Lisø PhD Dissertation Manuscript - NTNU

Lisø, K.R./ Building envelope performance in harsh climates: Methods for geographically dependent design
A driving rain exposure index for Norway (Paper XI)
Driving rain represents one of the greatest challenges in the design and construction
of outer wall structures in Norway. There is, however, no practical tool available for
assessing driving rain exposures in the planning and design of the built environment,
and there has been little progress made during the past 50 years in the quantification
and presentation of driving rain values at weather stations around the country, despite
the fact that the nature and quantity of driving rain varies significantly.
Weather data at most observing stations in Norway are not recorded as hourly values
and are therefore not suitable for existing standardised methods for the determination
of driving rain exposure requiring hourly values of rainfall and mean directional
wind speed. An alternative method for assessing driving rain exposures based on
multi-year records of synoptic observations of present weather, wind speed and
direction is therefore developed. Distributions of numbers of rain observations and
wind speeds versus wind direction combined with average annual rainfall totals has
yielded quantitative information about driving rain exposures at stations, providing a
representative picture of the relative frequency of driving rain from different
directions. A complete national driving rain map is presented in Figure 3.
The principal advantages with the presented method are that the angular distributions
of driving rain loads obtained are high resolution in terms of wind direction and that
the results are based on long-term series of climate data that are readily available.
Where the 1955 driving rain map for Norway yielded driving rain intensities in only
four principal directions (north, south, east and west), the new method, with 36
directions, gives a much more detailed picture of the directional dependence of winddriven
rain at a weather station. And, driving rain indices presented in this work are
from 30 years of synoptic observations representing the most recent climate.
The method can also be used to evaluate changes in driving rain loads due to climate
change under global warming, through the incorporation of data from climate change
scenarios into the methodology.
Effects of wind exposure on roof snow loads (Paper XII)
International standards do not necessarily reflect national distinctive climate
characteristics and topography features. The preparation of national appendices
associated with various types of climatic impact is necessary in order for these
characteristics to be considered. Again, local climate data needs to be analysed in
order to evaluate the validity of proposed methods and tools for assessment and
differentiation of climatic loads for various architectural or engineering purposes. In
the Norwegian Standard NS 3491-3 “Design of structures – Design Actions – Part 3:
Snow loads” from 2001 a new coefficient was introduced, making it possible to take
into account the effect of wind exposure on roof snow load. The definition of this
exposure coefficient is based on ISO 4355 “Bases for design of structures –
Determination of snow loads on roofs”. It is defined as a function of the mean
temperature in the coldest winter month and number of days with a wind velocity
above 10 m/s. Three winter temperature categories and three winter wind categories
are defined, resulting in 9 exposure coefficient values varying from 0.5 to 1.0
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