International Symposium on Mitigative Measures against Snow ...

<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