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The loads acting on a tower are essentially the same as those acting on buildings and other structures and may be placed in three main classifications: live, dead and erection. Wind is by far the most critical live load. The design wind load is usually set up from references to U. S. Weather Bureau reports and maps for each locality. It varies from a recorded 132 m.p.h. in Miami to 49 m.p.h. in Los Angeles. Velocity is converted into pounds per square feet in accordance with accepted formulas, taking into account the increase of wind velocity with height.(1) Wind tunnel tests have shown that the total wind load should be based on the projected area of 1 tower faces on square towers. The wind on round members may be figured as two-thirds the load on flat members. Thus the load on a 3-inch rod will be equivalent to the load on a 2-inch flat. bar or angle. No attempt, by the way, should be made to design a tower to resist a tornado as the chance of a direct hit is remote and there is no assurance that even a fantastically heavy structure will survive.
Ice load is another important live load. While its occurrence is not as frequent as high wind load, a heavy ice storm or freezing rain can be very disastrous to a tower. High winds seldom occur at the same time as heavy icing. On the other hand, fairly strong winds with light ice and moderate wind with heavy ice are common. Ice from % inch to 2 inches thick is the usual range used for design in the continental United States. However, ice with a thickness of 12 inches or more occurs in some isolated spots. Naturally, the presence of ice on tower members increases the projected area exposed to the wind and the weight of the ice adds to the dead load.
Earthquake load must be considered in some localities, particularly on the West Coast. This load acts horizontally and is a function of the weight or mass of the structure. Although earthquakes occur infrequently, their threat cannot be ignored.
Erection loads are also very important, especially in the case of guyed towers. Wind on the tower in some stages of erection can subject certain members to loads greater than they will receive in fully erected condition. Loads from large and heavy gin poles add to the burden of erection loads.
Dead loads include the weight of the tower members, antenna, transmission lines, ladders and platforms.
The term "safety factor" is a much misunderstood and sometimes misleading term. Generally, the term is intended to mean the number obtained by dividing the failure stress of the material by the allowable stress. A more realistic definition of the term would be a relation between the elastic limit of the material and the allowable stress. The elastic limit of the material is that stress below which the material will not take a permanent set or deformation. If a material is repeatedly loaded above the elastic limit it will fail at a load far below the failure limit. A safety factor of 2½ for a guy wire indicates that the breaking strength of the guy is 2½ times the working load.
The guyed tower depends entirely on the guys to hold it vertical and, therefore, the design of the guys is of prime importance. For a tower with one set of guys, an angle of 45 degrees to the horizontal is good practice. Tall towers with multiple guys require a steeper angle in the top guy. One anchor can then serve several guys and the angle of the lowest guy will not be too flat.
Towers should be inspected at regular intervals, the length of time between inspections depending on weather conditions. If the tower is located in a section of the country where windy seasons have regular cycles, inspections should regularly precede these seasons. The first step in any inspection is a check of the tower connections. Almost all towers have bolted connections with some means of locking the bolts in place. During this climbing inspection the paint should be observed for rust spots. If the tower is galvanized, the coating should be inspected.
The condition of the guys should also be checked periodically. Practically all guys are made of galvanized strand and are very durable even without additional protective treatment. However, if signs of rust appear, protective treatment is a must.
New England Professional Engineer.