Lisø PhD Dissertation Manuscript - NTNU

Effects of wind exposure on roof snow loads
Vivian Meløysund *,a,b , Kim Robert Lisø a,c , Hans Olav Hygen d , Karl V. Høiseth b and
Bernt Leira e
a SINTEF Building and Infrastructure, P.O. Box 124 Blindern, NO-0314 Oslo, Norway,
E-mail: vivian.meloysund@sintef.no, fax +47 22 69 94 38.
b Norwegian University of Science and Technology (NTNU), Department of Structural Engineering, NO-7491
Trondheim, Norway.
c Norwegian University of Science and Technology, Department of Civil and Transport Engineering, NO-
7491 Trondheim, Norway.
d Norwegian Meteorological Institute, P.O. Box 43 Blindern, NO-0313 Oslo, Norway.
e Norwegian University of Science and Technology, Department of Marine Technology, NO-7491 Trondheim,
Norway.
Abstract
This paper presents results from an investigation of the suitability of the exposure
coefficient as defined in ISO 4355 ”Bases for design of structures – Determination of snow
loads on roofs”, based on thorough analyses of weather data from 389 weather stations in
Norway for the reference 30-year period 1961-1990. First, the physics of snow transport
and the historical background of the exposure coefficient are examined. Field
investigations of snow loads on the roofs are also evaluated. Next, values for the exposure
coefficients in Norway are calculated according to ISO 4355. Finally, possible approaches
aiming at improving calculations of wind exposure on roof snow loads are suggested. It is
shown that the exposure coefficient as defined in ISO 4355 dos not reflect the actual effects
of wind exposure on roof snow loads in Norway, the main reasons being
oversimplifications in the definition of the coefficient and the extreme variations of the
climate in Norway. The definition is based on coarse simplifications of snow transport
theories, and must be revised and improved to serve as an applicable tool for calculations
of design snow loads on roofs in Norway.
Keywords: buildings, roofs, snow, snow loads, structural design, wind loads.
1. Introduction
In the current Norwegian snow load standard NS 3491-3 “Design of structures - Design
actions - Part 3: Snow loads” [1] snow loads on roofs are defined as
s e t
= μ ⋅ C ⋅ C ⋅ s (1)
where s0 is snow loads on the ground. The parameters μ, Ce and Ct describe conditions on
the roof. The exposure coefficient Ce takes into account that wind removes snow from flat
roofs. Using this coefficient the snow load on a sheltered roof becomes twice as large as
the snow load on a windswept roof. The shape coefficient μ describes the distribution of
snow load on the roof due to geometry. The thermal coefficient Ct defines the reduction of
* Corresponding author
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