In the present era the technology in communications has developed to a very large extent. The communication industries have seen a tremendous increase in last few years which have resulted in installation of large number of towers to increase the coverage area and network consistency. In wireless communication network these towers play a significant role hence failure of such structure in a disaster is a major concern. Therefore utmost importance should be given in considering all possible extreme conditions for designing these towers.
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3. CPM & BIM
11. ETABS Software
Based on structural action Self Supporting Tower- The towers that are supported on ground or on buildings are called as self-supporting towers. Though the weight of these towers is more they require less base area and are suitable in many situations. Most of the TV, MW, Power transmission, and flood light towers are self-supporting towers. Guyed Tower- Guyed towers are normally guyed in three directions. These towers are much lighter than self supporting type but require a large free space to anchor guy wires. Whenever large open space is available, guyed towers can be provided. There are other restrictions to mount dish antennae on these towers and require large anchor blocks to hold the ropes. Monopole Tower- It is single self-supporting pole, and is generally placed over roofs of high raised buildings, when number of antennae required is less or height of tower required is less than 9m.
Based on the type of material sections Angular and hybrid towers- 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. Lattice towers- Lattice towers are freestanding and segmentally designed with rectangular or triangular base steel lattices.
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. The following are the steps involved in design of communication tower: Selection of configuration of tower- like any other exposed structure, has a super structure shaped, dimensioned and designed to suit the external loads and self- weight Selection of configuration of a tower involves fixing of top width, bottom width, number of panels and their heights, type of bracing system and slope of tower. Computation of loads acting on tower- In case of communication towers self-weight of tower is most important component of tower design. The tele communication steel tower is a pin-jointed light structure, It is still assumed that their behavior is similar to simple truss. The percentage of openings in Tower structure will be more than 30%, so wind loads acting on the tower will be of less magnitude compared to chimneys, but the major cause of failures of telecommunication tower throughout the world though still remains to be high intensity winds.
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Analysis of tower for appropriate loading conditions- The loadings which are considered are: 1. Dead loads or Vertical loads ( i.e. self weight of tower members, Self weight of antennas, labour and equipment during construction and maintenance.) 2. Transverse loads (Wind load on exposed members of the tower and antenna.) Wind load on tower. Design of tower members according to codes of practices- Suitable steel sections are initially assumed as members of the tower for analyzing the structure. Once the analysis is done members are finalized based on the stresses developing in them, following the codal provisions provided by Indian Standards. Design of foundation according to codes of practices- Design of foundation is as per IS 800:2007, the depth and width of the foundation depends on the size of the tower.
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- 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. Double web or Warren system- 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 diagonal braces are connected at their cross points. Pratt system- This system also contains diagonal cross bracings and, in addition, it has horizontal struts. These struts are subjected to compression and the shear is taken entirely by one diagonal in tension, the other diagonal acting like a redundant member. Portal system- The diagonals are necessarily designed for both tension and compression and, therefore, this arrangement provides more stiffness than the Pratt system. The advantage of this system is that the horizontal struts are supported at mid length by the diagonals.
Software used for design and analysis
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