Mean Wind Velocity (v_m(z)): Section 4.3.1

The equation for mean wind velocity at a height above terrain is defined as follows:

v_{m} (z) = c_{r} (z) c_{o} (z) v_{b}

(4.3)

where

c_r(z)	=	roughness factor
c_o(z)	=	orography factor

Terrain Roughness Factor, c_r(z): Section 4.3.2

It accounts for the variability of the mean wind velocity at the site of the structure due to height above ground level and the ground roughness of the terrain upwind of the structure in direction of wind.

c_{r} (z) = \{\begin{matrix} k_{r} l n (\frac{z}{z_{o}}) z_{m i n} \leq z \leq z_{m a x} \\ c_{r} (z_{m i n}) z \leq z_{m i n} \end{matrix}

(4.4)

where

z_o

roughness length

k_r

terrain factor depending on the roughness length, z_o

k_{r} = 0.19 {(\frac{z_{o}}{z_{o, I I}})}^{0.07}

(4.5)

z_0,II

0.05m (terrain category II, Table 4.1)

z_min

the minimum height defined in Table 4.1

z_max

the maximum height (taken as 200m)

Terrain category	z_o (m)	z_min (m)
0	0.003	1
I	0.01	1
II	0.05	2
III	0.3	5
IV	1.0	10

Table 4.1 - Terrain categories and terrain parameters

Note: For buildings taller than 200m, z_max = 200m is assumed.

Terrain Orography Factor, c_o(z): Section 4.3.3

The orography factor reflects effects of hills and cliffs in wind velocity and the code refers to Annex 3 (A.3) for its calculation. Section NA.2.9 also recommends use of the method given in A.3.

If orography is not significant, c_o(z) = 1 should be used. Otherwise, it is calculated according to A.3.

c_{o} = 1 f o r Φ < 0.05

(A.1)

c_{o} = 1 + 2 s Φ f o r 0.05 < Φ < 0.3

(A.2)

c_{o} = 1 + 0.6 s f o r Φ > 0.3

(A.3)

where

Φ	=	upwind slope (i.e., Φ = H/L_u)
s	=	orographic location factor

A detailed procedure of calculation of orographic location factor, s, is given in Annex (A.3). Equations (A.4) - (A.13) are implemented based on chosen landform profile (upwind, downwind for cliffs & escarpments, and downwind for hills & ridges). For this reason, valid values for X, H, Lu, and Ld distances (see Figures A.2 and A.3 in Annex A) are expected.

It should be noted that if calculated value of c_o is less than 1.0, it is internally set to 1.0. Also note that c_o is always calculated at the building height. If orograph is not important, c_o = 1 should be used.

Mean Wind Velocity (vm(z)): Section 4.3.1

Terrain Roughness Factor, cr(z): Section 4.3.2

Terrain Orography Factor, co(z): Section 4.3.3

Mean Wind Velocity (v_m(z)): Section 4.3.1

Terrain Roughness Factor, c_r(z): Section 4.3.2

Terrain Orography Factor, c_o(z): Section 4.3.3