Glossary · Snow and wind
Limit design wind load Wm
The wind load used for strength checks: W_m = γ_fm · W₀ · C, where C covers aerodynamics, height, topography and dynamics.
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Definition
Clause 9.4 of DBN V.1.2-2:2006 gives the limit design wind load as the product of W₀, the reliability factor γfm and the factor C. Clause 9.7 splits C into six multipliers:
Wm = γfm · W0 · C, C = Caer · Ch · Calt · Crel · Cdir · Cd- Caer — the aerodynamic coefficient (cl. 9.8);
- Ch — the height factor (cl. 9.9);
- Calt — the altitude factor (cl. 9.10);
- Crel — topography; equals 1 unless the site is on a hill or slope (cl. 9.11);
- Cdir — direction; normally 1 (cl. 9.12);
- Cd — dynamic factor; 1 for structures with a natural period up to 0.25 s (cl. 9.13).
Example: Kyiv, ground table
W₀ = 370 Pa, a 50-year term (γfm = 1.00), array top at 2 m, terrain type II (Ch = 0.7), altitude below 0.5 km (Calt = 1). Pressure before the aerodynamic coefficient: 370 · 0.7 = 259 Pa. For a panel at 30°, Table 7.6 of EN 1991-1-4:2005 gives cf = −1.8 for uplift and +1.2 downwards, that is 466 Pa up and 311 Pa down.
The serviceability value We = γfe · W₀ · C (formula 9.2), with γfe = 0.21 at η = 0.02, is used for deflections.
How Solispec applies it
Solispec computes Wm = γfm · W₀ · Ch · Calt at the array top, takes Crel = Cdir = Cd = 1 and multiplies the result by the panel cf for the chosen tilt. The resulting uplift and downward forces feed the ballast, rail and truss checks.
Norms and clauses
- DBN V.1.2-2:2006, cl. 9.4–9.13, formulae (9.1)–(9.3)
- EN 1991-1-4:2005, Table 7.6
Related terms
- 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.
- Height factor ChThe wind correction for height above ground and terrain type, Table 9.01 of DBN V.1.2-2:2006: 0.7 at 5 m and 0.9 at 10 m for type II.
- 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.
- 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).
- Reliability factor for the limit value γfmThe multiplier on S₀ and W₀ that depends on the mean return period T: 1.00 at 50 years, 1.14 at 100 years (Tables 8.1, 9.1).
- Wind uplift and slidingThe two checks of a ballasted table: wind must neither lift the structure nor slide it across the ground or roof.
- 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.