There are four main methods of erection of steel transmission towers which a re described below:
- Build-up method or Piecemeal method.
- Section method.
- Ground assembly method.
- Helicopter method.
Build Up Method of Transmission Tower Erection
This method is most commonly used in India for the erection of 6.6 kV, 132 kV, 220 kV and 400 kV transmission line towers due to the following advantages :
- Tower materials can be supplied to site in knocked down condition which facilitates easier and cheaper transportation.
- It does not require any heavy machinery such as cranes etc.
- Tower erection activity can be done in any kind of terrain and mostly throughout the year.
- Availability of workmen at cheap rates.
This method consists of erecting the towers, member by member. The tower members are kept on ground serially according to erection sequence to avoid search or time loss. The erection progresses from the bottom upwards. The four main corner leg members of the first section of the tower are first erected and gaurd off. Sometimes more than one contiguous leg sections of each corner leg are bolted together at the ground and erected.
The cross braces of the first section which are already assembled on the ground are raised one by one as a unit and bolted to the already erected corner leg angles. First section of the lower thus built and horizontal struts (belt members) if any, are bolted in position. For assembling the second section of the tower, two gin poles are placed one each on the top of diagonally opposite corner legs. These two poles are used, for raising parts of second section. The leg members and braces of this section are then hoisted and assembled. The gin poles are then shifted to the corner leg members on the top of second section to raise the parts of third section of the lower in position for assembly. Gin poles are thus moved up as the tower grows. This process is continued till the complete tower is erected. Cross-arm members are assembled on the ground and raised up and fixed to the main body of the tower. For heavier towers, a small boom is rigged on one of the tower legs for hoisting purposes. The members/sections are hoisted either manually or by winch machines operated from the ground. For smaller base towers/vertical configuration towers one gin pole is used instead of two gin poles. In order to maintain speed and efficiency, a small assembly party goes ahead of the main erection gang and its purpose is to sort out the tower members, keeping the members in correct position on the ground and assembling the panels on the ground which can be erected as a complete unit.
Section Method of Transmission Tower Erection
In the section method, major sections of the tower are assembled on the ground and the same are erected as units. Either a mobile crane or a gin pole is used. The gin pole used is approximately 10 m long and is held in place by means of guys by the side of the tower to be erected. The two opposite sides of the tower section of the tower are assembled on the ground. Each assembled side is then lifted clear of the ground with the gin or derrick and is lowered into position on bolts to stubs or anchor bolts.
One side is h held in place with props while the other side is being erected. The two opposite sides are then laced together with cross members and diagonals; and the assembled section is lined up, made square to the line. After completing the first section, gin pole is set on the top of the first section. The gin rests on a strut of the tower immediately below the leg joint. The gin pole then has to be properly guyed into position.
The first face of the second section is raised. To raise the second face of this section it is necessary to slide the foot of the gin on the strut of the opposite face of the tower. After the two opposite faces are raised, the lacing on the other two sides is bolted up. The last lift raises the top of the towers. After the tower top is placed and all side lacing have been bolted up all the guyed are thrown off except one which is used to lower the gin pole. Sometimes whole one face of the tower is assembled on the ground, hoisted and supported in position. The opposite face is similarly assembled and hoisted and then the bracing angles connecting these two faces are fitted.
Ground Assembly Method of Tower Erection
This method consists of assembling the tower on ground, and erecting it as a complete unit. The complete tower is assembled in a horizontal position on even ground. The tower is assembled along the direction of the line to allow the cross arms to be fitted. One slopping ground, however, elaborate packing of the low side is essential before assembly commences. After the assembly is complete the tower is picked up from the ground with the help of a crane and carried to its location, and set on its foundation. For this method of erection, a level piece of ground close to footing is chosen from the tower assembly. This method is not useful when the towers are large and heavy and the foundations are located in arable land where building and erecting complete towers would cause damage to large areas or in hilly terrain where the assembly of complete tower on sloping ground may not be possible and it may be difficult to get crane into position to raise the complete tower.
In India, this method is not generally adopted because of prohibitive cost of mobile crane, and non-availability of good approach roads to tower locations.
Helicopter Method of Transmission Tower Erection
In the helicopter method, the transmission tower is erected in section. For example bottom section is first lifted on to the stubs and then the upper section is lifted and bolted to the first section and the process is repeated till the complete tower is erected. Sometimes a completely assembled tower is raised with the help of helicopter. Helicopters are also used for lifting completely assembled towers with guys from the marshaling yards where these are fabricated and then transported one by one to line locations. Helicopter hovers over the line location while the tower is securely guyed. The ground crew men connect and tighten the tower guys. As soon as the guy wires are adequately tensioned the helicopter disengages and files to the marshaling yard. This method is adopted where approach is v very difficult or to speed up the construction of the transmission line.
Tightening of Nuts and Punching of Threads and Tack Welding of Nuts of Transmission Towers
All nuts shall be tightened properly using correct sized spanners. Before tightening it is ensured that filter washers and plates are placed in relevant gaps between members, bolt of proper size and length are inserted and one spring washer is inserted under e each nut. In case of step bolts, spring washer shall be placed under the outer nut. The tightening shall be carried on progressively from the top downwards, care being taken that all bolts at every level are tightened simultaneously. It may be better to employ four persons, each covering one leg and the face to his right.
The threads of bolts shall be projected outside the nuts by one to two threads and shall be punched at three positions on the top inner periphery of the nut and bolt to ensure that the nuts are not loosened in course of time. If during tightening a nut is found to be slipping or running over the bolt threads, the bolt together with the nut shall be changed outright.
Painting of Joints of Transmission Tower
For galvanized towers is coastal or highly polluted areas, the joints shall be painted with zinc paint on all contact surfaces during the course of erection.
Checking the Verticality of Erected Transmission Towers
The finally erected tower shall be truly vertical after erection and no straining is permitted to bring it in alignment. Tolerance limit for vertical shall be one in 360 of the tower height.