Lisø PhD Dissertation Manuscript - NTNU

4.4. Main findings
Historical field investigations show that the effects of wind on roof snow loads are of
significance for a large part of buildings. According to snow transport theories snow
drifting occurs even for low wind velocities. Norway has a climate with low winter
temperatures, large snow amounts and high frequency of wind. The definition of the
exposure coefficient found in ISO 4355 is too conservative and does not manage to
differentiate the buildings in settled areas. The stations found to be most windswept are
situated in areas where no or a very few buildings are located. Other stations situated in
areas known as windswept and with high snowloads are found in the same category as
more shielded stations. According to the definition in ISO 4355 the buildings in the field
investigations of Høibø [5] and Løberg [6] have all exposure coefficient equal to 0.8
although the documentation shows exposure coefficients considerably lower for the
investigated buildings.
Norway has large variations in snow loads (Fig. 5). The lowest snow loads are due
to heavy snowfalls over a short period while the higher snow loads are a result of
accumulation over a long winter season. It seems reasonable that areas with low snow
loads have higher exposure coefficients than areas with high snow loads. This is taken into
account in ISO 4355 when differentiating the exposure coefficient according to mean
temperatures, but the limits chosen are not substantiated thoroughly through research
results.
Another way of taking into consideration the accumulation length is to include the
length of the winter season when deciding the exposure coefficient for a specific building
site. In the procedure presented by Otstavnov [4] both the length of the winter season and
number of days with snowfalls are included. Number of days with average wind velocities
above 10 m/s in the three coldest months defines the wind category according to ISO 4355.
When selecting three months, it is indirectly assumed that this is the length of the winter
season. Other periods should be considered when evaluating areas with lower or higher
accumulation period.
Mean temperatures for the coldest winter month are needed when determining the
value of the exposure coefficient. At first thought this temperature could also be considered
as a value taking into account the possibility of snow to be transported by wind actions. In
this situation the actual length of the winter season should be chosen. As mentioned above
it can also be a measure of the size of the snow ground load. This correlation should then
be scientifically documented.
5. Discussion and further work
Meteorological stations are located to enable a good representation of regional climate.
Typical locations are in agricultural and settled areas, airports and lighthouses. I.e. these
areas have a better representation than mountainous regions.
Maximum snow loads on the roof often do not appear simultaneously with
maximum snow loads on the ground. In the measurements reported by O’Rourke [8]
maximum snow load on roofs are measured independent of maximum snow loads on the
ground. In the measurements performed by Høibø [5] the roof and the ground are measured
simultaneously.
It also seems reasonable that the exposure coefficients decrease as the return period
of the characteristic snow loads on the ground increases. When measuring snow loads the
exposure coefficients therefore are expected to be higher in a normal year compared with a
year when extreme loads occur. In measurements reported by Taylor [16] snow loads on
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