Elsevier

Building and Environment

Volume 42, Issue 10, October 2007, Pages 3726-3736
Building and Environment

Effects of wind exposure on roof snow loads

https://doi.org/10.1016/j.buildenv.2006.09.005Get rights and content

Abstract

This paper presents results from an investigation of the suitability of the exposure coefficient as defined in ISO 4355 “Bases for design of structures—Determination of snow loads on roofs”, based on thorough analyses of weather data from 389 weather stations in Norway for the reference 30-year period 1961–1990. First, the background of the exposure coefficient is examined. Historical field investigations of snow loads on roofs are also evaluated. Next, values for the exposure coefficients in Norway are calculated according to ISO 4355. Finally, possible approaches aiming at improving calculations of wind exposure on roof snow loads are suggested. It is shown that the exposure coefficient as defined in ISO 4355 does not reflect the actual effects of wind exposure on roof snow loads in Norway, the main reasons being oversimplifications in the definition of the coefficient and the extreme variations of the climate in Norway. The definition is based on coarse simplifications of snow transport theories, and must be revised and improved to serve as an applicable tool for calculations of design snow loads on roofs in Norway.

Introduction

In the current Norwegian snow load standard NS 3491-3 “Design of structures—Design actions—Part 3: Snow loads” [1] snow loads on roofs are defined ass=μCeCts0,where s0 is snow loads on the ground. The parameters μ, Ce and Ct describe conditions on the roof. The exposure coefficient Ce takes into account that wind removes snow from flat roofs. Using this coefficient the snow load on a sheltered roof becomes twice as large as the snow load on a windswept roof. The shape coefficient μ describes the distribution of snow load on the roof due to geometry. The thermal coefficient Ct defines the reduction of 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.

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.

Section snippets

Snow load on roofs according to ISO 4355

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.

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=s0CeCtμb.

The slope reduction coefficient,

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 [2]. 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 in 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

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 [2] for building sites with mean temperature between −2.5 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

Discussion and further work

Meteorological stations are located to enable a good representation of regional climate. Typical locations are in agricultural and settled areas, airports and lighthouses. That is, these areas have a better representation than mountainous regions.

Maximum snow loads on the roof often do not appear simultaneously with maximum snow loads on the ground. In the measurements reported by O’Rourke et al. [8] maximum snow loads on roofs were measured independent of maximum snow loads on the ground. In

Conclusions

It is shown that the exposure coefficient as defined in an informative annex of ISO 4355 does not reflect the actual effects of wind exposure on roof snow loads in Norway, the main reasons being oversimplifications in the definition of the coefficient and the extreme variations of the climate in Norway. The definition is based on coarse simplifications of snow transport theories. It must be revised and improved to serve as an applicable tool for calculating design snow loads on roofs, using the

Acknowledgements

This paper has been written within the ongoing SINTEF Research and Development Programme “Climate 2000—Building Constructions in a More Severe Climate” (2000–07), strategic institute project “Impact of Climate Change on the Built Environment”. The authors gratefully acknowledge all construction industry partners and the Research Council of Norway. A special thanks to Dr. Kristoffer Apeland for valuable comments on the text.

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