- Transmission Tower Foundation Definition: A transmission tower foundation is defined as the base that supports the tower and transfers loads to the ground.
- Types of Loads: Foundations experience compression, tension, and lateral forces.
- Soil Parameters: Soil bearing capacity, density, and angle are crucial for foundation design.
- Stability Analysis: Stability checks are performed to prevent overturning, sliding, and tilting.
- Resistance Against Uplift: Uplift resistance involves the weight of earth and concrete.
A transmission tower foundation transfers compression, uplift, shear and overturning actions from the electrical transmission system structure into the ground. Its type and dimensions must suit the tower loads, ground conditions, groundwater, construction constraints and the governing design standard.
Common options include spread footings, drilled shafts, piles and anchors. River crossings, weak ground, scour risk or large uplift and lateral loads can make a deep foundation more suitable than a shallow footing.
Type of Loads on Foundation
A tower foundation is normally checked for three broad force components:
- The compression or downward thrust.
- The tension or uplift.
- The lateral forces of side thrusts in both transverse and longitudinal directions.
The designer must derive foundation actions from the governing tower load cases and apply the load and resistance factors required by the project standard. A fixed 10% increase is not a universal design rule.
The base slab must resist the factored axial force, shear and bending produced by the governing load combinations.
Foundation self-weight and any permitted soil weight may contribute to resistance, but the design must use the applicable geotechnical model and groundwater conditions.
Foundation design requires site-specific geotechnical parameters, including:
- Limit bearing capacity of soil.
- Density of soil.
- Angle of earth frustum.
These values should come from a geotechnical investigation that also identifies soil and rock layers, groundwater, settlement risk and hazards such as scour or liquefaction where relevant.
Stability Analysis of Transmission Tower Foundation
Strength and serviceability checks must address bearing failure, uplift, sliding, overturning, settlement and rotation. The available soil resistance depends on the foundation geometry, ground profile, groundwater and the displacement that the tower can tolerate.
Resistance Against Uplift of Transmission Tower Foundation
Uplift resistance may include foundation self-weight, permitted soil weight, shaft or pile resistance and anchors. An inverted soil-frustum calculation is one simplified method for some spread footings, but its angle, overlap treatment and resistance factors must come from the applicable standard and verified soil parameters. The designer must also account for groundwater, disturbed soil and group effects. The fixed overload factors sometimes quoted for suspension, angle, dead-end or special towers are project-specific and must not replace the governing load combinations.
Resistance Against Down Thrust of Transmission Tower Foundation
Bearing and structural checks should include the following effects:
- 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.
The base slab must be designed for the governing factored combinations and the resulting non-uniform contact pressure. Any increase in allowable bearing pressure, including the 25% increase sometimes used for short-duration loading, is permitted only when the governing geotechnical standard and project criteria allow it.
Resistance Against Side Thrust of Transmission Tower Foundation
The foundation chimney or pedestal must resist combined axial and shear forces plus the bending moment from the governing load cases. Reinforced-concrete design should follow the applicable limit-state standard, including its rules for cracking, reinforcement, anchorage and concrete tensile resistance.
Resistance Against Uprooting of Stub of Transmission Tower Foundation
The stub and its anchorage must transfer the factored uplift force into the footing without pull-out, splitting or reinforcement failure. Load factors for suspension, angle, dead-end, anchor and special towers must come from the governing standard and utility criteria rather than a universal set of overload factors.
