AS 4100 Wind Design Worked Example — Portal Frame, AS/NZS 1170.2, 20m Span

Wind dominates the design of steel portal frame buildings in Australia. Unlike gravity loads, wind produces uplift on the roof and suction on the walls, reversing the normal bending moment pattern in the frame and potentially governing member sizes, base fixity, and foundation design. AS/NZS 1170.2 sets out how to determine wind pressures at every point on the building envelope. This page works through a complete wind design for a 20 m span portal frame shed, with the full Cfig calculation, pressure coefficients, and wind actions ready for AS 4100 member design.

PRELIMINARY — NOT FOR CONSTRUCTION. This is an educational worked example. All designs must be independently verified by a licensed Professional Engineer before use in any project.

Design Brief

Step 1 — Regional Wind Speed Vr

Per AS/NZS 1170.2 Figure 3.1(A), Western Sydney is in Region A2:

Vr = V1000 (3-second gust, 10 m height, flat open terrain, 1000-year return)
Regional wind speed for Region A2: Vr = 43 m/s (from Table 3.1 for Region A2)

Actually, the standard values are: Region A1 (non-cyclonic, most populated areas): Vr = 39 m/s; A2: 43 m/s; A3: 39 m/s; A4: 45 m/s; A5: 37 m/s. Region B: 26-34 m/s; Region C: 49-61 m/s; Region D: 69-88 m/s.

Step 2 — Importance Level and Wind Direction Multiplier

For IL2 (500-year return period), AS/NZS 1170.2 Table 3.1:

Md = 1.0 (for Region A2, 500-year return, all directions)

Note: For IL3 (1000-year), Md would be approximately 1.05. For IL4 (2000-year), Md ≈ 1.10.

Step 3 — Terrain/Height Multiplier Mz,cat

Terrain Category 3 corresponds to suburban terrain with numerous obstructions of heights 3-10 m (houses, trees, industrial buildings). The multiplier Mz,cat varies with height above ground per Table 4.1(B).

For reference height z = h (the average roof height), which for a pitched roof is:

h = mean roof height = eave height + (ridge height - eave height)/3
                     = 6,000 + (7,763 - 6,000)/3
                     = 6,000 + 588
                     = 6,588 mm
                     ≈ 6.6 m

From AS/NZS 1170.2 Table 4.1(B), Terrain Category 3, at z = 6.6 m:

Interpolating between z = 5 m (Mz,cat = 0.79) and z = 10 m (Mz,cat = 0.83):
Mz,cat = 0.79 + (6.6 - 5)/(10 - 5) × (0.83 - 0.79)
       = 0.79 + 1.6/5 × 0.04
       = 0.79 + 0.013
       = 0.803

Mz,cat = 0.80 (rounded to two decimal places for design).

Step 4 — Site Wind Speed

Vdes,θ = Vr × Md × Mz,cat × Ms × Mt
       = 43 × 1.0 × 0.80 × 1.0 × 1.0
       = 34.4 m/s

This is the design 3-second gust wind speed at the mean roof height.

Step 5 — Aerodynamic Shape Factor Cfig

Cfig is calculated separately for each building surface per AS/NZS 1170.2 Section 5.

External Pressure Coefficients Cp,e

For a rectangular enclosed building with h ≤ 25 m (our h = 6.6 m), the external pressure coefficients are from Table 5.2(A) for walls and Table 5.2(B) for roofs.

Walls (wind direction θ = 0°, long face to wind):

The building has dimensions: span = 20 m (depth), length = 48 m, h = 6.6 m.

Building proportions:

• h/d = 6.6/20 = 0.33 (wall pressure depends on this ratio)

From Table 5.2(A), for h/d ≈ 0.33:

Windward wall (surface):    Cp,e = +0.7   (0 to h from ground)
Leeward wall:               Cp,e = -0.3   (based on h/d ≈ 0.33)
Side walls:                 Cp,e = -0.65  (near windward edge, within h from corner)
Side walls (remainder):     Cp,e = -0.5

Roof (θ = 0°, wind normal to ridge):

Roof pitch α = 10°. For a pitched roof with h/d ≈ 0.33, from Table 5.2(B):

The roof is divided into zones. For a two-span roof (20 m total, 10 m per side):

Upwind slope (windward roof):
  Leading edge (0 to h from eave, i.e., first 6.6 m):  Cp,e = -0.9, -0.4
  Remainder:                                            Cp,e = -0.5, 0.0

Downwind slope (leeward roof):
  Leading edge (first 6.6 m from ridge):                Cp,e = -0.5, 0.0
  Remainder:                                            Cp,e = -0.3, +0.1

Note: the two values represent minimum and maximum pressure for the zone. The most adverse combination must be selected for each load case.

Roof (θ = 90°, wind parallel to ridge):

When the wind blows parallel to the ridge (gable-end wind), the roof experiences suction across the entire surface:

Roof (full surface, θ = 90°):    Cp,e = -0.7 (uplift)
                                  Cp,e = -0.3 (minimum)

Area Reduction Factor Ka

The area reduction factor accounts for the fact that peak pressures do not act simultaneously over large areas. For a portal frame at 6 m centres:

Tributary area per frame = 6 m × (rafter length) ≈ 6 × 11 m ≈ 66 m²
Per AS/NZS 1170.2 Table 5.2 footnote: Ka = 1.0 for A ≤ 10 m², reducing to 0.8 for A ≥ 100 m².
Interpolating for A ≈ 66 m²: Ka ≈ 1.0 - (66-10)/(100-10) × 0.2 = 1.0 - 0.124 = 0.876
Use Ka ≈ 0.90 for simplification.

Combination Factor Kc

For wind on two or more surfaces acting together (walls + roof), AS/NZS 1170.2 Clause 5.4.3 requires a combination factor to account for non-simultaneous peak pressures:

For buildings with h ≤ 25 m: Kc = 0.90 for most surface combinations
                          Kc = 1.00 for single-surface checks

We'll use Kc = 0.90 for the portal frame analysis (combined wall and roof load case), and Kc = 1.00 for checking individual cladding or purlin loads.

Local Pressure Factor Kl

Local pressure factors apply near edges, ridges, and corners. For the portal frame at 6 m spacing, the local zone extends h = 6.6 m from the eave/ridge. The first purlin bay typically falls within the local pressure zone.

Wall corners (within h from corner): Kl = 1.5
Roof edges (within h from eave/ridge): Kl = 1.5
Roof remainder: Kl = 1.0

For our portal frame analysis, the local pressure governs the first purlin spans near the eave, but for the overall frame analysis, we use Kl = 1.0 (area-averaged). We note that the eave purlins and girts need separate local pressure design.

Cfig Summary (θ = 0°, wind normal to ridge):

Per Clause 5.4.1: Cfig = Cp,e × Ka × Kc × Kl × Kp. With Kp = 1.0 (non-porous cladding), Kl taken as 1.0 for primary frame members:

Windward wall:   Cfig = +0.70 × 0.90 × 0.90 × 1.0 = +0.567
Leeward wall:    Cfig = -0.30 × 0.90 × 0.90 × 1.0 = -0.243
Side wall:       Cfig = -0.65 × 0.90 × 0.90 × 1.0 = -0.527
Roof upwind:     Cfig = -0.90 × 0.90 × 0.90 × 1.0 = -0.729  (worst suction)
Roof downwind:   Cfig = -0.50 × 0.90 × 0.90 × 1.0 = -0.405

Step 6 — Internal Pressure Coefficient Cp,i

Internal pressure depends on the size and distribution of openings (doors, windows, vents) in the building envelope. For an enclosed building with no dominant opening, AS/NZS 1170.2 Clause 5.3.1:

Cp,i = +0.2 (positive internal pressure — doors/windows on windward face open)
Cp,i = -0.3 (negative internal pressure — dominant opening on leeward/side face)

Cfig (internal) = Cp,i × Kc = +0.2 × 1.0 = +0.20  or  -0.3 × 1.0 = -0.30

For the portal frame, we check both internal pressure cases:

• Case 1: Cp,i = +0.2 (windward opening): Reduces net suction on walls/roof — less critical for uplift.
• Case 2: Cp,i = -0.3 (leeward opening): Increases net pressure on windward wall and increases net uplift on roof — typically governs.

Step 7 — Design Wind Pressure

Per Clause 2.4.1: p = 0.5 × ρ_air × V²_des,θ × Cfig × Cdyn

Where:

• ρ_air = 1.2 kg/m³ (standard air density)
• Cdyn = 1.0 (dynamic response factor, applicable for h < 50 m and natural frequency > 1 Hz)

Dynamic pressure: qz = 0.5 × 1.2 × (34.4)² = 0.6 × 1,183.4 = 710 Pa

Design pressures (Cases 1 and 2), using net Cfig = Cfig,ext - Cfig,int:

Case 1 — Internal pressure +0.2 (Cp,i = +0.20):

Case 2 — Internal pressure -0.3 (Cp,i = -0.30):

Case 2 gives higher pressure on the windward wall (+616 Pa vs +261 Pa) but lower uplift on the roof (-305 Pa vs -660 Pa). Case 1 governs roof uplift.

Step 8 — Wind Actions on Portal Frame (θ = 0°)

The line load per metre on the portal frame = p × portal spacing:

Portal spacing = 6,000 mm = 6.0 m

Case 1 — Roof Uplift (Governing for uplift):

Windward column:  w = 0.261 × 6.0 = 1.57 kN/m (inward, →)
Rafter upwind:    w = -0.660 × 6.0 = -3.96 kN/m (uplift, acting normal to roof)
Rafter downwind:  w = -0.430 × 6.0 = -2.58 kN/m (uplift, acting normal to roof)
Leeward column:   w = -0.315 × 6.0 = -1.89 kN/m (outward, ←)

Case 2 — Windward Wall Pressure + Roof Uplift (Governing for inward frame racking):

Windward column:  w = 0.616 × 6.0 = 3.70 kN/m (inward, →)
Rafter upwind:    w = -0.305 × 6.0 = -1.83 kN/m (uplift)
Rafter downwind:  w = -0.075 × 6.0 = -0.45 kN/m (uplift)
Leeward column:   w = 0.040 × 6.0 = 0.24 kN/m (inward, →, negligible)

Step 9 — Load Combinations for AS 4100 Design

Wind actions must be combined with gravity loads per AS/NZS 1170.0:

Combination 1: 1.2G + Wu (uplift case — roof suction):

The uplift from wind can reverse the bending moment in the rafters. For a typical portal frame with dead load of approximately 0.4 kPa (roof sheeting, purlins, insulation) giving a line load of 0.4 × 6 = 2.4 kN/m downward:

Net uplift on rafter (Case 1):
  Upward:   1.0 × Wu = 3.96 kN/m ↑
  Downward: 0.9 × G  = 0.9 × 2.4 = 2.16 kN/m ↓
  Net:      3.96 - 2.16 = 1.80 kN/m ↑ (uplift governs!)

The rafter experiences net uplift, which reverses the bending moment — the bottom flange goes into compression, requiring fly braces on the bottom flange or a check that the bottom flange is restrained against LTB.

Combination 2: 1.2G + 1.5Q + Ws (gravity + serviceability wind):

This combination typically governs member sizes and deflections.
Wind is taken at serviceability level (25-year return instead of 500-year).

Combination 3: 1.2G + Wc + ψc × Q (ultimate lateral — wind as primary):

Wc = ultimate wind (500-year). Governs frame stability, base connections, and foundation overturning checks.

Step 10 — Key Design Implications for the Portal Frame

1. Rafter size: The rafter section must be checked for the reversed-moment case (net uplift). A section that works for gravity loading (bottom flange in tension) may be inadequate when the bottom flange is in compression during wind uplift. The critical case is often Case 1 where 3.96 kN/m uplift exceeds the 2.16 kN/m downward dead load.

2. Column size: The windward column carries 3.70 kN/m horizontal wind pressure (Case 2) in addition to vertical load from the rafter. This induces significant bending moment. The column base connection must resist both shear and overturning moment.

3. Base fixity: Wind uplift on the roof reduces the compression in the leeward column (or induces tension), which affects the base plate and anchor bolt design. Under Case 1, the leeward column may experience net tension, requiring anchor bolts designed for uplift.

4. Bracing: Longitudinal wind (θ = 90°) requires separate roof bracing and wall bracing checks. The end portal frames carry wind on the gable end wall plus bracing forces from the roof diaphragm. A separate analysis for θ = 90° is essential.

5. Serviceability: AS/NZS 1170.0 Appendix C requires checking deflections under serviceability wind (typically 25-year return, approximately 0.75 × Wu). Portal frame deflections are typically limited to h/150 for eave drift and span/250 for rafter sag/uplift.

Wind Pressure Summary Table (Case 1 — Roof Uplift Critical)

Positive pressure = acting toward the surface (inward). Negative = acting away from the surface (suction/uplift).

Frequently Asked Questions

Why do I need to check so many wind directions?

A rectangular building presents different aerodynamic profiles to wind from different directions. Wind normal to the long side (θ = 0°) typically produces the highest total drag force on the building (more surface area exposed). Wind normal to the gable end (θ = 90°) produces higher local pressures on the roof near the ridge. And quartering winds (θ = 45°) can produce torsional effects on the building because the pressure centre does not align with the shear centre. AS/NZS 1170.2 Clause 2.5.2 explicitly requires checking at least four orthogonal directions.

What is the difference between Cfig for primary frame members and for cladding/purlins?

For primary structural frames (columns, rafters), the area reduction factor Ka (typically 0.85-0.95) and combination factor Kc (0.90) reduce the pressure coefficients to account for non-simultaneous peak pressures. For cladding and individual purlins with small tributary areas (A < 10 m²), Ka = 1.0, Kc = 1.0, and the local pressure factor Kl (up to 1.5 at edges) is applied — resulting in pressures that can be 2-3 times higher than the frame-level pressures. This is why edge purlins and eave girts are often heavier sections than interior ones.

How does terrain category affect the wind speed multiplier Mz,cat?

Terrain Category 1 (exposed, flat, open coastline) gives the highest Mz,cat because there are no obstructions to slow the wind. Category 4 (dense urban, high-rise) gives the lowest because the ground roughness and surrounding buildings dissipate wind energy. For a given height of 6 m: TC1 → Mz,cat ≈ 0.96; TC2 → Mz,cat ≈ 0.90; TC3 → Mz,cat ≈ 0.80; TC4 → Mz,cat ≈ 0.64. The choice of terrain category is a significant design decision — a building on a coastal plain (TC1) experiences roughly 44% higher wind pressure than the same building in a dense urban area (TC4) due to the squared relationship between wind speed and pressure.

Do I need to consider topographic effects (Mt > 1.0)?

Mt > 1.0 applies when the building is on a hill, ridge, or escarpment where the wind accelerates as it flows over the topography. AS/NZS 1170.2 Section 4.4 provides the topographic multiplier based on hill slope, height, and building position. For a portal frame shed on flat ground in western Sydney, Mt = 1.0. But if the same shed were perched on top of a 50 m high ridge with a 1:5 slope, Mt could be 1.3-1.5, increasing wind pressures by 70-125%. Topography can easily double the wind load on an otherwise benign site.

What changes for a building with large roller doors (dominant openings)?

If a building has large openings that can remain open during a windstorm (e.g., roller doors on one face that are likely to be open), the internal pressure coefficient changes dramatically. Per AS/NZS 1170.2 Clause 5.3.2, if the area of openings on one wall exceeds the combined openings on all other walls by a factor of 2 or more, that wall is a dominant opening. The internal pressure then takes the external pressure coefficient from the wall containing the dominant opening: Cp,i = 0.90 × Cp,e of that wall. This can increase internal pressure from ±0.2/-0.3 to as high as +0.63 (if the windward wall has large roller doors), drastically increasing net pressures on the roof and leeward walls.

This worked example is for educational purposes only. Verify site-specific wind speeds, terrain categories, and all multipliers against the current editions of AS/NZS 1170.0, AS/NZS 1170.2, and the project site conditions. All wind designs must be independently checked and certified by a licensed Professional Engineer.