| OCR Text |
Show II. THE KIND AND MAGNITUDE OF THE OBSERVED PRESSURE EFFECTS. The observed destructive effects may be summarized as follows: 1. The forces, which in the situations investigated, caused the damages in the avalanche path, originate from the resistance which the object presents to the flowing snow mass. The observations have shown that, in the cases investigated, the rise of the forces to the maximum value ensued relatively slowly. The effect on the objects must not be examined according to impact theory, but the forces acting can be explained according to static principles. 2. As a result of the flow process accompanying the damming of the snow, forces act upon the objects in the direction of flow of the avalanche, as well as diagonally, particularly upwards, in the course of which the pressure maxima of the various forces do not coincide in time. The resultant impact energy of the avalanche acts upwards as a rule, especially on slopes of small inclination and in ravines ( Dalaas, Monstein), but it also can act downwards below very steep slopes ( Vals). In the cases investigated the pressure effect in the thrust direction of the avalanches seldom exceeded 5 tons/ m2 on the surface of the object struck, where it was principally a question of loose snow or powder avalanches. Only in three special cases in Switzerland was it concluded that the maximum pressure effects were from 10 to 20 tons/ m2, where it was a question of steep slope inclination, 45 to 100%, and in part of ground avalanches ( Grund-lawinen). The vertical forces, occurring as a result of the damming of an avalanche by an obstruction in the slide path, can, in the normal design of houses, reach about half the value of the horizontal force. Their effectiveness is dependent upon the sliding resistance occurring on the surface of impact, i. e., friction on rough facades ( building stones, shingles) or openings in the facade, window jambs, roof eaves, etc. For the case in Blons the ratio was Pv/? h ~ °- 2, in Dalaas for the least favorable case ~ 0.5. In the event at Vals the pressure had acted downward. Pressure effects directed downward were also detected frequently in Switzerland at the foot of steep slopes ( Andermatt, Zuoz). 3. Surfaces which are overrun by avalanches, are stressed by the weight of snow according to the avalanche or dammed up depth occurring under the existing circumstances. 4. From debris, stones, trees, etc., carried along, ram effects can occur, which in the case of a new installation could be allowed for by design or utilization of materials with a high degree of toughness. 5. The destructive action of the air blast, which precedes the avalanche, was small in the situations investigated and only sufficient to break windows and light doors, for which, empirically, a pressure of 500 kg/ m2 is enough. On the other hand the turbulence of a powder avalanche front could take effect similar to the pressure and suction effect accompanying a compression shock in air. Damages, which might have been attributed to a typical suction action, could be identified neither in the damages of the Vorarlberg nor from the available data in Switzerland. 15 |
