Wind loading is one of the most dynamic and complex actions a structural engineer must account for. Unlike gravity loads, wind pressures fluctuate based on geography, local terrain, and the specific geometry of the building. Eurocode 1 (EN 1991-1-4) provides the standard methodology for converting wind speeds into design pressures.
Understanding the transition from basic wind velocity to localized peak pressure is essential for the safety of cladding, secondary members, and primary structural frames.
1. Basic Wind Velocity ($v_b$)
The calculation begins with the basic wind velocity ($v_b$), which is the 10-minute mean wind velocity at 10m above ground in terrain category II. This value is adjusted for seasonal effects and direction:
Where $v_{b,0}$ is the fundamental value of basic wind velocity, often obtained from National Annex maps.
2. Peak Velocity Pressure ($q_p$)
To find the peak velocity pressure at a height $z$, we account for the terrain's roughness and orography (hills/ridges). This pressure includes the mean wind speed and the short-term fluctuations caused by turbulence.
Where $\rho$ is the air density and $I_v(z)$ is the turbulence intensity at height $z$.
3. External and Internal Pressures
The actual force on a surface is the difference between the external pressure and the internal pressure. These are determined using pressure coefficients ($c_{pe}$ and $c_{pi}$). Buildings are divided into zones (A, B, C, D, E for walls; F, G, H, I for roofs) because wind pressure is never uniform.
Zone F (corners) typically experiences the highest suction forces, which is why cladding fixings often fail at the edges of a roof during a storm.
Key Takeaways
- The "Size Effect": Coefficients for small areas ($c_{pe,1}$) are higher than for large areas ($c_{pe,10}$). Always use $c_{pe,1}$ when designing fixings.
- Internal Pressure Matters: A broken window or an open door can significantly increase the internal pressure ($c_{pi}$), potentially doubling the net load on the roof.
- Orography Factors: Buildings on top of cliffs or steep hills experience much higher wind speeds than those on flat ground.
Navigating the various charts and coefficients in EC1-4 is time-consuming. Our StrucTalogue Wind Load Calculator automates the zoning, velocity profiles, and pressure derivations, giving you precise ULS results in seconds.