Wind Loads

Part of Structural Engineering

How to estimate wind pressure on structures and design buildings and towers to resist overturning and racking.

Why This Matters

Wind kills structures. A building that carries its own weight and occupant loads easily can fail spectacularly when a storm strikes, because the forces involved are entirely different in character. Gravity loads push straight down and are predictable. Wind loads push sideways and upward, try to overturn, rack, and uplift, and fluctuate rapidly in ways that generate dynamic effects far exceeding the average pressure.

In a rebuilding context, wind matters for every tall structure: chimneys, water towers, windmills, communication towers, and tall buildings. It also matters for ordinary buildings in exposed sites — an isolated farmhouse on an open hillside experiences far higher wind loads than a townhouse sheltered by neighboring structures. Getting this wrong means your windmill tower collapses in the first storm, or your barn roof peels off and kills livestock.

You cannot measure wind speed precisely without instruments, but you can estimate design wind speeds from local knowledge, observe damage patterns on natural features, and apply conservative pressure formulas. Traditional builders developed rules of thumb over generations of observing what failed and what survived. This article gives you the engineering basis behind those rules so you can adapt them to unusual situations.

Estimating Wind Speed

Without an anemometer, use the Beaufort scale and physical evidence to bracket the design wind speed for your site.

Beaufort Force	Description	Wind Speed	Observed Effects
6	Strong breeze	39–49 km/h	Large branches moving, whistling in wires
7	Near gale	50–61 km/h	Whole trees in motion, walking difficult
8	Gale	62–74 km/h	Twigs break off trees
9	Strong gale	75–88 km/h	Slates blow off roofs
10	Storm	89–102 km/h	Trees uprooted, structural damage
11	Violent storm	103–117 km/h	Widespread damage
12	Hurricane	>118 km/h	Extreme destruction

Design wind speed selection:

• Temperate, inland, sheltered site: design for Force 9 (85 km/h = 24 m/s)
• Exposed hilltop, coastal, or storm-prone region: design for Force 10–11 (100 km/h = 28 m/s)
• Tropical cyclone zone: design for Force 12+ (140+ km/h = 39+ m/s)

Always design for the highest wind you can reasonably expect in a 50-year period, not the typical annual maximum. Ask local farmers what the worst storm in living memory did to buildings.

Wind Pressure Calculation

The dynamic pressure of wind on a flat surface is:

q = ½ × ρ × V²

Where:

• q = pressure in Pascals (N/m²)
• ρ = air density ≈ 1.25 kg/m³ at sea level
• V = wind speed in m/s

At 24 m/s (85 km/h): q = ½ × 1.25 × 576 = 360 Pa (36 kg/m²) At 28 m/s (100 km/h): q = ½ × 1.25 × 784 = 490 Pa (49 kg/m²) At 39 m/s (140 km/h): q = ½ × 1.25 × 1521 = 950 Pa (95 kg/m²)

This is the pressure on a flat plate perpendicular to the wind. For actual building shapes, multiply by a pressure coefficient Cp:

Surface	Cp	Effect
Windward wall (perpendicular)	+0.8	Inward pressure
Leeward wall	−0.5	Outward suction
Side walls	−0.7	Outward suction
Windward roof slope < 30°	−0.9	Upward suction
Windward roof slope > 45°	+0.4	Inward pressure
Leeward roof	−0.4 to −0.7	Upward suction

Net wind load per unit area = q × (Cp_windward − Cp_leeward)

For a wall: 360 × (0.8 − (−0.5)) = 360 × 1.3 = 468 N/m² at 24 m/s

For a low-pitched roof (suction dominates): 360 × (−0.9 − (−0.4)) = 360 × 0.5 = 180 N/m² uplift — but this is additive to gravity loads being absent (the roof is trying to fly off), so the net effect is severe.

Overturning and Sliding

A rectangular building or tower acts as a lever. Wind pressure on the windward face creates an overturning moment that tries to tip the structure. Gravity (the building’s weight) creates a restoring moment.

Overturning moment = Total wind force × Height to centroid of load For a uniform wall: centroid = H/2, so moment = (q × Cp × W × H) × H/2 = q × Cp × W × H²/2

Restoring moment = Building weight × half-width = W_total × B/2

For stability: Restoring moment > Overturning moment × Safety factor (typically 1.5)

Example: A 4 m × 4 m × 6 m tall stone building. Wind on 4 m wide face.

• Wind force = 468 N/m² × 4 m × 6 m = 11,232 N ≈ 11.2 kN
• Overturning moment = 11.2 kN × 3 m = 33.6 kN·m
• Building weight (stone, 2,200 kg/m³, 0.5 m walls): roughly 200 kN
• Restoring moment = 200 × 2 = 400 kN·m
• Ratio = 400/33.6 = 11.9 — very stable

A tall slender tower tells a very different story. A stone tower 2 m × 2 m × 15 m:

• Wind force = 468 × 2 × 15 = 14,040 N = 14 kN
• Overturning moment = 14 × 7.5 = 105 kN·m
• Tower weight (solid stone): ~220 kN
• Restoring moment = 220 × 1 = 220 kN·m
• Ratio = 220/105 = 2.1 — barely adequate; foundation anchor required

Towers narrower than 1:5 (width:height) almost always need guy wires, buttresses, or a very wide foundation.

Racking (Lateral Deformation)

Even a structure that does not overturn can be destroyed by racking — the tendency of a rectangular frame to distort into a parallelogram under horizontal load. A wall that sheds water fine under gravity can fail completely in racking because the connections were never designed for horizontal forces.

Resistance methods:

Diagonal bracing: A single diagonal in a rectangular frame converts racking into axial load in the diagonal (tension or compression depending on wind direction). Use a cross-brace (two diagonals) to handle wind from both sides.

Shear panels: Boards nailed diagonally across stud walls, or overlapping planks, create a shear-resistant panel. The nails transfer shear between boards; use plenty of nails (every 150 mm along each framing member).

Masonry returns: A stone or brick wall extending at right angles to the wind-loaded wall acts as a buttress. The classic gable end wall stiffened by a returning side wall is inherently racking-resistant.

Tie rods: Iron rods connecting opposite walls through the building, tensioned with a turnbuckle or wedge, prevent the walls from spreading. Visible on many old stone buildings as the circular or star-shaped iron plates on exterior walls.

Roof Uplift and Anchoring

Low-pitched roofs on exposed buildings are vulnerable to uplift. The wind creates suction on top of the roof AND pressure under the eaves — both try to lift the roof off the walls. The failure mode is catastrophic: once the roof lifts even slightly, wind gets underneath and the suction increases dramatically.

Calculate uplift force: Uplift = q × Cp_uplift × Roof area At 24 m/s on a 10 m × 6 m roof: 360 × 0.9 × 60 = 19,440 N ≈ 2,000 kg of uplift force

Anchoring the roof to the walls:

• Hurricane straps — bent iron straps nailed to each rafter and bolted into the wall plate below. Space at maximum 600 mm.
• Wire ties — heavy-gauge wire looped around the rafter and tied to a wall anchor bolt, tightened by twisting.
• Through-bolts — long bolts passing through the wall plate and into the masonry below with a large washer plate.
• Heavy purlins — if the roof structure is heavy enough (slate or clay tile on heavy timbers), gravity alone may resist uplift, but verify with the calculation above.

Site Shielding and Exposure

Local terrain dramatically affects wind loads. A building in the lee of a forested hill experiences perhaps half the wind pressure of an identical building on an exposed ridgeline. Conversely, a gap between two hills can accelerate wind to 1.3–1.5× the open-country speed (venturi effect).

Exposure categories for design:

• Sheltered (surrounded by trees or buildings, open countryside beyond): use 0.7× calculated pressure
• Open (flat farmland, no obstruction within 500 m): use 1.0× calculated pressure
• Exposed (hilltop, coastal, or gap in hills): use 1.3–1.5× calculated pressure

Planting windbreaks — a row of trees or dense shrubs on the prevailing wind side — is one of the most effective structural interventions for an existing building. A windbreak 10 m tall reduces wind loads for 100 m downwind. This is why old farms universally had trees on the northwest or north side (in temperate northern hemisphere locations where prevailing winds are from the southwest but the worst storms come from the northwest).

The lesson of wind engineering is that orientation and siting matter as much as structural strength. Orient the narrowest building face to the prevailing wind, shelter structures behind natural features, and use diagonal bracing in every wall panel. These measures together can reduce wind-induced forces by 70% compared to an unprotected, unbraced rectangular structure.