Glossary · Snow and wind
Aerodynamic coefficient Caer (cf)
The multiplier turning wind pressure into force on a shape; for panels Solispec takes c_f from Table 7.6 of EN 1991-1-4:2005, as cl. 9.8 of the DBN allows.
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Definition
In the Wm formula Caer describes the shape: the same wind acts differently on a wall, a roof and a tilted panel. Under clause 9.8 of DBN V.1.2-2:2006 Caer comes from Annex I. A plus sign means pressure towards the surface, minus away from it.
For a free-standing tilted panel with wind flowing on both sides, Annex I gives no uplift coefficient. The same clause 9.8 (Change 2) helps here: where the annex does not cover a case, coefficients may be taken from reference and experimental data. The closest documented shape is the monopitch canopy of Table 7.6 of EN 1991-1-4:2005, overall force coefficients cf for an empty canopy (φ = 0):
| α | cf, uplift | cf, downward |
|---|---|---|
| 0° | −0.5 | +0.2 |
| 10° | −0.9 | +0.5 |
| 20° | −1.3 | +0.8 |
| 25° | −1.6 | +1.0 |
| 30° | −1.8 | +1.2 |
Example: Kyiv, 30°
At 259 Pa at 2 m (type II) a 30° panel gets 1.8 · 259 = 466 Pa of uplift and 1.2 · 259 = 311 Pa downwards. Uplift is larger, so for ground tables it governs the ballast or the pile depth.
Table 7.6 ends at 30°. For steeper angles the coefficients in the engine’s table are extrapolated, and the calculation flags it.
How Solispec applies it
The Solispec engine interpolates cf by panel tilt from Table 7.6 of EN 1991-1-4:2005 and multiplies Wm by it. The uplift resultant acts d/4 from the windward edge (7.3(6), Figure 7.16 of the same standard), so the rear support takes the larger share. For roof systems the engine also applies its own roof-zone multipliers (middle 1, edge 1.5, corner 2) — reference multipliers of the engine, not set by the norm.
Norms and clauses
- DBN V.1.2-2:2006, cl. 9.8 (Change 2), Annex I
- EN 1991-1-4:2005, 7.3, Table 7.6, Figure 7.16
Related terms
- Limit design wind load WmThe wind load used for strength checks: W_m = γ_fm · W₀ · C, where C covers aerodynamics, height, topography and dynamics.
- Wind uplift and slidingThe two checks of a ballasted table: wind must neither lift the structure nor slide it across the ground or roof.
- BallastDead weight (concrete blocks, gravel trays) that keeps a PV table from wind uplift and sliding without anchoring into the ground or roof.
- Panel tilt angleThe angle between the panel plane and the horizontal; it affects yield, snow (μ), wind (c_f) and row pitch. Kyiv’s optimum is about 36°.
- Snow shape coefficient μThe conversion from ground snow to roof snow: μ = 1 up to 25°, 0 from 60°, linear in between (DBN V.1.2-2:2006, Annex Zh, scheme 1).
- DBN V.1.2-2:2006 “Loads and actions”: what matters for a solar plantNorms explained
Solispec’s calculation is preliminary: for a design it has to be checked by an engineer. The values and clauses follow the current editions of the norms on the date of the page.