Titel_SS06

where:

Q: wind velocity pressure

1 : weight density ( 1 =1.25 kg/m^3 for standard air)

U: wind velocity

The reference mean velocity pressure Qref is a 10 minutes average taken at an elevation of

10m above ground in horizontal open terrain exposure (z 0 =0.03m) and is modelled by a
Weibull distribution with scale parameter k close to 2. The annual maximum wind velocity
pressure is Gumbel distributed. Because of the relation between the wind speed U and
velocity pressure Q given by Equation (7.17), the maximum annual velocity pressure is also

Gumbel distributed.

According to the JCSS (2001) the factors for gust effects, terrain roughness and the
aerodynamic shape can be assumed Lognormal distributed. Table 7.3 summarizes their
coefficients of variation. The coefficient of variation of the wind action can then be estimated
by Equation (7.18) or (7.19). For the extreme cases, the coefficient of variation of the wind
load is 0.26 and 0.53 respectively. As a representative value a coefficient of variation equal to
0.37 might be appropriate.

Table 7.3: Probabilistic model for wind load.
2222
VVVVwc cC arQref (for non-dynamic sensitive buildings) (7.18)

and
22222
VVVVVwc c cC darQref (dynamic sensitive buildings) (7.19)

Snow Loads

According to Rackwitz (2000) the snow load can be defined as the product between the

ground snow load

Sr

Sg, a ground to roof conversion snow load factor r and a term taking into

account the climate and altitude in which the building is situated. Sr may thus be written as:

r

h

h

SSrkrg (7.20)

where

Sg the snow load on ground at the weather station

r conversion factor of snow load on ground to snow load on roofs