Towers - Structural Types.

Steel towers can be constructed in a number of ways but the most efficient use of material is achieved by using an open steel lattice. Typical  arrangements for microwave radio and transmission towers are shown in Fig. 5.1. The use of an open lattice avoids presenting the full width of structure to the wind but enables the  construction of extremely lightweight and stiff structures.Most power transmission, telecommunication and broadcasting structures fall into this class.

Lattice towers: (a) microwave tower. (b), (c) and (d) transmission towers
Fig. 5.1 Lattice towers: (a) microwave tower. (b), (c) and (d) transmission towers

Lattice towers are typically square or triangular and have low  redundancy. The legs are braced by the main bracings: both of these are often propped by additional secondary bracing to reduce the effective buckling lengths.The most common forms of main bracing are shown in Fig. 5.2.

Fig. 5.2 Main bracing arrangements

Lattice towers for most purposes are made of bolted angles. Tubular legs and  bracings can be economic, especially when the stresses are low enough to allow  relatively simple connections. Towers with tubular members may be less than half the weight of angle towers because of the reduced wind load on circular sections.

However, the extra cost of the tube and the more complicated connection details can exceed the saving of steel weight and foundations.

Connections are usually arranged to allow site bolting and erection of relatively small components. Angles can be cut to length and bolt holes punched by machines as part of the same operation. Where heavy-lift cranes are available much larger segments of a tower can be erected but often even these are site bolted together.

Guyed towers provide height at a much lower material cost than self-supporting towers due to the efficient use of high-strength steel in the guys. Guyed towers are normally guyed in three directions over an anchor radius of typically 2 – 3 of the tower height and have a triangular lattice section for the central mast. Tubular masts are also used, especially where icing is very heavy and lattice sections would ice up fully.

A typical example of a guyed tower is shown in Fig. 5.3.

The range of structural forms is wide and varied. Other examples are illustrated in Figs 5.4 and 5.5. Figure 5.4 is a modular tower arrangement capable of extension for an increased number of antennas.The arrangement shown in Fig. 5.5 is adopted for supporting flare risers where maintenance of the flare tip is carried out at ground level.

A significant influence on the economics of tower construction is the method of erection, which should be carefully considered at the design stage.

Guyed tower
Fig. 5.3 Guyed tower

Modular tower
Fig. 5.4 Modular tower

Flare tower
Fig. 5.5 Flare tower

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