Does The Icing Map Provide a Reliable Basis for Engineering Design? Does The Icing Map Provide a Reliable Basis for Engineering Design?

Icing on structures occurs when super-cooled liquid droplets impinge on a cold surface in liquid form and then freeze. The creation of super-cooled droplets requires a specific type of air temperature structure having a layer of cold air near the ground of sufficient depth to cool the liquid rain droplets to below freezing temperatures overlaid by a warm air layer of sufficient depth and moisture content to create the liquid droplets as ice crystals fall to ground.

If the cold layer is deep enough and cold enough the super-cooled droplets will refreeze before hitting the surface and fall as ice pellets or sleet. If the cold layer is too shallow the precipitation will hit the surface as wet snow or rain or a mixture of precipitation types.

The maximum accumulation amount for an event then depends on the precipitation rate and the duration of the conditions favourable to icing. The accumulation of ice ends when the freezing precipitation stops. Once the air temperature rises to above freezing the accumulated ice on structures starts to melt which can take some time. Occasionally the freezing precipitation restarts before all ice is melted and adds to the remaining accumulated ice.

Airport observing sites provide hourly reports of precipitation amount and type, with some stations having more than 40 years of continuous record. Although researchers have developed models of icing formation and accumulation taking into account the meteorology, vertical temperature profile, and thermodynamics of ice formation, these cannot be easily run for 40 years of hourly data for a large number of sites.

Simplified models have been developed, such as the Jones Simple Icing model which can use the observational data routinely available to calculate the resultant accumulation for each hour and the total for an event. The Simple Icing model was used to perform the calculations for several hundred stations in the US for the ASCE7 Map. Similarly the Chaine and Skeates Model was used by Yip to evaluate 300 stations across Canada for the NBCC Map.

The number of icing events at a site in a given year varies from 0 to 5 or more depending on geographical location. The annual maxima of accumulation for a site form a set of modeled values which is then subjected to extreme value analysis to project the 50 year return period accumulation (in Canada) or the 500 year return period accumulation (in the US). The return period icing is then mapped for purposes of the NBCC (minimum of 10 mm to 45 mm) or for the ASCE7 (minimum of 0 inches to a maximum of 3 inches). The TIA 222-H uses the ASCE7 icing maps and procedures.

The ASCE7 also provides a companion wind speed (concurrent wind speed for the maximum accumulation) to be used with the 500 year return accumulation for design of structures. The S37 recommends that the concurrent wind load be set to 50% of the return period wind load. This is equivalent to the concurrent wind speed being set to 70% of the return period wind speed.

Ativan is an older medication that has been used for a very long time. Topographical Influences on Ice Accumulation

Since the airports are located predominantly in flat areas, the maps do not account for effects of elevated terrain on icing accumulation. Elevated terrain changes the wind speed but also the temperature near the ground as well as the depth or even presence of the cold layer. This is the reason for the notes on the ASCE7 maps:


1 Ice thickness on structures in exposed locations at elevations higher than the surrounding terrain and in valleys may exceed the mapped values.
2 In the mountain west, indicated by the shading, ice thickness may exceed the mapped values in the foothills and passes. However at elevations above 5000 ft, freezing rain is unlikely.
3 In the Appalachian Mountains, indicated by the shading, ice thickness may vary significantly over short distances.

Since the icing accumulation depends on wind speed, the ASCE7 recommends that the icing amount be incremented with height using the increase of wind speed with height (Kzt) to the power of 0.35.

The S37 includes an escalation factor of ice with height which increases as (H/10)0.1. This formulation does not account for the speed-up factor by topographic features, which can increase the ice load at the base of the tower by 25%.

lidir. Rime Icing (In-cloud icing)

Rime icing or in-cloud icing occurs when a structure is tall enough or is located on a high elevation that occasionally places it within the cloud layer and the temperature at cloud height is cold enough to provide super-cooled droplets which can impinge on the structure where they freeze rapidly. Rime ice is less dense than glaze ice due to incorporation of air during the rapid freezing process.

For tall towers or towers on elevated terrain the S37 recommends that a site specific assessment of rime icing (in-cloud) accumulation be done for the site but does not provide a methodology for such an evaluation.

The ASCE7 does not provide any guidance on estimating in-cloud icing, and does not call for its inclusion, although the TIA 222-H suggests a site specific assessment for rime ice accumulation potential.

ICE Inc. uses the approach recommended in the ISO 12494 to estimate the potential for rime ice accumulation. We use the airport hourly observations of Ceiling Height, which is the height of the lowest cloud layer, and the temperature at the airport adjusted for moist adiabatic temperature drop with height to determine the hours when the highest point on the tower (or other chosen point) is within the cloud. Any icing for the hour will be added to already accumulated ice, or if the temperature is above the range required for icing a melted amount will be calculated. The event ends when there is no ice left on the tower.

Ativan is an older medication that has been used for a very long time. Using Site Specific Wind and Ice Assessment to Improve Reliability

ICE Inc. has performed hundreds of glaze and rime ice assessments for towers on hills and mountains in the US and Canada as well as tall towers on level ground. We find that icing can increase by 25% or more on hills compared to airport level, and have found airports reporting freezing precipitation in the high elevation regions above the 5000 ft level.

We also find that the companion wind on the ASCE7 map is under-estimating the concurrent winds because it does not include high winds occurring after the precipitation has stopped while the ice accumulation on structures is at its highest. Most of the damage in ice storms occurs when a precipitation event has stopped and the ice accumulation is at maximum with winds picking up.

Our estimates of rime icing show that the rime accumulation can be as significant as freezing rain accumulation especially for tall towers and high elevations. More importantly, the rime ice accumulation is largest at higher levels of the tower increasing to the top of the tower. This is the reverse of the ice profile for glaze icing events and needs to be considered in the tower design.

The NBCC tabulation requires a minimum of 10 mm glaze ice in all regions. We find that in some regions airport observations do not show accumulations above 2 mm. There are many regions where the 50 year ice accumulation is greater than the 45 mm upper limit.