Tower Design

Design of Steel Structures

Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar

7.4 Tower Design

Once the external loads acting on the tower are determined, one proceeds

with an analysis of the forces in various members with a view to fixing up their

sizes. Since axial force is the only force for a truss element, the member has to

be designed for either compression or tension. When there are multiple load

conditions, certain members may be subjected to both compressive and tensile

forces under different loading conditions. Reversal of loads may also induce

alternate nature of forces; hence these members are to be designed for both

compression and tension. The total force acting on any individual member under

the normal condition and also under the broken- wire condition is multiplied by

the corresponding factor of safety, and it is ensured that the values are within the

permissible ultimate strength of the particular steel used.

Bracing systems

Once the width of the tower at the top and also the level at which the

batter should start are determined, the next step is to select the system of

bracings. The following bracing systems are usually adopted for transmission line

towers.

Single web system (Figure 7.29a)

It comprises either diagonals and struts or all diagonals. This system is

particularly used for narrow-based towers, in cross-arm girders and for portal

type of towers. Except for 66 kV single circuit towers, this system has little

application for wide-based towers at higher voltages.

Indian Institute of Technology Madras

Design of Steel Structures

Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar

Double web or Warren system (Figure 7.29b)

This system is made up of diagonal cross bracings. Shear is equally

distributed between the two diagonals, one in compression and the other in

tension. Both the diagonals are designed for compression and tension in order to

permit reversal of externally applied shear. The...