Lisø PhD Dissertation Manuscript - NTNU

The hurricane (Beaufort number 12) which occurred in northwest Norway on New Year’s Day
1992 caused damage to buildings costing somewhere in the region of $ 200 million. Wind velocities
of 62 - 63 m/s were recorded, the highest wind velocities ever recorded on mainland Norway. The
bulk of the damage was related to roofs and roofing, primarily due to insufficient anchoring. Most of
the damage that appeared could have been avoided if the existing building regulations and Codes of
Practice had been adhered to (National Office of Building Technology and Administration et al.
1993). Investigations into the damage taking place in 1992 showed that during the period 1950-1991
the likelihood of structures in new buildings having too low load carrying capacity was increasing.
The investigations also demonstrated that if the safety levels in the regulations were to be conformed
to, it would be necessary to update the wind action provisions. 55 % of the recorded cases of damage
caused by the hurricane in northwest Norway in 1992 involved buildings erected before 1970
(National Office of Building Technology and Administration et al. 1993). The fact that a building has
served well for many years is thus no guarantee that it will withstand subsequent hurricanes.
Principal objectives and delimitations
The principal objective of the investigation has been to obtain a reliable indicator as to whether
existing buildings in Norway meet current regulatory requirements concerning safety against collapse
owing to snow loads and/or wind actions, and also to establish a basis for the analysis of future
climate change impacts on the Norwegian building stock. The analysis encompasses design
documentation investigations and field studies of 20 existing buildings in five high-snowfall and five
high-wind municipalities in Norway (Siem et al. 2003; Meløysund et al. 2004). Statistical data for
e.g. building types, year of construction and geographical localization of the approximately 3.7
million registered buildings in Norway are available in the Ground Property, Address and Building
Register (GAB). Special attention has been paid to exposed types of buildings, and the buildings have
been randomly selected within the exposed building categories. Assessments of whether the
regulations are satisfactory, and theoretical parameter studies of the regulations, are not included in
the investigation. The investigation focuses on assessing the buildings’ main load-bearing structures
and, to a lesser extent, their secondary load-bearing structures.
BUILDING REGULATIONS AND DESIGN CODES
The development of design codes for snow loads and wind actions
The building regulations of 15 December 1949 (National Office of Building Technology and
Administration 1949) referred to a general snow load on roofs corresponding to 1.5 kN/m 2 . This
value could be reduced or increased by the individual building authority with the Ministry’s approval.
The importance of the shape of the roof for the size of the snow load on the roof was calculated in a
simple way. Structures should normally be designed for a wind pressure equal to 1.0 kN/m 2 , while a
wind pressure equal to 1.5 kN/m 2 should be used in exposed areas. In heavily exposed areas, building
authorities could increase these basic values with the Ministry’s approval. The sum of the wind shape
factors for lee and windward walls for a closed building was 1.2 (i.e. 1 + 0.2).
In NS 3052 (Standards Norway 1970) snow maps were introduced showing zones with roof snow
loads with values of up to 1.5 kN/m 2 , between 1.5 and 2.5 kN/m 2 and above 2.5 kN/m 2 . Four curves
for the wind pressure were introduced: the curves A, B, C and D as seen in Figure 2. The code quoted
many more detailed rules for the wind shape factors than the building regulations of 1949. The sum
of the shape factors for the lee and windward walls was in the code also set to 1.2 (i.e. 0.7 + 0.5).
Compared to the building regulations of 1949, the changes in NS 3052 largely implied a reduction in
the wind velocity pressures in exposed areas. In NS 3052 the partial factor method was introduced.
The partial factor for snow loads was set to 1.6 while the partial factor for wind actions was set to
1.5.
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