Lisø PhD Dissertation Manuscript - NTNU
Lisø PhD Dissertation Manuscript - NTNU Lisø PhD Dissertation Manuscript - NTNU
of roof slope and heating were included. Based on this study, the average ground-to-roof conversion factors for unheated flat roofs are recalculated by the authors of this paper to be 0.76 for the sheltered roofs, 0.57 for the semi-sheltered roofs and 0.55 for the windswept roofs. In the European Snow Load Program 1997-1999, roof snow load were measured for 55 pitched roofs and 26 flat roofs in Switzerland, Italy, Great Britain and Germany in the winter season 1998-99 [9]. The roof-to-ground ratio for flat roofs was calculated to 0.90 for sheltered roofs, 0.74 for semi windswept roofs and 0.58 for windswept roofs. When selecting the buildings for this project, unheated or very high isolated roofs were required (whether this requirement is met is not considered). The findings of the investigations, which indicate the effect of wind blowing snow from the roof, are summarized in Table 2. Table 2. Exposure coefficients for flat roofs Reference Exposure coefficient Sheltered Semi-sheltered Windswept Otstavnov (1989) a) 0.98 0.72 0.46 Lutes (1970) 0.90 Taylor (1979) 0.80 b) - O’Rourke (1983) c) 0.76 0.57 Høibø (1988) s 0 = 1.0 kN/m 2 s 0 = 3.5 kN/m 2 0.82 d) 0.62 d) Løberg (1976) - 0.55 Com. Eur. Comm (1999) 0.90 0.74 Page 6 of 17 0.60 0.30 a) Assumed snow cover for 3.5 months. Average winter wind velocity in sheltered, semi-sheltered and windswept area are assumed to be respectively 2 m/s, 4 m/s and 6 m/s b) Snow ground load with 30-year return period is used when calculating roof-to-ground ratio c) Values are recalculated in order to apply unheated roofs d) Degree of wind exposure is not registered. s 0 – ground snow load. In investigations performed by Irwin et al. [10], the effect of roof size was studied. It was concluded that there is a trend towards increased uniform snow loads on flat roofs with increasing size. It was recommended to account for roof size when considering roofs with characteristic lengths above respectively 75 m and 200 m for sheltered and open wind exposure (characteristic length equals width * (2 – width/length)). ISO 4355 defines wind categories and temperature classes in connection with determination of the exposure coefficients Ce. The justification of the recommendations is somewhat vague. According to Otstavnov [4] drifting occurs at average wind velocities above 4 m/s during snowfall and above 6.5 m/s with no snowfall. Other studies have focused on a more instant threshold wind velocity and not a wind velocity averaged over a longer period. According to Mellor [11] threshold wind velocities of 3 to 8 m/s at a height of 10 m are needed in order to transport loose and unbounded snow. If the surface snow is densely packed and firmly bounded threshold wind velocity above 30 m/s may occur. According to Kind [12] the threshold wind velocity is approximately 5 m/s at a height of 10 m for fresh dry snow, 11 m/s for slightly aged or hardened snow and 23 m/s for snow hardened by very strong winds. - - - 0.55 - - 0.27 0.58
Li and Pomeroy [13] evaluated hourly observations from the period 1970 to 1976 at 16 meteorological stations in the Canadian prairies. Based on this studies threshold wind velocities were recorded and presented as a function of temperature. It was concluded that threshold wind increased nonlinearly with ambient air temperature above –25 °C. An average threshold wind velocity of 9.9 and 7.7 m/s was observed for respectively wet and dry snow transport. An average threshold wind velocity of 7.5 and 8.0 m/s was observed for respectively fresh and aged snow. Results from field investigations show a reduction in roof snow load with increasing wind exposure (Table 2). The values of the exposure coefficients vary, possibly as a result of differences in the definitions of the categories. Although it can be concluded that wind exposure is of large importance for the resulting snow loads on roofs. 3. The exposure coefficients for Norway according to ISO 4355 Data from 389 meteorological stations in Norway is used in order to derive temperature zones and wind categories as defined in ISO 4355. Within the normal period (1961 – 1990), stations with at least 15 years of data are used. Temperature zones are based on reference grids for the normal period developed by the Norwegian Meteorological Institute. As seen on Fig. 4, almost none of the stations have mean temperatures above 2.5 °C in the coldest winter month. Temperature category A (as defined in ISO 4355) is only represented at small offshore islands in the southern part of Norway, and it is therefore not visible on the map in Fig. 4. Mean temperatures between –2.5 and 2.5 °C are mainly found in the coastal areas in south and west. For a majority of the stations, mean temperatures below – 2.5 °C are registered for the coldest winter month. Almost none of the meteorological stations situated in the zone with mean temperatures between −2.5 and 2.5 °C have less than one day with wind velocity above 10 m/s. In south and west, stations situated at places highly exposed to wind (e.g. at lighthouses on islands/peninsulas) have more than 10 days with wind velocities above 10 m/s. In areas settled with buildings, the number of days with wind velocities above 10 m/s is mainly between 1 and 10. In the northern part of Norway, the number of days with wind velocities above 10 m/s exceeding 10 days is found for this temperature zone also in settled areas. Considering the areas with mean temperatures below –2.5 °C in the coldest winter month, some characteristics can be observed. In the inland of southern Norway, the stations situated in the mountainous areas have mainly between 1 and 10 days with wind velocity above 10 m/s in the three coldest winter months. For lower regions, where most of the people are settled, the number of days with wind velocity above 10 m/s is mainly below 1. Further north the number of days with wind velocity above 10 m/s is mainly between 1 and 10 days. At some of the stations highly exposed to wind, the number of days with wind velocity above 10 m/s exceeds 10 days. These stations are mainly situated in areas close to the sea where the building density is low. Page 7 of 17
- Page 121 and 122: The hurricane (Beaufort number 12)
- Page 123 and 124: The extensive revisions of the code
- Page 125 and 126: The number of buildings investigate
- Page 127 and 128: As is apparent from the values in t
- Page 129 and 130: climate change is now dominated by
- Page 131: TABLE 3. Summary of findings Struct
- Page 134 and 135: Lisø, K.R./ Building envelope perf
- Page 136 and 137: temperature of -4°C or less. See F
- Page 138 and 139: Today, frost resistance of brick, c
- Page 140 and 141: degree of saturation, freezing will
- Page 142 and 143: accelerated frost damage or frost d
- Page 144 and 145: A possible objection to the present
- Page 146 and 147: [17] Dührkop H., Saretok V., Sneck
- Page 149 and 150: Decay potential in wood structures
- Page 151 and 152: Wood decaying fungi will only be ac
- Page 153 and 154: to promote decay prevails. Two of t
- Page 155: Acknowledgements This paper has bee
- Page 158 and 159: (a driving rain gauge), however, is
- Page 160 and 161: Fig. 1. Map of Norwayshowing the lo
- Page 162 and 163: Frequency 25 20 15 10 5 0 highest p
- Page 164 and 165: interesting to compare those result
- Page 167 and 168: Effects of wind exposure on roof sn
- Page 169 and 170: In ISO 4355 ”Bases for design of
- Page 171: variety of roofs and wind exposures
- Page 175 and 176: Fig. 5. Exposure coefficients for 3
- Page 177 and 178: meteorological stations are expecte
- Page 179 and 180: Many of the meteorological stations
- Page 181 and 182: the ground with return period of 30
- Page 183: References [1] Standards Norway. De
Li and Pomeroy [13] evaluated hourly observations from the period 1970 to 1976 at<br />
16 meteorological stations in the Canadian prairies. Based on this studies threshold wind<br />
velocities were recorded and presented as a function of temperature. It was concluded that<br />
threshold wind increased nonlinearly with ambient air temperature above –25 °C. An<br />
average threshold wind velocity of 9.9 and 7.7 m/s was observed for respectively wet and<br />
dry snow transport. An average threshold wind velocity of 7.5 and 8.0 m/s was observed<br />
for respectively fresh and aged snow.<br />
Results from field investigations show a reduction in roof snow load with<br />
increasing wind exposure (Table 2). The values of the exposure coefficients vary, possibly<br />
as a result of differences in the definitions of the categories. Although it can be concluded<br />
that wind exposure is of large importance for the resulting snow loads on roofs.<br />
3. The exposure coefficients for Norway according to ISO 4355<br />
Data from 389 meteorological stations in Norway is used in order to derive temperature<br />
zones and wind categories as defined in ISO 4355. Within the normal period (1961 –<br />
1990), stations with at least 15 years of data are used. Temperature zones are based on<br />
reference grids for the normal period developed by the Norwegian Meteorological Institute.<br />
As seen on Fig. 4, almost none of the stations have mean temperatures above 2.5 °C in the<br />
coldest winter month. Temperature category A (as defined in ISO 4355) is only represented<br />
at small offshore islands in the southern part of Norway, and it is therefore not visible on<br />
the map in Fig. 4. Mean temperatures between –2.5 and 2.5 °C are mainly found in the<br />
coastal areas in south and west. For a majority of the stations, mean temperatures below –<br />
2.5 °C are registered for the coldest winter month.<br />
Almost none of the meteorological stations situated in the zone with mean<br />
temperatures between −2.5 and 2.5 °C have less than one day with wind velocity above 10<br />
m/s. In south and west, stations situated at places highly exposed to wind (e.g. at<br />
lighthouses on islands/peninsulas) have more than 10 days with wind velocities above 10<br />
m/s. In areas settled with buildings, the number of days with wind velocities above 10 m/s<br />
is mainly between 1 and 10. In the northern part of Norway, the number of days with wind<br />
velocities above 10 m/s exceeding 10 days is found for this temperature zone also in settled<br />
areas.<br />
Considering the areas with mean temperatures below –2.5 °C in the coldest winter<br />
month, some characteristics can be observed. In the inland of southern Norway, the stations<br />
situated in the mountainous areas have mainly between 1 and 10 days with wind velocity<br />
above 10 m/s in the three coldest winter months. For lower regions, where most of the<br />
people are settled, the number of days with wind velocity above 10 m/s is mainly below 1.<br />
Further north the number of days with wind velocity above 10 m/s is mainly between 1 and<br />
10 days. At some of the stations highly exposed to wind, the number of days with wind<br />
velocity above 10 m/s exceeds 10 days. These stations are mainly situated in areas close to<br />
the sea where the building density is low.<br />
Page 7 of 17
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