Lisø PhD Dissertation Manuscript - NTNU

the snow load on the roof as a function of the heat flux through the roof. An equivalent
expression can be found in ISO 4355 ”Bases for design of structures – Determination of
snow loads on roofs” [2].
In practice, it has turned out difficult for consultants in structural engineering to
determine the exposure coefficient Ce. The main reason is the meteorological input needed.
According to an informative annex in ISO 4355 and NS 3491-3, the exposure coefficient is
a function of the mean temperature, θ, in the coldest winter month and number of days, N,
with a wind velocity above 10 m/s where N is defined as an average for the three coldest
months of the year (see Table 1). Mean values for “many years” are recommended (usually
30 years). This meteorological information is available merely at advanced weather
stations. If a building site happens to be located near such a station, the data needed is still
not easily accessible.
Table 1. Exposure coefficient according to ISO 4355
Winter temperature Winter wind category
Category Mean temp. (ºC) I
II
III
N < 1 1 ≤ N ≤ 10 N > 10
A
θ > 2.5 1,0 1,0 0,8
B
C
–2.5 ≤ θ ≤ 2.5 1,0 0,8 0,6
θ < –2.5 0,8 0,8 0,5
In this paper weather data from meteorological stations in Norway for the reference
30-year period 1961-1990 is used to determine the exposure coefficient Ce according to the
definition in ISO 4355. First, historical field investigations studying snow loads on roofs
are evaluated giving the background of the exposure coefficient. Next, values for the
exposure coefficients are calculated for 389 meteorological stations, and the suitability of
the definition in order to describe the effects of wind exposure is discussed. Finally,
possible approaches aiming at improving calculations of wind exposure on roof snow loads
are suggested.
2. Background
2.1. Snow load on roofs according to ISO 4355
Fig. 1. Balanced snow load sb and drift snow load sd on pitched roofs.
Page 2 of 17