Steel Buildings in Europe

Part 3: Actions 3 - 23 2. Basic wind velocity v b v b = c dir c season v b,0 where: c dir is the directional factor c season is the seasonal factor The recommended value is 1,0 for both c dir and c season but the National Annex may give other values. 3. Basic velocity pressure The basic velocity pressure q b is calculated as follows: q b 2 b 2 1 v   where:  is the air density = 1,25 kg/m 3 (recommended value but the National Annex may give another value) 4. Terrain factor k r 0,07 0, II 0 r 0,19          z z k where: z 0 is the roughness length according to the terrain category z 0,II is the roughness length for the terrain category II: z 0,II = 0,05 m z max = 200 m Terrain categories and terrain parameters are defined in EN 1991-1-4 Table 4.1, but the National Annex may give other values. 5. Roughness factor c r ( z ) c r ( z ) = k r ln( z / z 0 ) for z min ≤ z ≤ z max c r ( z ) = c r ( z min ) for z ≤ z min where: z is the reference height defined by EN 1991-1-4 Figure 7.4. z min depends on the terrain category, EN 1991-1-4 Table 4.1. 6. Orography factor c o ( z ) The orography consists of the study of the shape of the terrain in the vicinity of the construction.