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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 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 the 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.

Year of Construction, Loads, and Geographical Location Design loads on buildings have changed considerably in the period from 1949 to 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 regulation 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 and wind action are also important for the snow loads that the building experiences. Construction Process

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Prefabricated structures are often imported. It has been claimed that 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. Selected Buildings Based on the assessments above, 20 buildings were selected Table 3 lists the municipality in which the buildings were selected, the building type, and the requirement that currently applies to characteristic snow load on the ground and to the reference wind velocity. As shown in Table 3, attempts have been made to keep the selected buildings as anonymous as possible. Problems in obtaining the necessary documentation implied that an investigation of only one building was conducted in two of the municipalities, while this was extended to three buildings in two other municipalities. Three of the buildings were constructed in the period before 1970, eight were built in the period 1970-79, and nine were built in the period after 1979. This implies that the loads are determined by the 1949 building regulations for three of the buildings, by NS 3052 for the buildings, and by NS 3479 for nine of the buildings.

Project Documentation Investigation and Field Study Calculation models, loads, forces, and solutions used when the buildings were constructed have been investigated. The forces in the structure were then determined in accordance with new load requirements, and the capacities checked in accordance with new load requirements. In light of these analyses, the structure’s utilization ratio has been determined in accordance with new calculation rules, and the need for reinforcement assessed. On site, whether the structures have defects or deficiencies that are not apparent from the project documentation of whether or not the construction was in accordance with the documentation, and whether or not there were weaknesses in the structure owing to reduced durability or due to reconstruction.

Results

Geometry and Material Data External dimensions, maximum spans, and the material of the main load-bearing structures are shown in Table 3. The building’s external dimensions are quoted as width, length, height, and roof slope. The height indicates the cornice height for buildings with other roof shapes. Additions or extensions that are not included in the assessments have not been included in the dimensions. As is apparent from the values in the table, the buildings selected can be characterized as medium-sized buildings with medium spans. The roof slope varies between 0 and 26°. All the buildings are of low height relative to their width and length. Essentially, the buildings included in the investigation are light-weight constructions, because buildings of this type are empirically expected to be most vulnerable. Availability and Scale of the Documentation When the investigations started, the writers 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 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

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documentation so that built-in structures were known from the documentation. If there were links between available documentation, such selection criteria would lead to the buildings most extensively planned being included in the investigation. Buildings that were 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. Changes in Design Snow Loads and Wind Actions for Selected Buildings Current requirements for characteristic snow loads on the ground and characteristic gust velocity pressure against the selected buildings are presented in Table 3. In Table 3, Andoy 2, Frana 1, and Nittedal 1 are quoted with “a” and “b” versions. Here, “a” means the original building and “b” means additional (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 experienced reduced design snow loads, one experienced an unaltered load level, while the rest experienced 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 slopes of between 15 and 60°have been given reduced shape factors for snow loads on the lee side of the roof. For the seven buildings with roof slopes>15°, 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 wind action rules have not been as important as the change in the snow load rules for the design loads on the buildings in 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 municipalities of Andoy and Frana. 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 NS3491-4, while the factor may become 1.5 for a high building. In earlier coeds, 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.

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). Ninety percent of the buildings investigated have too low a capacity when compared with current design rules. Thus, potentially 4.5% of the total bulk of buildings in Norway may have too low a capacity

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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. Fifty-five percent of the investigated buildings have a 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 a 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.

Conclusions The principal objective has been to obtain reliable indicators as to whether existing buildings in Norway meet current regulatory requirements concerning safety against collapse as a result of snow loads and/or wind actions. Some clear indications of aspects that ought to be considered as a represented as a representative trend for the building types investigated have been found. Eighteen out of 20 buildings have a utilization ratio of more than 1.0 (90% of the buildings investigated). The design requirements for 95% of the buildings have increased since they were built. Nevertheless, one would assume that the buildings had built-in reserve capacities resulting in fewer buildings experiencing a utilization ratio of more than 1.0. Scenarios for future climate change indicate both increased winter precipitation and increased temperature, and will result in changes regarding snow loads on roofs in parts of the country. An increase in frequency of strong winds in areas also exposed today is also estimated. According to these scenarios the future reliability of buildings in these areas could decrease.

Acknowledgments This paper has been written within the ongoing SINTEF Research and Development Programme “Climate 2000-Building Constructions in a More Severe Climate” (2000-2006), strategic institute project “Impact of climate Changer on the Built Environment” (Liso et al. 2005). The writers aratefully acknowledge all construction industry partners and Research Council of Norway. Special thanks are extended to Professor Jan Vincent. Thus, Professor Karl Vincent Hioseth, and Professor Tore Kvande for comments on the text.

References

Karl, T. R., and Trenberth, K.E. (2003). “Modern global climate change.” Science, 302 1719-1723

National office of Buildings Techolgy and Administration. (1993). “Orkan 1992.” Norwegian Building Research Insititue, Oslo, Norway (in Norwegian).

Standards Norway. (1970). Beregninger av belasninger, NS 3052, 1st Ed., Standard Norway, Oslo, Norway (in Norwegian).

Standards Norway. (1970). Prosjektering av bygningskonstruksjoner Dimensjonerende laster, NS 3479, 1st Ed., Standard Norway, Oslo, Norway (in Norwegian).

Standards Norway. (1999). Design of structures Requirements to reliability, NS 3490, 1st Ed., Standard Norway, Oslo, Norway (in Norwegian).

Standards Norway. (2002a). Design of structures Design actions1st Ed., Standard Norway, Oslo, Norway (in Norwegian).

McCarthy, J.J., Canziani, O.F., Leary, N.A., Dokken, D.J., and White, K.S., eds. (2001). Climate change 2001: Impacts, adaptation and vulnerability, Cambrige University Press, Cambriged, U.K.

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