Basic Concept of Transmission Tower Foundation
Foundation of any structure plays an important role in safety and Satisfactory performance of the structure as it transmits mechanical.
Loads of the electrical transmission system to earth. A transmission structure without having a sound and safe foundation, it cannot perform the functions for which it has been designed. The foundations in various types of soils have to be designed to suit the soil conditions of particular type.
In addition to foundations of normal towers, there are situations where considering techno- economical aspect for special towers required or river crossing which may be located either on the bank of the river or in the mind stream or both, pile foundation may be provided.
Type of Loads on Foundation:
The foundation of towers are normally subjected to three types of forces.
- The tension or uplift
- The lateral forces of side thrusts in both transverse and longitudinal directions.
- The magnitude or limit loads for foundations should be taken 10% higher than these for the Corresponding towers.
- The base slab of the foundation shall be designed for additional moments developing due to eccentricity of the loads.
- The additional weights of concrete in the footing below ground level over the earth weight and the full weight of concrete above ground level in the footing and embedded steel parts also be taken into account adding to the down-thrust.
Design of Foundation of Transmission Towers in Different Soils
- All foundation shall be of RCC. The design and construction of RCC structures shall be carried out as per IS:456 and minimum grade of concrete shall be M-20.
- Limit state method of design shall be adopted.
- Cold twisted deformed bars as per IS: 1786 or TM T bars shall be used as reinforcement.
- Foundations shall be designed for the critical loading combination of the steel structure and or equipment and/or superstructure.
- If required protection to the foundation, shall be provided to take care of any special requirements for aggressive alkaline soil, black cotton soil or any soil which is detrimental/harmful to the concrete foundations.
- All structures shall be checked for sliding and overturning stability during both construction and operating conditions for various combination of loads.
- For checking against overturning, weight of soil vertically above tooting shall be taken and inverted frustum of pyramid of earth on foundation should not be considered.
- Base slab of any underground enclosure shall also be designed for maximum ground water table. Minimum factor of safety of 1.5 against bouncy shall be ensured.
- The tower and equipment foundations shall be checked for a factor of safety of 2.2 for normal condition and 1.65 for short circuit condition against sliding, overturning and pullout.
Methods of Transmission Tower Erection:
There are four main methods of steel transmission towers, which are described below:
- Build up method or place meal method
- Section method
- Ground assembly method
Build Up Method of Transmission Tower Erection:
This method is most commonly used in India for the erection of 6.6 KV, 132 K V, 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.
Section Method of Transmission Tower Erection
Ground Assembly Method of Tower Erection
Handling of Conductor and Earth wire
- Handling and transporting of the conductor and accessories shall be carried out in such a manner as to minimize the possibility of damages from abrasion through rough handling or dirt and grit and getting into the reel of the conductor by touching or rubbing against ground or objects, causing injury to the conductor etc. Particular care shall be taken at all times to ensure that the conductor do not become kinked twisted or abraded in any manner. If the conductor is damaged, the section affected shall be replaced or repaired by putting joint or using repair sleeves or polishing with emery cloth, so as to give satisfactory performance
- At all stage of construction proper care shall be taken so that the conductor surface is smooth enough to be given satisfactory corona and radio interference performance. All equipment used in handling or transporting the conductor such as grips, pulleys slings, cable care etc. shall be so designed and maintained that the surface which may contact the conductor, are kept free of foreign or defects.
- Care shall be taken while running out the conductors such that the conductors do not touch or rub against the ground or objects which could scratches or damage to the strands. The conductor shall not be over strained during erection. The conductor shall be run out of the drums from the top in order to avoid damage due to chaffing. Drum battens shall not be removed until conductor drums are properly mounted at the drum station on the line, and battens shall be immediately refitted on the drum if any surplus conductor is left thereon. Drums will be transported and positioned on station with the least possible amount of rolling, immediately after running out, the conductor shall be raised at the supports to the level of the clamps and placed into the running blocks. The grooves of the running blocks shall be of a design that the seat is semi-circular and larger that the diameter of conductor/earth wire and it does not slip over or rubs against the sides. The grooves shall be lined with hard rubber or neoprene to avoid damage to conductor and shall be lined with hard rubber or neoprene to avoid damage to conductor and shall be mounted on well-oiled bearings. At all stages of construction proper care shall be taken so that the conductor surface is smooth enough to give satisfactory corona and radio interference performance