# 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.

The base slab of the foundation shall be designed for additional moments developing due to eccentricity of the loads.

The additional weight 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.

Soil parameters For designing the foundations, following parameters are required.

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. These are:

- The compression or downward thrust.
- The tension or uplift.
- The lateral forces of side thrusts in both transverse and longitudinal directions.

The base slab of the foundation shall be designed for additional moments developing due to eccentricity of the loads.

The additional weight 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.

Soil parameters For designing the foundations, following parameters are required.

- Limit bearing capacity of soil.
- Density of soil.
- Angle of earth frustum.

### Stability Analysis of Transmission Tower Foundation

In addition to the strength design, stability analysis of the foundation shall be done to check the possibility of failure by over turning, uprooting of stubs, sliding and tilting of foundation etc. The following primary type of soil resistance shall be assumed to act in resisting the loads imposed on the footing in earth.#### Resistance Against Uplift of Transmission Tower Foundation

The uplift loads shall be assumed to be resisted by the weight of earth in an inverted frustum of a pyramid of earth whose sides make an angle equal to the angle of report of the earth with the vertical in average soil. The volume of earth computation shall be as per enclosed drawing (Fig.3) The weight of concrete embedded in earth and that above the ground level shall also be considered for resisting the uplift. In case where the frustum of earth pyramid of two adjoining legs overlaps each other, the earth frustum shall be assumed truncated by a vertical plane passing through the center line of the tower base. Over load factor (OLF) of 10% (Ten percent) shall be considered over the design load i.e. OLF = 1.10 for suspension tower and 1.15 for angle including dead end and anchor tower. However, for special tower OLF shall be 1.20.#### Resistance Against Down Thrust of Transmission Tower Foundation

The following load combinations shall be resisted by the bearing strength of the soil:- The down thrust loads combined with an additional weight of concrete above earth are assumed to be acting on the total area of the bottom of the footing.
- The moment due to side thrust forces at the bottom of the footing.

#### Resistance Against Side Thrust of Transmission Tower Foundation

The chimney shall be designed as per limit state method for the combined action of axial forces, tension and compression and the associated maximum bending moment. In these calculations, the tensile strength of concrete shall be ignored.#### Resistance Against Uprooting of Stub of Transmission Tower Foundation

OLF of 10% (Ten percent) shall be considered i.e. OLF = 1.10 for normal suspension towers and 1.15 for angle tower including Dead end/anchor tower. For special towers OLF shall be 1.20.Closely Related Articles Electrical Transmission Tower Types and DesignMethods of Transmission Tower ErectionDesign of Foundations of Transmission Towers in different SoilsMore Related Articles Electrical Power Cable Types of Overhead ConductorTesting of Electrical Power Cable | Type Test | Acceptance Test | Routine TestConductor Resistance Test of Electrical Power CablesTest for Thickness of Insulation of Power CableAnnealing Test for Wires and Conductors Tensile Test of ConductorsPersulphate Test of ConductorWrapping Test for ConductorsCapacitor Bank | Reactive Power CompensationTypes of Capacitor BankTesting of Capacitor BankSpecifications or Rating of Power Capacitor BankShunt Capacitor Switchable Capacitor Bank or Switched Capacitor BankLocation of Shunt CapacitorsElectrical Insulator | Insulating Material | Porcelain Glass Polymer InsulatorTypes of Electrical Insulator | Overhead InsulatorInsulation Coordination in Power SystemElectrical Insulator Testing | Cause of Insulator failureDielectric Properties of InsulationElectrical Power Substation Engineering and LayoutElectrical Bus System and Electrical Substation LayoutMobile Substation | Portable Substation | Mobile TransformerLoad Curve | Load Duration Curve | Daily Load CurveHigh Voltage Direct Current Transmission | HVDC TransmissionCorona Effect in Power SystemFerranti Effect in Power SystemAdvantages of Three Phase System over Single Phase SystemInductance in Single Conductor Power Transmission LineInductance in Three Phase Transmission LineWhy Supply Frequency 50 or 60 Hz not Other Values than these?Power System StabilityLoad Flow or Power Flow AnalysisTransient Stability in Power SystemFlexible AC Transmission Systems | FACTSTariff of Electricity in IndiaPower Factor | Calculation and Power Factor ImprovementSkin Effect in Transmission LinesInductance of Two Wire Single Phase Transmission LineAuto Reclosing Scheme of Transmission SystemLoad Flow and Y BusEqual Area CriterionSteady State StabilityElectrical Power Transmission System and NetworkTransmission Line in Power SystemVoltage in Power Electric LinesShort Transmission LineMedium Transmission LineLong Transmission LinePerformance of Transmission LineABCD Parameters of Transmission LineSag in Overhead ConductorSurge Impedance Loading or SILAdmittanceAdvantages of Bundled ConductorsBundled Conductors Used in Transmission LineGround Clearance of Different Transmission LinesNew Articles Ring CounterDischarging a CapacitorCharging a CapacitorElectric PotentialParity Generator