Glossary · Snow and wind
Terrain type I–IV
The 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).
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Definition
The terrain type describes how much the surface slows the wind and, through the factor Ch, directly affects the wind load. Clause 9.9 of DBN V.1.2-2:2006 (Change 1) has four types:
- I — open sea and lake surfaces and flat plains without obstacles over at least 3 km;
- II — countryside with fences, small structures, houses and trees;
- III — suburban and industrial areas, extensive forests;
- IV — urban areas where at least 15 % of the surface is covered by buildings over 15 m high on average.
The type is set separately for each design wind direction. A structure counts as standing on a given terrain if that terrain extends 30Z in that direction (for structures below 60 m) or 2 km for taller ones. On a boundary or in doubt, the norm requires the type with the larger Ch.
Example: Kyiv, 2 m table
With W₀ = 370 Pa and a 2 m height, the pressure before the aerodynamic coefficient is: type I 333 Pa, type II 259 Pa, type III 148 Pa, type IV 74 Pa. Open land by a reservoir and a city block differ fourfold, so the type deserves an honest choice: a ground plant in a field outside a village is II, not III.
How Solispec applies it
In Solispec the terrain type is an input with four options, II by default. The engine feeds it into Table 9.01 together with the array top height. One type applies to all wind directions, so for a plot with different surroundings on different sides choose the type with the larger Ch, as clause 9.9 requires.
Norms and clauses
- DBN V.1.2-2:2006, cl. 9.9 (Change 1), Table 9.01
Related terms
- 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.
- 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.
- 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.
- 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.