Defense structures in avalanche starting zones - SLF

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$N e w s l e t t e r N a t u r g e f a h r e n Schneeverfrachtung - SLF$

Technical Guideline for Defense Structures in Avalanche Starting Zones FOEN / WSL 2007 32 3.6 Height of structure 3.6.1 Definition of height of structure The height of the structure HK is defined as the average vertical distance from the upper edge of the supporting surface to the ground. Definitions of the different types of structure are given in Section 5 (snow bridge: Section 5.8.1.3.3; snow rake: Section 5.8.2.3.2 and snow net: Section 5.8.3.4). 3.6.2 Determination of height of structure The height of the structure HK must be at least as great as the extreme snow height anticipated at the site of the supporting structure. H ≥ [m] (7) K Hext This is the fundamental condition to be fulfilled to provide protection from avalanches during natural catastrophes, and dictates the procedures for dimensioning the defense structures. Where HK>Hext is chosen, the supporting structures must be dimensioned based on HK throughout. Note that depending on the design of the supporting structures and the wind conditions, these may have a substantial influence on the quantity of snow deposited. 3.6.3 Definition of grate and net heights The grate or net height BK is defined as the average width of the supporting surface normal to the contour line. It is bounded at the lower end by the surface of the ground (Fig. 12). The effective grate or net height DK is defined in a similar way to the snow thickness as the average distance of the upper edge of the supporting surface from the ground normal to the line of slope. Fig. 12 > Grate and net heights. Grate height Net height
3 > Planning of supporting structures 33 3.7 Distance between structures in the line of slope 3.7.1 Determination of the distance between structures The distance between structures and rows of structures in the line of slope should be so dimensioned that in addition to fulfilling the objective of the supporting structure according to Section 3.2.1, the following three conditions are all met: > the structures should suffer no damage from the static effect of the maximum snow pressure > likewise, the dynamic loads resulting from snow movement should be sustained without damage > the velocity of snow movement within the supporting structure should not exceed a certain limiting value. The energy of motion is limited by the structure to a value below that which would cause damage to buildings etc. lying below the structure. 3.7.2 Calculation of distance between structures 3.7.2.1 Calculation of the distance in the line of slope The distance L in the line of slope is calculated from: L = fL ⋅HK [m] (8) The distance factor fL depends on the inclination of the slope and – in accordance with the three conditions in Section 3.7.1 – on the angle of friction ϕ between the ground and the snow, on the glide factor N and on the height of the structure HK. Fig. 13 shows fL as a function of the parameters mentioned. The value of fL under the given conditions may be obtained from the 3 families of curves tan ϕ, N and HK = const. The distances L (in the line of slope) and L' (plan view) may also be obtained directly as a function of DK from Tables 2.1 and 2.2, or 3.1 and 3.2, respectively. 3.7.2.2 Safety requirements and ground surface > With smooth ground (N > 2) or for higher safety requirements, values of tan ϕ = 0.55 and 0.50 should be used. > For rough ground (N < 2) and where no particular safety requirements are imposed, tan ϕ should be chosen as 0.60. 3.7.2.3 Maximum permissible values of the distance factor The curves for tan ϕ = 0.60, N > 1.3 and fL = 13 give the highest permissible values for the distance factor fL.
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Defense structures in avalanche starting zones - SLF

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