Steel Buildings in Europe

Part 3: Actions 3 - 24 The effects of orography may be neglected when the average slope of the upwind terrain is less than 3°. The recommended value of c o ( z ) is 1,0, but the National Annex may give the procedure to calculate the orography factor. Annex A3 of EN 1991-1-4 gives the recommended procedure to determine c o for hills, cliffs, etc. 7. Turbulence factor k l The recommended value is 1,0 but the National Annex may give other values. 8. Peak velocity pressure q p ( z )   ( ) 2 ( ) 1 7 ( ) 1 2 m v p v z I z q z    where: I v ( z ) is the turbulence intensity which allows to take into account the contribution from short-term fluctuations / ) ( ) ln( ( ) 0 o l v c z z z k I z  for z min ≤ z ≤ z max ( ) ( ) v min v I z I z  for z < z min z max = 200 m v m ( z ) is the mean wind velocity at height z above the terrain: v m ( z ) = c r ( z ) c o ( z ) v b Alternative for step 8: For single-storey-buildings, the determination of the mean wind velocity v m ( z ) is not absolutely necessary. The peak velocity pressure can be directly obtained from the exposure factor c e ( z ): b e p ( ) ( ) q z c z q  where: ( ) ( ) ( ) ( ) ( ) 1 7 2 r 2 o r o l r e c z c z c z c z k k c z         For flat terrain ( c o ( z ) = 1) and for turbulence factor k l = 1, the exposure factor c e ( z ) can be directly obtained from Figure 4.2 of EN 1991-1-4, as a function of the height above terrain and a function of terrain category. 8.2.2 Wind pressure on surfaces – Wind forces There are three types of wind forces acting on a building:  External forces F w,e (see 8.2.2.1)  Internal forces F w,i (see 8.2.2.2)  Friction forces F fr (see 8.2.2.3).