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ANNEX H: COMMENTARY ON ICE DESIGN CRITERIA FOR COMMUNICATION STRUCTURES

1 INTRODUCTION

The meteorological phenomenon of ice accumulation is very difficult to predict with certainty. For tower and pole structures, ice accumulation can be one of the predominant
applied loads.

The first task in developing ice design criteria is to determine if the proposed or existing site is susceptible to icing. If the site has a history of ice accumulation, the frequency, thickness, type and duration of icing must be determined. Potential sources of this information include the National Weather Service (NWS), local weather agencies, owners of existing towers at the same site or nearby sites, local landowners, and consulting meteorologists.

Judgment must be exercised to determine if reported icing events are frequent or rare occurrences. Likewise, in some geographical areas, seasonal high winds and icing occur simultaneously. For these situations, simultaneous application of maximum wind and ice loadings may be required.

The effect of icing on a tower generally relates to the type and size of tower and to the type and thickness of icing. For example, a 1/'2-inch radial ice accumulation will have more impact on a short tower with small members than a tall tower with larger members. Very tall towers may experience large thicknesses of in-cloud icing over portions of the mast. Solid or clear glaze ice has a higher density than that of rime ice or hoarfrost. Consequently, the effects of increased dead weight from ice accumulation will vary depending on the type of ice. Large accumulations of radial ice can dramatically increase the projected wind area of tower members and antennas.

 

2 TYPES OF ICING O) (2) (3)

There are several types of icing which can accumulate on communication structures. It is important to understand where and how they form.

2.1 Hoarfrost

Hoarfrost is a fluffy or feathery deposit of interlocking ice crystals formed on objects, usually those of small diameter freely exposed to the air, such as tree branches, wires, etc. The deposition of hoarfrost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing. It forms when air, with a dew point below freezing, is brought to saturation by cooling. Hoarfrost has densities less than 19 lb/f0 [3 kN/m3].

2.2 Rime Ice

Rime ice is a white or milky granular deposit of ice formed by the rapid freezing of super cooled water drops as they impinge upon an exposed object. It is denser and harder than hoarfrost, but lighter, softer, and less transparent than glaze. Rime is composed essentially of discrete ice granules and has densities ranging from 56 to
19 lb/ft3 [9 to 3 kN/m3].

Rime is often described as soft or hard. Soft rime is a white, opaque coating of frae rime deposited especially on points and edges of objects. It is usually formed in super cooled fog. On the windward side, soft rime may grow to very thick layers, long feathery cones, or needles pointing into the wind and having a structure similar
to hoarfrost.

Hard rime is an opaque, granular mass of time formed by a dense super cooled fog. Hard rime is compact and amorphous and may build out into the wind as glazed cones or feathers. The icing of ships and shoreline structures by super cooled spray usually has the characteristics of hard rime.

2.3 Glaze Ice

Glaze ice is a coating of ice, generally clear and smooth, but usually containing some air pockets. It is formed on exposed objects by the freezing of a fdm of super cooled water, usually deposited by rain or drizzle. Glaze is denser, harder, and more transparent than either rime or hoarfrost. Its density may be as high as 561b/ft3
[9 kN/m3].

(1) Atmospheric Icing on Structures. Boyd & Willism.q.

(2) Draft Guidelines for Transmission Line Structural Loadings. ASCE.

(3) Tattelman, P., and Gringorten, I.L, "Estimatext Glaze Ice and W'md Loads at the F-anh's surface for the

Contiguous United States", Air Force Cambridge Research Laboratories, Bedford, Massachusetts, 1973.

 

3 CONDITIONS OF ICE FORMATION

The type of ice formed is determined by combinations of air temperature, wind speed, drop size, and liquid water content or rainfall intensity. The icing problem, therefore, can be classified either by the meteorological conditions that produce the formation of ice or by the type of ice that is formed.

3.1 Precipitation Icing

This is the most common icing mechanism and can occur in any area subject to freezing rain or drizzle. The ice is formed when warm, moist air is forced over a sub freezing, denser layer of air at the ground surface. As the warm air rises and condenses, rain falls through the colder air and freezes on objects near the ground. This frozen deposit is a clear glaze type of ice. Since this kind of weather is caused by frontal activity, it usually doesn't last more than a day or two.

Because it is necessary for excess water to be present for glaze to form on exposed surfaces, often the excess water may freeze into icicles or other distended shapes. In actual practice, glaze ice can be seen to form on cables and guys in a variety of shapes ranging from the classical smooth cylindrical sheath, through crescents on the windward side and icicles hanging on the underside to large irregular protuberances spaced along the cable. In most eases, glaze ice develops on structures as a fairly smooth layer on the windward surfaces with icicles forming below horizontal members. The shape of the glaze is apparently dependent on a combination of factors, such as wind speed, variations in wind speed, the angle of the wind, the turbulence of the flow, variations in air temperature and duration of the storm. Since most of these factors vary from storm to storm, and even during the storm, a cylindrical shape of equivalent weight is assumed for design purposes.

3.2 In-Cloud Icing
This type of icing condition is caused by the impingement of super-cooled water droplets of a cloud on the  cable. This is rime ice. It can occur in mountainous areas where clouds exist above the freezing level or in a super-cooled fog at lower elevations produced by a stable air mass with a strong temperature
inversion. These conditions can last for days or weeks.

The total amount of in-cloud ice deposited is dependent on wind speed. Since wind speed increases with height above ground, larger amounts of ice will occur towards the top of taller towers and on the cables that support or are mounted on taller towers.

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