Lisø PhD Dissertation Manuscript - NTNU
Lisø PhD Dissertation Manuscript - NTNU Lisø PhD Dissertation Manuscript - NTNU
30 meteorological stations (8 %) achieve values of the exposure coefficient equal to 0.6. 19 of these stations are situated at lighthouses. Seven stations are situated at small island communities on the edge of the coastline heavily exposed to the weather. Only four stations (1 %) are situated in settled areas: Ørland III, Bodø, Andøya and Loppa. None of the meteorological stations in this category is situated in the eastern part of Norway where the building density is highest. 22 meteorological stations (6 %) achieve values of the exposure coefficient equal 1.0. These stations are placed shielded from the wind typically at the farther end of the long fjords of western Norway. None of the stations in northern Norway is in this category. 331 meteorological stations (85 %) achieve values of the exposure coefficient equal to 0.8. Almost all of the stations in settled areas are found in this category. Exceptions are one station with an exposure coefficient of 0.5, four stations with exposure coefficients of 0.6 and 22 stations with exposure coefficients of 1.2 as mentioned above. 4. Evaluation of the exposure coefficients for Norway 4.1. Historical field investigations Results from field investigations show a reduction in roof snow load with increasing wind exposure (see Table 2). The calculated values of the exposure coefficient according to ISO 4355 for building sites with mean temperature between –2.5 °C and 2.5 °C are in fairly good agreement with these results, and to the conservative side. But there is no available research supporting ISO’s description of wind categories. In regions with a mean temperature above 2.5 °C, ISO 4355 allows a reduction of the snow loads on the roof is only permitted at building sites with more than 10 days of wind velocity above 10 m/s. This seems not to be justified by field investigations. It is nevertheless reasonable considering the fact that high temperatures reduce the ability of wind actions to transport snow. Whether this temperature limit should be 2.5 °C is uncertain. In regions with a mean temperature below –2.5 °C, ISO 4355 recommends a reduction of snow loads also when the building is completely shielded. An exposure coefficient equal to 0.8 for this situation agrees with some of the research results (see Table 2). A question remains: were the buildings in the historical investigations completely shielded? Is it possible to obtain a completely shielded building? For a completely shielded building the snow load on a flat roof is expected to be equal to the snow load on the ground. 4.2. Snow transport theories According to snow transport theories drifting occurs even for light winds (0.3 – 1.5 m/s). At higher wind velocities (1.6 – 3.3 m/s) the snow particles move more horizontally than vertically. Drifting affects the deposition of snow; particles are transferred through areas with high wind velocities and accumulate in areas with low wind velocities. At wind velocities between 3.4 and 5.4 the snow moves considerably faster horizontally than vertically, and significant redistribution may occur. Higher winds often blow the snow off the roofs leaving them almost bare ([14], [7]). The limit of 10 m/s chosen for the wind categories seems unreasonable considering the fact that drifting occurs at wind velocities as low as 0.3 to 1.5 m/s. A larger number of Page 10 of 17
meteorological stations are expected to achieve a value of the exposure coefficient below 0.8. In Canada heavy snowfalls often coincidence with high wind velocities according to [15]. This is not the pattern in Norway. In Norway heavy snowfalls may occur at low wind velocities as well as at higher wind velocities. 4.3. The Norwegian climate Norway is a country with large variations in mean temperatures and wind velocities. In Fig. 6 mean winter temperatures are given (December – February). There is a pattern of low winter temperatures in the mountainous regions of southern Norway and the inland regions in the far north. Coastal areas in southwest have temperatures between 0 and – 2.5 °C, while the inland in the far north has winter temperatures less than –15 °C. In Fig. 7 the number of days with wind stronger than 5 m/s for months with normal temperature less than 1 °C is presented. 1 °C was chosen as a limit to consider all months with high probability of snow and snowdrift. The map also shows number of months with normal average temperature less than 1 °C. Approximately 90 % of the Norwegian mainland has six or more months with average temperature less than 1 °C. I.e. six or more months with possible snow and snowdrift. In Fig. 7 there is a clear pattern of higher probability of high winds in the costal areas. It should be noted that the costal areas do not always have the highest number of occurrences, but these regions also have fewer months with temperatures below 1 °C. In the middle of Norway (approximately 62° – 67° north) this pattern is most pronounced. In this region the coastal areas have at least one month with mean temperatures below 1 °C less than the inland, but a higher number of days with wind above 5 m/s. Page 11 of 17
- 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 and 172: variety of roofs and wind exposures
- Page 173 and 174: Li and Pomeroy [13] evaluated hourl
- Page 175: Fig. 5. Exposure coefficients for 3
- 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
30 meteorological stations (8 %) achieve values of the exposure coefficient equal to<br />
0.6. 19 of these stations are situated at lighthouses. Seven stations are situated at small<br />
island communities on the edge of the coastline heavily exposed to the weather. Only four<br />
stations (1 %) are situated in settled areas: Ørland III, Bodø, Andøya and Loppa. None of<br />
the meteorological stations in this category is situated in the eastern part of Norway where<br />
the building density is highest.<br />
22 meteorological stations (6 %) achieve values of the exposure coefficient equal<br />
1.0. These stations are placed shielded from the wind typically at the farther end of the long<br />
fjords of western Norway. None of the stations in northern Norway is in this category.<br />
331 meteorological stations (85 %) achieve values of the exposure coefficient equal<br />
to 0.8. Almost all of the stations in settled areas are found in this category. Exceptions are<br />
one station with an exposure coefficient of 0.5, four stations with exposure coefficients of<br />
0.6 and 22 stations with exposure coefficients of 1.2 as mentioned above.<br />
4. Evaluation of the exposure coefficients for Norway<br />
4.1. Historical field investigations<br />
Results from field investigations show a reduction in roof snow load with increasing wind<br />
exposure (see Table 2). The calculated values of the exposure coefficient according to ISO<br />
4355 for building sites with mean temperature between –2.5 °C and 2.5 °C are in fairly<br />
good agreement with these results, and to the conservative side. But there is no available<br />
research supporting ISO’s description of wind categories.<br />
In regions with a mean temperature above 2.5 °C, ISO 4355 allows a reduction of<br />
the snow loads on the roof is only permitted at building sites with more than 10 days of<br />
wind velocity above 10 m/s. This seems not to be justified by field investigations. It is<br />
nevertheless reasonable considering the fact that high temperatures reduce the ability of<br />
wind actions to transport snow. Whether this temperature limit should be 2.5 °C is<br />
uncertain.<br />
In regions with a mean temperature below –2.5 °C, ISO 4355 recommends a<br />
reduction of snow loads also when the building is completely shielded. An exposure<br />
coefficient equal to 0.8 for this situation agrees with some of the research results (see Table<br />
2). A question remains: were the buildings in the historical investigations completely<br />
shielded? Is it possible to obtain a completely shielded building? For a completely shielded<br />
building the snow load on a flat roof is expected to be equal to the snow load on the<br />
ground.<br />
4.2. Snow transport theories<br />
According to snow transport theories drifting occurs even for light winds (0.3 – 1.5 m/s).<br />
At higher wind velocities (1.6 – 3.3 m/s) the snow particles move more horizontally than<br />
vertically. Drifting affects the deposition of snow; particles are transferred through areas<br />
with high wind velocities and accumulate in areas with low wind velocities. At wind<br />
velocities between 3.4 and 5.4 the snow moves considerably faster horizontally than<br />
vertically, and significant redistribution may occur. Higher winds often blow the snow off<br />
the roofs leaving them almost bare ([14], [7]).<br />
The limit of 10 m/s chosen for the wind categories seems unreasonable considering<br />
the fact that drifting occurs at wind velocities as low as 0.3 to 1.5 m/s. A larger number of<br />
Page 10 of 17
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