Lisø PhD Dissertation Manuscript - NTNU

Year of construction, loads and geographical location
Design loads on buildings have changed considerably in the period from 1949 till today. The year of
construction may therefore tell something about the building’s safety level. In general, older
buildings in high-snowfall areas may have a lower safety with respect to snow loads than newer
buildings. The difference in safety level with respect to wind action is probably somewhat less.
The safety level is probably affected mostly in areas that are heavily exposed to the environmental
loads, when snow loads and wind actions in the regulations are increased from general loads that
have applied to the entire country to differentiated loads that are adjusted to the actual environmental
load variation in Norway. Increased wind actions therefore probably have the greatest consequences
for coastal areas from northwest Norway northward. Locally, roughness of terrain and topography
may be of great significance for the wind actions that the buildings experience. Inland areas high
above sea level are most vulnerable to increased snow loads. Locally, topography and wind action is
also important for the snow loads that the building experiences.
The construction process
Questions may be raised as to whether buildings erected by way of voluntary communal work or that
are self-built, are more vulnerable to collapse than other buildings. It has been claimed that savings
on design costs and technical expertise in the construction phase are often made for such buildings.
Such savings can lead to little or no documentation of the structures being produced, and there may
be a lack of people with the necessary technical competence to take overall responsibility for the
execution of the work. This can lead to unfortunate improvisations on site that is assessed by the
person implementing the project as “good enough”. It is therefore reasonable to believe that there is a
greater probability of weak points that can result in the sudden collapse of such buildings.
Prefabricated structures are often imported. It has been claimed that the design calculations do not
always meet the design rules set out in Norwegian codes and that many structures have been designed
for relatively small snow loads compared to Norwegian requirements. Structures have been imported
from countries such as Denmark that are designed for snow loads well below those required in
Norway.
The history of buildings
Reconstruction or rebuilding may lead to significant changes in both design loads and load-bearing
capacity. Columns and beams in outer walls (for example) may be centrally arranged during
rebuilding, and thus assume greater loads. The need to strengthen structural elements, including
foundations, may be extensive and is not always met. When walls are removed or new openings
established, the stabilising elements of columns can change, or the anchoring may change, resulting
in reduced load-bearing capacity.
Modifying the construction of roof structures may result in load change. For example, roofs that
are changed from e.g. roofing membranes to tiles acquire an additional load of about 0.5 kN/m 2 .
Additional insulation of roofs in older buildings may lead to reduced snow melting. Assuming the
same precipitation pattern, this may result in larger quantities of snow remaining on the roof than
before the additional insulation.
Changing the use of buildings, which may lead to greater consequences in the event of collapse,
makes it necessary to reassess the capacity of the structures. In the case of a change of use from for
example a storage building to a building for public use, the requirement concerning maximum annual
probability of failure becomes more stringent, and there may be a need to strengthen the building.
Selection criteria
It would be desirable to identify a selection of buildings that could at large be characterised as
communal halls, sports halls, buildings used by many people, lightweight structures, prefabricated
buildings, buildings with large spans, buildings supplied as package solutions, buildings with
modified applications, buildings erected via communal voluntary work, temporary structures and
converted buildings. These are building types all of which are regarded as being especially exposed
to increasing snow loads and wind actions.
Manuscript No. ST/2005/024694 6 of 13