Lisø PhD Dissertation Manuscript - NTNU

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the snow load on the roof as a function of the heat flux through the roof. An equivalent expression can be found in ISO 4355 ”Bases for design of structures – Determination of snow loads on roofs” [2]. In practice, it has turned out difficult for consultants in structural engineering to determine the exposure coefficient Ce. The main reason is the meteorological input needed. According to an informative annex in ISO 4355 and NS 3491-3, the exposure coefficient is a function of the mean temperature, θ, in the coldest winter month and number of days, N, with a wind velocity above 10 m/s where N is defined as an average for the three coldest months of the year (see Table 1). Mean values for “many years” are recommended (usually 30 years). This meteorological information is available merely at advanced weather stations. If a building site happens to be located near such a station, the data needed is still not easily accessible. Table 1. Exposure coefficient according to ISO 4355 Winter temperature Winter wind category Category Mean temp. (ºC) I II III N < 1 1 ≤ N ≤ 10 N > 10 A θ > 2.5 1,0 1,0 0,8 B C –2.5 ≤ θ ≤ 2.5 1,0 0,8 0,6 θ < –2.5 0,8 0,8 0,5 In this paper weather data from meteorological stations in Norway for the reference 30-year period 1961-1990 is used to determine the exposure coefficient Ce according to the definition in ISO 4355. First, historical field investigations studying snow loads on roofs are evaluated giving the background of the exposure coefficient. Next, values for the exposure coefficients are calculated for 389 meteorological stations, and the suitability of the definition in order to describe the effects of wind exposure is discussed. Finally, possible approaches aiming at improving calculations of wind exposure on roof snow loads are suggested. 2. Background 2.1. Snow load on roofs according to ISO 4355 Fig. 1. Balanced snow load sb and drift snow load sd on pitched roofs. Page 2 of 17
In ISO 4355 ”Bases for design of structures – Determination of snow loads on roofs” [2] the snow load on the roof is defined as the sum of a balanced load sb, a drift load part sd and a slide load part ss (see Fig. 1): s = sb + sd + ss (2) The balanced load sb is uniformly distributed on the roof (except for curved roofs) and a function of characteristic snow load on the ground s0, exposure coefficient Ce, thermal coefficient Ct and slope reduction coefficient μb: sb = s0·Ce·Ct·μb The slope reduction coefficient, μb, defines the reduction of the snow on the roof due to roof slope and surface material. High slopes and smooth surface materials make the snow slide from the roof. In Fig. 2 slope reduction coefficient μb is shown for a single pitched roof with non-slippery surface. Fig. 2. Slope reduction coefficient μb for simple pitched roofs with non-slippery surface. The thermal coefficient Ct defines the reduction of the snow load on the roof as a function of the heat flux through the roof, causing snow melting. The exposure coefficient Ce defines the balanced load on a flat horizontal roof of a cold building, as a fraction of the characteristic snow load on the ground. The coefficient includes the effect of snow being removed from flat roofs by wind. According to an informative annex it is a function of the mean temperature, θ, in the coldest winter month Page 3 of 17 (3)
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Building envelope performance asses
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Lisø, K.R./ Building envelope perf
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BUILDING RESEARCH &INFORMATION (200
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LisÖ et al. on how to cope with an
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LisÖ et al. the exposure to contin
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LisÖ et al. business premises. The
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LisÖ et al. construction industry
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Research in Building Physics, Carme
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4 CLIMATE CHANGE SCENARIOS FOR NORW
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with large amounts of precipitation
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will most likely be worse, even wit
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Graves, H.M. & Phillipson, M.C. 200
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766 Nordvik and Lisø The topic of
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768 Nordvik and Lisø into the futu
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770 Nordvik and Lisø climate chang
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772 Nordvik and Lisø 0 Â Â = p d
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774 Nordvik and Lisø probably to a
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BUILDING RESEARCH &INFORMATION (200
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cost of repairing process-induced b
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change in quality (Mehus et al., 20
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on the impacts of different climati
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different strategies: risk-based, p
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Learning from experience - an analy
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An important aspect of the programm
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Concrete walls 32 % LECA masonry 10
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construction industry, and academic
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BUILDING RESEARCH &INFORMATION (JAN
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greater variations (Lisø et al., 2
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Typical problem areas and recommend
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Figure 3b Recommended design of win
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Figure 7 Example showing the recomm
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Figure 11a Poor solution for a £as
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Acknowledgements This paper was wri
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Climate adapted design of masonry s
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2. Masonry defects in Norway 2.1. S
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Fig. 2. Example illustrations of th
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The presented results are in good a
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appropriate in a more severe type o
Page 117 and 118: screen in a two-stage weatherproofi
Page 119 and 120: INCREASED SNOW LOADS AND WIND ACTIO
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: Effects of wind exposure on roof sn
Page 171 and 172: variety of roofs and wind exposures
Page 173 and 174: Li and Pomeroy [13] evaluated hourl
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
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