Icing of Elevated Structures

Weather systems which produce icing events in North America exhibit large geographic variability, as the conditions which lead to icing require the production of supercooled droplets for transport onto an exposed surface, which in turn requires a specific temperature structure of the atmosphere and the availability of sufficient moisture in the air.

This variability is shown in the maps of icing such as in the ASCE 7 Code, or the NBCC and S37 Code Tables. Neither of the codes provides mapping of potential for rime icing, although they acknowledge that it may be more important than glaze icing in mountainous terrain, suggesting that local studies should be used to quantify the icing accumulations.

In the case of tall and elevated structures such as towers additional large variations are caused by topographic effects due to the speed increase on hills and mountains as well as the lower temperatures with height which provide the opportunity for super-cooling of the available droplets at higher elevations.

The ASCE 7 code explicitly requires that the wind profile with height, including speed-up by topography be applied to the mapped icing given on the maps (with an exponent of 0.35). However, there is no accounting taken of the temperature effect with elevation in creating the necessary freezing conditions, except for a statement on the maps indicating that it is unlikely for freezing rain to occur above elevations of 5000 ft (1500 m).

The ICE Site Specific assessment provides glaze ice accumulation values based on site conditions and wind profile. In addition, for elevations greater than 300 m, it is assumed that the airport observations of solid precipitation indicate that the elevated site is in the supercooled droplet phase, adding to the accumulation of glaze icing. The temperature at the site following an icing period is then used to determine the persistence of accumulated ice in order to determine the maximum wind for the event as well as allow for the further cumulative accumulation if a new event is encountered before all the ice has melted.

As an example of the typical differences in icing on elevated ground, an Ontario Canada airport data was used to determine the glaze icing accumulation at the base of a tower located on level ground, the same tower on the crest of a 290 m hill, and the same tower on the crest of a 310 m hill. The table below shows the 50 year return icing for the three situations.

http://janrebel.eu/makelaars/woestijnelaan/ Hill Height (m)

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In this case the speed up by the hill produces a factor of 1.7 increase in speed at 10 m above tower base. The 50 yr return icing amount increases from 35 mm to 42 mm or a factor of 1.2 increase. In this case the increase in icing accumulation is equal to the 0.35 power of the speed increase. However, this accumulation increase is sensitive to the amount of precipitation vs wind speed parameter in the icing equation, so it will vary for different situations.

This case also shows an increase from 42 mm to 48 mm (14% increase) for a situation where the hill height is greater than 300 m and it is assumed that frozen precipitation was likely to have been in a supercooled liquid state at the 10 m level of the tower and higher.

The ICE Site Specific assessment also estimates the accumulation of rime ice on an elevated tower or for a tall tower for different heights on the tower. This uses the ISO 12494 recommended procedure and airport observations of cloud ceiling and temperature to determine when the tower is protruding into the cloud layer. The temperature at the height is used to determine whether supercooled droplets are available as well as to determine if melting of the accumulated ice is occurring in order to determine cumulative superposition of the accumulations over time. It is found that at high elevations and for very tall towers on level ground, the rime ice accumulations can have a bigger impact on the tower than does the glaze icing.