International Symposium on Mitigative Measures against Snow ...

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<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches Egilsstaðir, Iceland, March 11–14, 2008 3. SNOW SHED GUIDELINES OF 2007 3.1 Overview The goal of the guidelines is to define a procedure to determine avalanche actions on snow sheds and to set up uniform basics for the structural design. The loading of a snow shed depends strongly on its geometry (Fig. 4). Because the geometry is often optimized during the design process the guidelines propose that the avalanche expert determines the flow height dL and velocity of the design avalanche vL at an interface position, which is located at a distance of at most 100 m upside of the structure (Fig. 5). The design engineer calculates the determining avalanche actions in relation of the deviation angle α and the inclination β of the snow shed roof . At the location of the snow shed the avalanche expert defines the height of the natural snow cover dS, the height of avalanche deposits dA and the flow width of the avalanche. These parameters are determined by a hazard analysis including information from the avalanche history, terrain analysis, climatic conditions and avalanche dynamics calculations. Based on the results of the experimental investigations the guidelines from 1994 were revised by a working group under the lead of the Swiss Federal Roads Office (ASTRA/SBB, 2007). 3.2 Basics It is fundamental that a snow shed has to cover the full width of an avalanche path. Insufficient lengths are the most common reasons for failures. Sometimes it is possible to reduce the width of an avalanche by constructing lateral dams or walls. According to the guidelines the geometry of the shed should be chosen so that the deviation of the avalanche flow is as small as possible. If this is not possible the deviation point of slope inclination should be positioned at a distance of more than 6 times the flow height uphill of the shed. The outside wall should be closed if the terrain below the snow shed is not much inclined so that avalanche snow might flow into the snow shed. 3.3 Load cases In the guidelines (ASTRA/SBB, 2007) eight different load cases are distinguished (Tab. 1). For the verification of the structural safety avalanches with a return period of 30 years are regarded as variable actions and with a return period of 300 years as accidental actions. Table 1 Load cases to determine the actions induced by snow and avalanches Case 1: Avalanche slides on snow shed The actions consist of the moving avalanche and the deflection. The without snow deposit deflection and friction force are maximal. Case 2: Avalanche slides on snow shed Similar to case 1 however the weight of the natural snow cover has to covered with snow be added. Case 3: Avalanche slides on snow shed Similar to case 1 however the weight of the avalanche deposit has to covered with avalanche deposits be added. Because of the deposit the deviation angle is reduced. Case 4: Avalanche deposit on snow shed At locations with huge deposits often the determining load case. Case 5: Static snow pressure on the If a snow shed is completely covered by avalanche deposits the static outside wall of snow shed snow pressure can load the outside wall. Case 6: Dynamic avalanche pressure on Avalanches from the opposite valley side can impact the outside wall the outside wall of snow shed of the shed. Case 7: Snow pressure on the roof If snow shed is situated below a steep slope. Case 8: Avalanche impact on the roof of If the avalanche jumps on the roof or if the deviation angle α is the snow shed bigger than 60° 34 New findings on the design of snow sheds
<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches Egilsstaðir, Iceland, March 11–14, 2008 All actions from avalanches like friction, normal loads and deviation loads are combined as? either leading action or accompanying action. The actions from avalanche deposits and sliding avalanches cannot be combined as? leading actions?. If snow sheds are loaded by avalanches and rockfall the two actions must not be combined. qq pS pS Interface position Avalanche Avalanche α β Deviation point Natural snow cover
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Sponsored by: • The Association o
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