Lisø PhD Dissertation Manuscript - NTNU

In NS 3479 (Standards Norway 1979) the section dealing with snow loads contained a direct
translation of ISO 4355 “Snow loads on roofs”. Characteristic snow loads on the ground for each
municipality (5-year return period) with values largely between 1.5 kN/m 2 and 3.5 kN/m 2 were
inctroduced. In the case of roofs from which snow was difficult to clear, a return period of at least 20
years needed to be accounted for. The code quoted snow shape factors for a number of typical roof
shapes. As a result of the hurricane that struck northwest Norway on New Year’s Day 1992, two new
wind curves were introduced for a selection of coastal municipalities in a revision of NS 3479
(Standards Norway 1994): curves E and F (see Fig. 2). The partial factor for wind actions was set to
1.6.
In NS 3491-3 (Standards Norway 2001) characteristic snow loads on the ground with a 50-year
return period was specified. The bulk of the municipalities now had a value for snow loads on the
ground of between 3.0 kN/m 2 and 4.5 kN/m 2 . A few coastal municipalities had values as low as 1.5
kN/m 2 , and in some inland municipalities values of up to 9.0 kN/m 2 were introduced. Pitched roofs,
shed roofs and curved roofs have also undergone certain changes associated with the shape factors.
In NS 3491-4 (Standards Norway 2002a) a classification of the whole country has been carried out
so that wind exposure for all 434 municipalities is defined. Exposure is defined by means of a
reference wind velocity (varies between 22 m/s and 31 m/s). Roughness of the terrain in an area 10
km against the wind direction is important for the wind pressure (in this code called the gust velocity
pressure). The code defines five such categories of terrain roughness. Other parameters of importance
for the gust velocity pressure are the wind direction, the height of the building site above sea level
and the topography.
In this regulation amendment process, NS 3490 (Standards Norway 1999) prescribes a 50-year
return period for environmental loads. The partial factors for environmental loads are set to 1.5. A
reduction factor kL (with a value of 0.8 – 1.0, depending on the reliability class) by which the partial
factor must be multiplied is also introduced.
FIG. 2. Wind pressure curves for non-exposed areas (A), exposed areas (B), dense-built environments in nonexposed
areas (C), dense-built environments in exposed areas (D) and especially exposed areas according to
specification in code (E and F) (figure from NS 3479/A1; Standards Norway 1994, printed with permission from
Pronorm AS 02/2006)
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