Modern codes and standards require that a transmitting tower be designed to resist dead and live wind and earthquake loads. Loads, forces, and stresses that result from temperature changes, movements due to differential settlements, or any combination of these should also be considered. Older towers should be carefully checked to assure that no significant changes in design criteria have been required, since their original design and erection, that would increase the possibility of failure.
Dead and Live Loads
Dead loads consist of the weights of the members of a tower and any equipment that is permanently attached to it, including any attached signs. AM transmitting towers support only their lighting equipment. The FM transmitting tower supports its transmission line, antenna, and lighting equipment. The microwave tower must support microwave dishes, reflectors, and lighting equipment. The TV transmitting tower is the most complicated, having to support its transmission line, antenna, and microwave facilities. In addition to the above, many larger towers of all types may be required to support deicer circuits, deicer control circuits, elevators, telephone circuits, power circuits, anti-climbing devices, andclimbing and working facilities. Some towers are used to transmit multiple signals, such as AM, FM, and TV signals, increasing the associated dead loads. Ice is considered a live load. Snow and rain are also considered live loads, but have no application to towers unless they are equipped with large platforms or similar installations where the precipitation can accumulate.
Wind loads have been responsible for the vast majority of tower failures. The stresses produced in the members of a tower by load combinations, that include wind loads, are usually the most critical and control the structural design. Therefore, it is important that the effects of wind be carefully considered during the design phase. Wind loads, sometimes referred to as “wind pressures,” are most commonly expressed in units of pounds per square foot (psf) (kgs/m2). They develop as air at some velocity (wind) moves past the members of a tower and its appurtenances, such as guy wires, antenna assemblies, transmission lines, reflectors, conduits, lighting, signs, anti-climbing devices, and climbing and working facilities. The magnitude of the wind pressure that is developed on a body will depend, among other factors, on the geometrical shape of its cross section. For a constant wind speed, the more a body is streamlined, the less wind pressure will be developed on it.
Therefore, a tower that is built of members with flat or angular cross sections will develop larger wind pressures than a similar tower built of members with circular cross sections. The total load in pounds (lbs) (kgs) on a member or appurtenance of a tower can be obtained by multiplying the wind pressure by the normal projected area of the member or appurtenance. The total wind load (lbs) acting on a section of a tower can be obtained by simply adding the total wind loads (lbs) on the members and appurtenances in that section. The magnitude of the wind pressure that is
developed will also depend on the speed of the wind. It follows that, in the design of a tower, the variation of the wind speed
is important, as well as the maximum wind speed to which it might be subjected. The Basic Wind Speed is often exceeded for a few seconds during gusts. To account for this phenomenon in design, the Basic Wind Speed can be multiplied by a “gust factor” that can vary with the height above ground. In addition to the variation of wind speeds with respect to geographical location, they also vary with respect to the height above ground, usually increasing with higher elevations. Engineers generally accept that the wind speed will increase by some nth power of the height above ground (i.e., hn where h is the height above ground). The factor “n” can vary depending on the type of terrain where the tower is to be built (for example, urban areas, flat, open country, or
mountainous). Towers are virtually never designed to resist the wind loads produced by tornadoes, even though no section of the country can be considered entirely free from them. It is generally felt that the probability of a tower being in the narrow path of the maximum wind velocity of a tornado is small, even in those regions of the country with the greatest tornado frequency. Further, to design towers for the full force of a tornado, which can amount to several hundred pounds per square foot, is uneconomical. Even if so designed, there is no guarantee that an extremely strong structure will survive a direct hit. Wind loads produced by hurricanes should always be considered in the design of a tower. Since hurricanes, unlike tornadoes, have wide paths of travel and can be anticipated in certain well defined regions of the country, their associated wind loads should be included in the design of towers in these regions. Hurricane wind speeds, while not as great as tornado, have been recorded in excess of 155 mph (249.45 k m/h)