RAM Structural System Help

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

The equation for mean wind velocity at a height above terrain is defined as follows:
vmz=crz  coz  vb(4.3)
where
cr(z)
=
roughness factor
co(z)
=
orography factor

Terrain Roughness Factor, cr(z): NA.2.11

Equations (4.4) and (4.5) do not apply. Instead, cr(z) is calculated from Figure NA.3 and NA.4 based on chosen terrain type (country or town).

Terrain Type: Country

cr(z) value is read from Figure NA.3.

Terrain Type: Town

cr(z) values from Figure NA.3 are multiplied with the value from Figure NA.4.

In order to use Figures NA.3 and NA.4, the following information is needed:
  • Distance to sea (NA.3 & NA.4)
  • Distance to town (NA.4)
  • z-hdis (NA.3 & NA.4)

hdis is referred to as obstruction distance. It is defined by BS EN 1991-1-4:2005 A.5. All of the above information is user-provided.

Terrain Orography Factor, co(z): Section 4.3.3 & NA.2.9

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, co = 1 should be used. Otherwise, it is calculated according to A.3.

co= 1                                for          Φ<0.05(A.1)
co= 1+2 s Φ                 for          0.05<Φ<0.3(A.2)
co= 1+0.6 s                  for             Φ>0.3(A.3)
where
Φ
=
upwind slope (i.e., Φ = H/Lu)
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 co is less than 1.0, it is internally set to 1.0. Also note that co is always calculated at the building height. If orograph is not important, co = 1 should be used.

Parent topic: Eurocode EN 1991-1-4:2005 UK NA