier than the windward side unless the frame is designed as a moment-resisting portal with redistribution.

Step 5 — Design implications:

• Rafter at ridge: The unbalanced moment due to the 3:1 load differential across the ridge requires a substantial ridge connection (moment connection or haunch).
• Column at eaves: The eccentric loading from the unbalanced snow produces a net moment at the column top, increasing the column section required.
• Foundation: The net vertical reaction from dead + snow + frame self-weight governs footing size. Uplift from wind is unlikely to govern at this snow load level.
• Serviceability: Long-term deflection under 6.18 kPa permanent snow load must be checked. Steel beam creep is negligible, but the elastic deflection under this permanent load may be significant.

Comparison with International Snow Load Standards

The Australian standard produces roof snow loads approximately 10-15% higher than ASCE 7 for equivalent conditions (due to the C_shape = 0.8 vs C_e × C_t × I_s × 0.7 base factor), but the difference is within typical design conservatism.

Frequently Asked Questions

Does snow load ever govern steel design in Australia?

For the vast majority of Australian buildings — no. Only buildings in alpine regions above approximately 1,200 m elevation (Snowy Mountains, Victorian Alps, Tasmanian highlands) need to consider snow loads. For all coastal cities (Sydney, Melbourne, Brisbane, Perth, Adelaide, Hobart) and most inland areas, snow loads per AS 1170.3 are taken as zero. In alpine regions, however, snow loads are the governing design case and can reach 7-10 kPa — equivalent to 4-6 storeys of additional building weight.

What is the roof snow load for a steel building at Thredbo?

At Thredbo (elevation 1,380 m), S_s = 4.5 kPa. For a heated building with normal exposure and a 15° roof pitch, the uniform roof snow load S = 4.5 × 0.8 × 1.0 × 1.0 + 0.4 = 4.0 kPa. At 5 m frame spacing, this is 20 kN/m on the rafter. The unbalanced case produces 1.2 × 4.5 × 1.0 × 1.0 = 5.4 kPa (27 kN/m) on the leeward side — heavy but manageable with standard UB sections for spans up to 15-18 m.

How does snow load combine with wind load in alpine areas?

AS 1170.0 Combination 7a uses 1.2G + ψ_l × Q + S for snow-governed combinations. For the rare case where both extreme snow and extreme wind occur simultaneously, AS 1170.0 allows the combined combination 0.9G + W_u + 0.7 × S. The 0.7 factor on snow reflects the low probability of concurrent extremes. This combination typically governs for uplift in alpine areas only when S_s < 2.0 kPa (lower alpine elevations). At Charlotte Pass with S_s = 7.1 kPa, the combined combination produces net downward load even at 0.7 × S.

What snow load should be used for existing steel buildings in alpine Victoria?

For existing buildings in alpine Victoria (Mount Hotham, Falls Creek, Mount Buller), refer to AS 1170.3 Appendix A for the ground snow load at the specific elevation. If the elevation is between tabulated values, linear interpolation is acceptable. Alternatively, consult the local council or building surveyor for the site-specific snow load used in the original design approval.

Related Pages

• AS 4100 Steel Design Overview — Australia — Full AS 4100 design reference
• AS 4100 Load Combinations — AS 1170.0 — Load combination guide for steel design
• AS 4100 Column Buckling Guide — Compression member design per AS 4100
• Australian Wind Load — AS 1170.2 — Wind load calculation guide
• Australian Steel Grades — AS/NZS 3678 & 3679.1 — Material properties
• 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

Educational reference only. Snow load methodology per AS 1170.3:2003. Verify ground snow load for your specific site elevation, exposure, and thermal conditions. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.