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Increased Snow Loads and Wind Actions on Existing Buildings: Reliability of the Norwegian Building Stock
Vivian Meloysund, Ph.D. ; Kim Robert Liso, Ph.D. ; Jan Siem ; and Kristoffer Apeland

Abstract: Results from an investigation of snow loads and wind actions on 20 existing buildings in Norway are presented. The objective has been to investigate to what extent existing buildings meet current regulatory requirements relating to safety against collapse owing to snow loads or wind actions. Eighteen buildings have a utilization ratio of more than 1.0 under current regulations. The new design rules have led to most of the buildings investigated having reduced safety against collapse owing to snow and greater safety against collapse owing to wind actions than the regulations now demand. The investigation indicates too low reliability for a considerable number of buildings according to current building regulations when evaluating the possible consequences of the conclusions in a national perspective. Scenarios for future climate change indicate both increased winter precipitation and increased temperatures, and thus changing the snow loads on roofs. Wind scenarios for the decades to come indicate an increase in frequencies of strong winds in areas also exposed today. Thus, the future reliability of the buildings in these areas could decrease.
DOL: 10.1061/(ASCE)0733-9445(2006)132:11(1831)
CE Database subject headings: Bearing capacity; Buildings; Climatic changes; Norway; Reliability; Snow loads; Structural design; Structural safety; Wind loads.

Introduction
Background
Large snow loads on during the winter of 1999/2000 led to the collapse of several buildings in northern Norway. The accident at Bardufoss Community Centre, where the roof caved in and claimed three lives, was the most serious of these accidents (Fig.1). The most important causes of this collapse were a faulty construction of the roof when the building was erected and larger snow loads on the roof than it was designed for.

Principal Objectives and Delimitations
The principal objective of the investigation has been to obtain a reliable indicator as to whether existing buildings in Norway meet current regulatory requirements concerning safety against collapse owing to snow loads and/or wind actions, and also to establish a basis for the analysis of future climate change impacts on the Norwegian building stock. The analysis encompasses design documentation investigations and field studies of 20 existing buildings in five high-snowfall and five high-wind municipalities in Norway (Siem et al. 2003; Meloysund et al.2004). Statistical data for building types, year of construction, and geographical localization of the approximately 3.7 million registered buildings in Norway are available in the Ground Property, Address and Building Register (GAB). Special attention has been paid to exposed types of buildings, and the buildings have been randomly selected within the exposed building categories. Assessments of whether the regulations are satisfactory and theoretical parameter studies of the regulations are not included in the investigation. The investigation focuses on assessing the buildings’ main load-bearing structures and, to a lesser extent, their secondary load-bearing structures.


Building Regulations and Design Codes
Development of Design Codes for Snow Loads and Wind Actions
The building regulations of December 15, 1949 referred to a general snow load on roofs corresponding to 1.5KN/m. This value could be reduced or increased by the individual building authority with the Ministry’s approval. The importance of the shape of the roof for the size of the snow load on the roof was calculated in a simple way. Structures should normally be designed for a wind pressure equal to 1.0 KN/m, while a wind pressure equal to 1.5 KN/m should be used in exposed areas. In heavily exposed areas, building authorities could increase these basic values with the Ministry’s approval. The sum of the wind shape factors for lee and windward walls for a closed building was 1.2.
In NS 3052 (Standard Norway 1970) snow maps were introduced showing zones with roof snow loads values of up to 1.5 KN/m, between 1.5 KN/mand 2.5 KN/m, and above 2.5 KN/m. Four curves for the wind pressure were introduced: Curves A, B, C, and D, as seen in Fig.2. The code quoted many more-detailed rules for the wind shape factors for the lee and windward walls was in the code also set to 1.2. Compared to the building regulations of 1949, the changes in NS 3052 largely implied a reduction in the wind velocity pressures in exposed areas. In NS 3052 the partial factor method was introduced. The partial factor for snow loads was set to 1.6 while the partial factor for wind actions was set to 1.5.
In NS 3497-4 (Standards Norway 2002a), a classification of the whole country has been carried out so that wind exposure for all 434 municipalities is defined. Exposure is defined by means of a reference wind velocity (varies between 22 m/s and 31 m/s). Roughness of the terrain in an area 10 km against the wind direction is important for the wind pressure (in the code called the gust velocity pressure). The code defines five such categories of terrain roughness. Other parameters of importance for the gust velocity pressure are the wind direction, the height of the building site above sea level, and the topography.
In this regulation amendment process, NS 3490 (standards Norway 1999) prescribes a 50-year return period for environmental loads. The partial factors for environmental loads are set to 1.5. A reduction factor kby which the partial factor must be multiplied is introduced.
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.
A thorough description of the historical development of design loads for wind actions and snow loads is presented by Meloysund 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, 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 numbers in Table 2 in which, under current rules, more stringent requirements are imposed on structures whose collapse may have major consequences.
Material Use and Geometry
For light roofs, the specific weight is open low compared to the snow load that the roof is required to withstand. If the snow load exceeds the design value, the 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 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
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 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 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.