Lisø PhD Dissertation Manuscript - NTNU

municipalities of Andøy and Fræna. The buildings included in the investigation were low in height
relative to their width and length. For buildings with this form, the sum of the shape factors against
the windward and lee wall is equal to 0.85 in NS 3491-4, while the factor may become 1.5 for a high
building. In earlier codes, the corresponding shape factor is 1.2, irrespective of the height of the
building. In other words, the shape factor has become significantly lower for the building form that
dominates the selected buildings, while it would not have dropped so low if the buildings had been
high relative to their length and width. The reduction in design wind action for the selected buildings
would therefore not apply for example to high-rise buildings.
Capacity exceedings compared with load increases
The highest utilization ratio found for important structural components for the buildings investigated
are summarised in Table 3. As the table shows, 18 buildings have a utilization ratio of more than 1.0.
This represents 90 % of the buildings that were investigated. Although the design requirements for 95
% of the buildings have increased since they were built, one would assume that they had built-in
reserve capacities that meant that fewer buildings experienced a utilization ratio of more than 1.0.
The design rules for the most important materials in the structures have changed since the
buildings investigated were planned. Essentially, the materials load-bearing properties can be utilised
to a higher degree now than in the past. If the buildings had been constructed in accordance with the
regulations when they were built, one would therefore have expected that fewer than 18 buildings
would have exceeded their capacity under current rules. The table also shows that 11 buildings have a
higher utilization ratio than a load increase. This indicates incorrect planning, incorrect construction
or rebuilding.
It is important that all owners of buildings know what value of snow load the roof is designed for,
and have routines for monitoring the snow load on the roof and clearing it when necessary. When
roofs are being cleared of snow, this may in the worst-case make the stresses on the structure greater
than before the snow clearance started. There are many examples of snow clearing leading to the
collapse of structures. Instructions on this are contained in “Snow loads on existing roof
constructions” (Lisø et al. 2000) and Report HO-1/2001 “Guidelines on snow loads on roofs”
(National Office of Building Technology and Administration 2001).
DISCUSSION
As mentioned earlier the selected buildings in the investigation are building types regarded as being
especially exposed to increasing snow loads and wind actions. The exposed building types amount to
5 % of the total bulk of buildings in Norway (11 % of total building floor area).
90 % of the buildings investigated have too low capacity when compared with current design
rules. Thus, potentially 4.5 % of the total bulk of buildings in Norway may have too low capacity
according to current regulations. The design snow loads have increased for 95 % of the investigated
buildings indicating an increase in design snow loads for 4.7 % of the total bulk of buildings. 55 % of
the investigated buildings have higher utilization ratio than load increase, which may indicate
incorrect planning, incorrect construction or rebuilding. Thus, potentially 2.8 % of the total bulk of
buildings in Norway have higher utilization ratio than load increase. However, the investigation
constitutes only 20 buildings, and thus has obvious quantitative weaknesses. It must nevertheless be
regarded as an important pointer on challenges concerning reliability.
The investigation indicates too low reliability for a considerable number of buildings according to
current building regulations. It could also be of interest considering the future reliability of the
building stock taking into account possible climate changes. In Norway the design life for buildings is
60 years in general and 100 years for monumental buildings.
Research establishments in several parts of the world are addressing the risks associated with
future climate change and the resulting impacts. According to Karl and Trenberth (2003) modern
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