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1、外文原文1Increased Snow Loads and Wind Actions on Existing Buildings: Reliability of the Norwegian Building StockVivian Meloysund, Ph.D. ; Kim Robert Liso, Ph.D. ; Jan Siem ; and Kristoffer ApelandAbstract: Results from an investigation of snow loads and wind actions on 20 existing buildings in Norway a
2、re 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 r
3、ules 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 curr
4、ent 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 c
5、ome 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.CE Database subject headings: Bearing capacity; Buildings; Climatic changes; Norway; Reliability; Snow loads; Structural design; Structural
6、 safety; Wind loads.IntroductionBackgroundLarge 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
7、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 DelimitationsThe principal objective of the investigation has been to obtain a reliable indicator as to whet
8、her 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 documenta
9、tion 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 registe
10、red 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 satisfact
11、ory 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 CodesDevelopment of Desig
12、n Codes for Snow Loads and Wind ActionsThe 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 Ministrys approval. The importance of the shape of the roof f
13、or 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 va
14、lues with the Ministrys 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
15、 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 imp
16、lied 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 countr
17、y 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 calle
18、d 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 (standard
19、s 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 regul
20、ations 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 level
21、s 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 MethodologyLimits of UseThe consequences of a collapse are greater in buildi
22、ngs 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
23、 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 GeometryFor light roofs, the specific weight is open low compared to the snow load that the roof is
24、 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
25、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
26、 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 start
27、ed. 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 LocationDesign loads on buildings have changed c
28、onsiderably in the period from 1949 to today. The year of construction may, therefore, tell something about the buildings 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 resp
29、ect 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
30、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
31、 the building experiences.Construction ProcessPrefabricated 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 req
32、uirements. Structures have been imported from countries such as Denmark that are designed for snow loads well below those required in Norway.Selected BuildingsBased on the assessments above, 20 buildings were selected Table 3 lists the municipality in which the buildings were selected, the building
33、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 inves
34、tigation 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. Th
35、is 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 StudyCalculation models, loads, forces, and solutions used when the buildings wer
36、e 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 structures utilization ratio has been determined in accordance with ne
37、w 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
38、structure owing to reduced durability or due to reconstruction.ResultsGeometry and Material DataExternal dimensions, maximum spans, and the material of the main load-bearing structures are shown in Table 3. The buildings external dimensions are quoted as width, length, height, and roof slope. The he
39、ight 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 me
40、dium 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.Availabil
41、ity and Scale of the DocumentationWhen 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. A
42、lthough 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
43、 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 d
44、ocumentation. 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 asse
45、ss 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 constructors (se
46、e 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 fo
47、r Selected BuildingsCurrent 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 buil
48、ding 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 an
49、d 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
