Lisø PhD Dissertation Manuscript - NTNU

The extensive revisions of the codes have increased the level of detail in the regulations
considerably. The objective is to achieve a safety level in accordance with Table 2. In other words,
the intention is to achieve a more uniform safety level for buildings that have the same reliability
class even if they are built in different places, and also to obtain different safety levels for structures
classified in different reliability classes.
TABLE 2. The link between reliability class, failure consequences, reliability index and maximum annual
probability of failure
Reliability
class
4
3
2
1
Consequence
(examples of construction)
Particularly considerable consequences
(atomic reactor).
Considerable consequences
(shopping centre)
Moderate consequences
(office building)
Limited consequences
(house)
Manuscript No. ST/2005/024694 5 of 13
Reliability
index β
Maximum annual
probability of
failure
4.26 10 -5
3.71 10 -4
3.09 10 -3
2.32 10 -2
A thorough description of the historical development of design loads for wind actions and snow
loads is presented by Meløysund et al. (2004).
SELECTION CRITERIA AND METHODOLOGY
Limits of use
The consequences of a collapse are greater in buildings in which many people are present than in
buildings with few people. A collapse in public buildings such as sports halls, shops and the like has
therefore greater consequences than, for example, in storage facilities in which it is less probable that
people will be present. This is also apparent from the reliability approach set out in figures in Table 2
in which, under current rules, more stringent requirements are imposed on structures whose collapse
may have major consequences.
Temporary structures are often more poorly planned and designed than permanent structures. This
is in accordance with Norwegian codes if the building is designed to have a short lifetime and the
return period for the environmental loads is adjusted for this. However, temporary structures often
remain in use for a much longer time than expected. This means that the safety level for such
structures in practice often is too low.
Material use and geometry
For light roofs, the specific weight is often low compared to the snow load that the roof is required to
withstand. If the snow load exceeds the design value, the total load has increased virtually the same
percentage as the snow load. If the specific weight had been high, the percentage increase would have
been much smaller. Lightweight structures are therefore more vulnerable to an increase in snow load
above the load for which the structure is designed than heavy structures. In other words, heavy
structures have greater built-in safety when the snow load increases beyond the load that the structure
is designed to withstand.
Another selection criterion is the maximum span of a building. The consequences of a collapse in
buildings with large spans are usually great.
A number of types of construction may be sensitive to unbalanced loads. When the structures are
being cleared of snow, this may in worst-case make the stresses in the structure larger than before the
snow clearance started. There are many examples of snow clearing leading to the collapse of
structures. It is therefore important to know whether the structure can carry the unbalanced load that
arises during snow clearance.