Glossary · Snow and wind
Wind uplift and sliding
The two checks of a ballasted table: wind must neither lift the structure nor slide it across the ground or roof.
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Definition
Wind on a tilted panel produces a force F normal to its plane. The vertical component Fz = F · cos α tries to lift the table (uplift), the horizontal Fx = F · sin α tries to slide it. F equals Wm times the uplift coefficient cf times the panel area.
Uplift
The table is held down by its own weight and the ballast. Since weight here works in favour of stability, it is taken with a reduced factor: Table 5.1 of DBN V.1.2-2:2006 gives 0.90 in brackets for concrete and for steel structures where self-weight makes up 50 % or more of the effects — precisely for overturning checks.
Where the resultant acts matters. For a monopitch canopy EN 1991-1-4:2005 (7.3(6), Figure 7.16) places it d/4 from the windward edge. With wind from behind, the windward edge is the top one, and the rear support line takes a larger share of the uplift than the front. Splitting the ballast evenly between supports is not enough.
Sliding
Sliding is resisted by friction between the support and the surface. Uplift reduces the normal force, so the required weight grows for both reasons at once. The friction coefficient depends on the surface: gravel, soil, concrete, a rubber mat on a membrane all differ.
Example: Kyiv, 30°
An uplift pressure of 466 Pa on 1 m² of panels gives Fz = 466 · cos 30° ≈ 404 N. A 12 kg/m² panel with the 0.9 factor holds ≈ 106 N. The ballast must supply the rest: (404 − 106) / (0.9 · 9.81) ≈ 34 kg per square metre of panels, before the uneven split between supports and sliding are considered.
How Solispec applies it
The Solispec engine computes ballast for uplift as (Fz − 0.9 · panel weight) / 0.9 and for sliding as (Fx / μ + Fz) / 0.9 minus the panel weight, taking 12 kg/m² for panels. The design friction μd = μk / 1.25 is chosen by surface: compacted gravel 0.35, sand 0.30, grass or soil 0.24, dry concrete 0.45, foil-faced mat 0.15; a value measured on site can be entered instead (up to 0.8). The engine also finds the share of uplift on the most loaded support line from the d/4 position of the resultant and checks the ballast under that line.
Norms and clauses
- DBN V.1.2-2:2006, Table 5.1 (values in brackets), cl. 9.8
- EN 1991-1-4:2005, 7.3(6), Figure 7.16, Table 7.6
Related terms
- BallastDead weight (concrete blocks, gravel trays) that keeps a PV table from wind uplift and sliding without anchoring into the ground or roof.
- 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.
- 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.
- Characteristic wind pressure W₀The mean wind pressure at 10 m height exceeded on average once in 50 years; Kyiv has 370 Pa under DBN V.1.2-2:2006.
- Terrain type I–IVThe wind classification of a structure’s surroundings under cl. 9.9 of DBN V.1.2-2:2006: from open water (I) to dense urban areas (IV).
- 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.