Snow-creep forces on masts

Abstract

ABSTRACT. Since 1975, the Nor wegia n Geotechnica l Institute has performed fi eld investigati ons o f snow-creep forces on m asts at t he ]-esea rch site in Grasd a len, Stry n mountain s in No rway. Two poles, with dia meters of 419 a nd 219 m.m , resp ec ti ve ly, we re erected at the site togeth er with a retaining structure. On both poles, stra in gauges were mounted in pai rs ever y 0.5 m to find the ax ia l stresses and the moments in d ifferent sections. In the middle secti on of the reta ining structure, the beams and supporters were instrumented to obtain measurem ents of the strains a nd stresses. Snow glide was controll ed by glide shoes mounted a t the rock surface above the structures.During the winter, snow profil es were made system aticall y; th ese incl uded meas urements of snow depth, density a nd temperature, a nd obse rvations of snow type a nd moisture content. By relating the meas ured stresses a nd moments in th e structures to the snow depth, it was possible to find the snow-press ure distribution. A compa riso n of the snow pressures with the "Body force index" (product of snow depth h, density p a nd accelerati on due to gravity g), show a close relationship for the wa ll element. For th e pole elements, the now temperature during the winter is a n added facto r of high importa nce a nd th e h ighest pressures on these elements occ ur in winters with long peri od s ofO°C isoth erm al snowpack. INTRODUCT IONBecause snow creep causes failure in m as ts almost every winter, the No r wegian Water Resourses a nd Electricity Board (NVE ), which is res ponsibl e [or the electricity transmission lines, was interested in co-opera ting with the Norwegia n Geotechnical Institute (NGI ) to develop a study of snow-creep fo rces on different mas ts. T he goal wa s to find new m ast typ es which could be used in terrain with deep snow a nd to find design cri teri a for different co nstructions.The resea rch site is a 40 m wide a nd 30 m long rock surface til ting at 25° to the ea st, a nd th e m as ts we re located at a n elevati on of 1150 m a .s. 1. in the lower pa rt of th is site. Th e investigati ons we re performed close to NGI's fi el d stati on in th e Sty n mountains, 100 km from the coast in western No rway, where th e coastal climate ha a strong inOuence on the weather and snow conditi ons. The average annual precipitation in thi s area is close to 3 m. At the site, the maximum snow depth, which occurs each winter between late M arch and early M ay, vari es between 2 and 6 m.Different types of mas ts a nd a retaining structure were erected in 1975 (La rsen, 1982). As Nature is unpredictable concerning weather and snow conditions, it took yea rs to discover design load situations. For th e retaining structure, a clear relati onship was di scovered between the snow pressure a nd the "Bod y force index" (product of snow depth h, density p a nd acceleration due to g ravity g) (M c Clu ng and others, 1984; La rsen and others, 1985). In the late 1980s and earl y 1990s, new relationships of int...