Fundamentals
Wind loads on structures: wind speed, pressure and load cases
How wind speed becomes pressure on a building, the difference between mean and 3-second gust speeds, windward and leeward pressures, drag on open frames, and wind in Struxt3D.
Wind is often the load that decides the bracing, the columns of tall buildings, the purlins and cladding of sheds, and nearly everything in towers. This tutorial explains how a wind speed turns into forces on a structure, why codes disagree on the speed you should enter, and how Struxt3D generates its wind load cases.
From wind speed to pressure
Moving air has kinetic energy. When it is stopped by a wall, that energy becomes pressure — the velocity pressure:
At 26 m/s this is ½ × 1.25 × 26² = 423 Pa ≈ 0.42 kPa. Because pressure goes with the square of the speed, a 10 % higher wind speed gives 21 % more load — the choice of speed matters a lot.
Wind grows with height
Near the ground, friction slows the wind; higher up it blows faster and gustier. Codes multiply the basic pressure by an exposure factor that grows with height and depends on the terrain (open country, suburbs, city centre). Struxt3D uses open-country exposure:
- Mean-speed codes (Eurocode, KDS, CSA): peak pressure q(z) = ½ρv² · ce(z), with ce = 2.35 (z/10)0.24 (EN 1991-1-4 terrain category II, z at least 5 m).
- Gust codes (AISC/ACI with ASCE 7, NSCP, AS/NZS): q(z) = 0.613 Kz V² G, with Kz = 2.01 (z/274)2/9.5 (ASCE 7 exposure C, z at least 4.6 m) and gust factor G = 0.85.
Mean speed or 3-second gust?
This is the most common source of mistakes. Codes define the "basic wind speed" in two different ways:
| Code | Speed definition | Struxt3D default | Wind factor in combinations |
|---|---|---|---|
| Eurocode (EN 1991-1-4) | 10-minute mean, vb | 26 m/s | 1.5 |
| KDS 41 | 10-minute mean (basic) | 28 m/s | 1.3 |
| CSA / NBCC | Mean (basic) | 25 m/s | 1.4 |
| AISC / ACI (ASCE 7) | 3-second gust, strength level | 50 m/s | 1.0 |
| AS/NZS 1170.2 | 3-second gust, regional VR | 45 m/s | 1.0 |
| NSCP 2015 | 3-second gust (Fig. 207A.5-1) | 69 m/s (250 km/h) | 1.0 |
A gust is much faster than the mean over 10 minutes (roughly 1.4–1.5 times in open country), so the numbers look very different for the same storm. Gust codes also already map their speed to a strength-level (rare) event, which is why their wind load factor is 1.0. Always enter the speed in the definition your code uses; Struxt3D labels the field "Basic wind speed" for mean-speed codes and "3-s gust" for gust codes.
The defaults are typical values, not your site's. Take the site wind speed from the code's map (for NSCP, Figure 207A.5-1 by occupancy category) and adjust for terrain, topography and importance where the code asks.
Worked example: wind on a 10 m high building
Eurocode, vb = 26 m/s, top of the wall at z = 10 m:
On a face 20 m wide and 10 m high (200 m²), the total horizontal force is about 1.29 × 200 ≈ 258 kN (characteristic). At ULS with the 1.5 factor: 387 kN, shared among the frames or walls by their stiffness.
ASCE 7, V = 50 m/s (gust), z = 10 m:
Open frames: drag on members
Towers, pipe racks, trusses and space frames have no walls for wind to press on; it flows around each member. Struxt3D then applies drag to every member: a uniform load equal to the pressure at the member's height × a force coefficient of 2.0 × the section's width facing the wind, using only the part of the wind that blows across the member (a member parallel to the wind gets none).
Shielding of members behind others is not taken into account, which is conservative for dense lattices.
Four wind directions
Wind can come from any side, so Struxt3D creates four load cases: Wind +X, Wind −X, Wind +Y and Wind −Y. Each wind rule in the combinations is applied with each direction in turn. On buildings with cladding, each wall facing the wind gets 0.8 q (pressure) and each wall facing away gets 0.5 q (suction), with q at the wall's top height; walls nearly parallel to the wind get nothing. Cladding passes its load to the members along its edges; structural walls take it as shells.
This is a simplified, sound first answer. It does not include roof uplift coefficients, internal pressure, local zones at corners, dynamic response of slender towers, or funnelling between buildings. Check those separately for final design.
Try it in Struxt3D
- Open a template such as Portal frame shed or Lattice tower.
- In What is it made of?, choose your Design code. The default wind speed follows the code.
- In How is it held and loaded?, switch Wind on. The note shows the speed definition and "±X and ±Y". Set Wind speed (m/s) for your site.
- Under Show on the model, pick Wind +X to see the loads drawn.
- Run the analysis and look at Displacement for a wind case: this is the sway. Compare it with a limit such as height/300 for the frame.
Summary
- Pressure grows with the square of the speed and with height.
- Mean-speed codes use load factors of 1.3–1.5; gust codes are already at strength level (1.0).
- Buildings get windward pressure and leeward suction; open frames get drag on each member.
- Struxt3D analyses four directions and combines each with the other loads.