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
a) at Ryggfonn test station b) steel reinforced base section c) tubular shafts (Photo: NGI)
Figure 9: Avalanche towers from Norway.
The study revealed that a singular, tubular steel shaft would be the best option in the impact
zone of the avalanche core. Factors that influenced this result were:
• Foundation cost is high, thus it is important to minimize the foundation load and
especially the overturning moment.
• All elements located within the impact zone of the avalanche core need to be compact
and able to withstand high local pressure.
• Avalanche directional variability − Many towers may be subjected to snow avalanches
from opposite hillsides of the valleys, in addition to directional variations in a flow from
one hillside.
• It is more economical to build strong tower than to build separate protection structure
uphill, i.e. plow- or wedge.
• Visual impact of transmission line towers is a matter of importance. Tubular shaft
towers are believed to have a relatively consistent and good appearance.
• Relative ease of adopting design to variable avalanche loading between tower sites.
From the structural systems investigated during the tender design, a Y-shaped, tubular tower
was selected as it appeared to minimize area exposed to the avalanche loading and thus reduce
overturning moments. This in turn would result in savings for both towers and foundations.
Brief cost comparisons also indicated economical advantages in having the concrete foundation
column terminate just above the ground level, as opposed to extend the foundation up to and
above the expected impact zone of the avalanche core. Factored herein was also the relative
speed and simplicity of construction using tubular steel base sections as opposed to concrete.
Tower locations were carefully selected in order to minimize avalanche loading. This further
resulted in having towers in the two lines located essentially side-by-side, as opposed to shifted
along the line. It was therefore important to maintain sufficient spacing between the parallel
lines such that a tower collapse in one line would not cause damage to the adjacent tower in the
other line. Apart from having to fulfil required electrical clearances above ground, the height of
the towers was also specified such that the average height of the conductor was maintained
above the saltation layer.
Transmission lines in avalanche areas are at times designed with all towers as dead-end
structures (i.e. no suspension tower) to minimize damage of adjacent towers if one tower fails.
172 Towers for snow avalanches in 420 kV transmission lines in Iceland