Russ Frith : CE 699 |
Snow load and snow depth geographical variables cannot be measured at all parts of space; thus, research involving those variables generally requires the use of interpolation techniques for the studies. Observations are taken at points and spatial interpolation is used to obtain full spatial coverage of Alaska's geographic extent. Spatially accurate estimates for those variables requires investigating the relation between snowfall accumulation and secondary data such as elevation data, wind effects, and sea effects. Incorporating those influences on the spatial estimates provides more accurate estimates than approaches based on one parameter like snowfall accumulation. This study treats the mapping of annual average snow depth for Alaska for sparce point data using co-kriging and geographic weighted regression. By using spatial relationships between meteorlogical observations and variables derived from digital elevation modeling, optimum spatial distributions of mean annual snow fall are aimed to be defined. Alaska's unique Arctic climate and topography introduce snow fall mechanisms and snow depth distributions which vary widely across the state. This study presents recommendations for determining snow loads on roof tops in Alaska. A three part procedure is proposed which includes establshing a design concept for dealing with roof snow loads, considering snow conditions at a construction site, and considering snow conditions on the roof itself.
Snow loads on roofs vary as a function of the characteristic of the snow load on the ground, climate (including temperature and wind speed during the winter), roof shape, roofing material, and from one winter to another. Most available snow load data comprises snow depth on the ground. The analysis in this project assumes that the snow load on the roof is proportional to that on the ground. The designer should set up the design concept for the roof snow load, whether or not an active control system is to be introduced to reduce the snow load accumulating on a roof.
Snow depth on the ground at a construction site is estimated by a statistical treatment and by an interpolation technique. The basic load value in this study is defined as one for the snow depth on the ground with a return period of 100 years. If a building is designed for another return period, then a conversion factor is introduced to modify the value. Snow load on the ground is calculated from snow depth multiplied by an equivalent snow density. In addition, snow weight may be estimated using observed precipitation and temperature.
Snow load on the roof is estimated from the snow load on the ground multiplied by a ground-to-roof conversion factor (shape coefficient). Shape coefficient varies according to roof shape, temperature, snow type, wind direction, and wind speed; subsequently,it is difficult to determine.