Lisø PhD Dissertation Manuscript - NTNU

As is apparent from the values in the table, the buildings selected can be characterised as mediumsized
buildings with medium spans. The roof slope varies between 0 and 26 degrees. All the buildings
are of low height relative to their width and length. Essentially, the buildings included in the
investigation are lightweight constructions, because buildings of this type are empirically expected to
be most vulnerable.
Availability and scale of the documentation
When the investigations started, the authors were prepared for the fact that it might be difficult to
obtain full documentation on the load-bearing structures in the buildings, which in this context have
been defined as design calculations and structural drawings. Although there were requirements in the
building regulations up to 1997 that design calculations should form part of the building licence
application, it is well known that many municipalities have not enforced this requirement.
In the light of the information supplied by the municipalities, a total of 20 buildings were selected.
Buildings with available documentation were given priority. It was decided at an early stage that
built-in structures would not be opened and investigated. It was therefore necessary to obtain the best
possible documentation so that built-in structures were known from the documentation. If there were
a link between available documentation in the municipality and existing documentation, such a
selection criteria would lead to the buildings most extensively planned being included in the
investigation. Buildings that are planned in detail are probably also those with the fewest defects. It
has not been possible to assess the significance of this aspect within the scope of this investigation.
A lack of important documentation for buildings included in the investigation can affect the
results. The calculations must then be based on our own assumptions and assessments, which may be
different from the constructor’s (see Table 3 for information on available structural calculations).
Deficient information on hidden, structural measures may then be significant. A lack of
documentation makes it difficult to uncover the reason for chosen structural designs unambiguously.
All buildings in the investigation were approved for erection by the local authority (the
municipality in which they are located). But, according to the available documentation, only 14 of the
selected buildings proved to be designed by professionals. The actual number of buildings designed
by professional is probably higher.
Changes in design snow loads and wind actions for selected buildings
Current requirements for characteristic snow loads on the ground and characteristic gust velocity
pressures against the selected buildings are presented in Table 3. In the table, Andøy 2, Fræna 1 and
Nittedal 1 are quoted with a) and b) versions. Here, a) means the original building and b) means
additions (or extensions). Furthermore, the changes in design loads on the buildings are shown, where
current requirements are compared with the requirements that applied when the building was being
designed. Table 3 shows that the changes in design snow loads vary between 0.8 and 2.7 and have a
mean value equal to 1.6. The changes in design wind action against the buildings vary accordingly
between 0.4 and 1.4 and have a mean value equal to 0.9. In other words, the design snow load has on
the average increased, while the design wind action has on the average been reduced.
As Table 3 indicates; only two buildings in two municipalities experience reduced design snow
loads, one experiences an unaltered load level, while the rest experience increased snow loads. The
changes in the rules for snow loads have therefore been of major importance to the requirement
concerning design snow loads on most of the buildings that have been investigated. Buildings with a
low roof slope dominate the investigation. Pitched roofs with roof slopes of between 15 º and 60 º have
been given reduced shape factor for snow loads on the lee side of the roof. For the seven buildings
with roof slopes > 15 º , the increase in design load is on the average 1.4, which is somewhat lower
than the mean value for all the buildings.
The changes in the wind action rules have not been as important as the change in the snow load
rules for the design loads on the buildings investigated. As Table 3 shows, the changes in the rules
have only resulted in a significant increase in the wind action on the buildings in the coastal
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