Pad foundations are square or rectangular slab that support load from single columns. They are often used when the bearing strata of the soil is close to the ground surface or where the load from columns are appreciably light. They are mostly reinforced concrete; however, they can be made of mass concrete if there is allowance and economic justification for enough concrete thickness to achieve a load dispersal angle of 45degrees.
Soil Pressure under Pad Foundations
The fundamental assumption in the design of a pad foundation is that it is a rigid body and soil pressure from the ground on the foundation is assumed to be linear. When only concentric vertical loading is applied, square pads are used, assuming a uniform pressure under the whole base area. If the loading is eccentric or if a moment is applied to the base, then it is more efficient to adopt a rectangular base. In this case the pressure under the base is assumed to vary linearly. Soil pressure acting on pad foundations can be classified as follows:
- Uniform Base Pressure
Uniform base pressure occurs when a concentric axial load acts on the foundation such that the line of action of the load coincides with the centroid of the foundation. The pressure in this case can be calculated using:
P = N/BD
Where;
P = soil pressure under base
N = Axial load from supported column
B = Breadth of the foundation
D = Depth of the foundation
Click here to study a Worked Example on the design of an axially loaded Pad Foundation
2. Varying Base Pressure: This occurs when the pressure on the base is not constant all through the width of the foundation. This might be due to any of the following:
- When Load from superstructure is not applied through the centroid of the base
- When there is fixity between the column and the base such that moment is transferred between them
- When there is lateral load being applied to the column supported by the base
The varying pressure due to the aforementioned reasons can either be:
- Varying Base Pressure in compression all through:
This is a situation of varying base pressure where pressure under the foundation is in compression throughout, hence the foundation is supported by the soil all through its length and breadth. The pressure under the base is the sum of pressure due to axial load and pressure caused by moment due to eccentric axial load or lateral load. The pressure can be estimated using:
N/BD ± 6M/BD²
Click here to study a worked example on the design of Pad Foundation subjected to axial load and moment.
2. Varying Base Pressure with compression and tension:
This happens when part of the base is in compression and the other remaining part is in tension. The part of the foundation bearing on the portion of the soil in compression is supported while that part of the foundation over the portion of the soil in tension is unsupported. This is displayed in the figure below by the absence of bearing pressure along some part of the foundation.
The pressure under the base can be estimated using simple elastic analysis.
2N/3B(D/2 – e)
The Middle Third Rule
It is a good practice to always size foundations such that tension does not develop in the soil beneath them. Soil is weak in tension, so when tension develops in a soil under a base then the soil lacks the capacity to develop bearing pressure to support the base. This means part of the foundation bearing over the portion of the soil in tension will be left unsupported, this can cause tilting or even ultimate collapse of the foundation.
To prevent failure, a middle third rule can be adopted to check whether the size of the footing would not permit the development of tension in the soil.
The development of tensile stress can be prevented in the soil by making the eccentricity of the load on the foundation not to be greater than one-sixth of foundation depth (D)
Ie: e > D/6; tension develops in the soil
e ≤ D/6; no tension in the soil
where, eccentricity (e) = M/N
Design of pad foundation for flexure
A pad foundation is designed as an inverted cantilever for flexure. The bending moment caused by the base pressure to the face of the column is calculated and reinforcement is provided to resist it.
Design of Pad Foundation for One-way Shear
The foundation is checked for one-way shear at a distance of d from the face of the column. The one-way shear is checked across the full width of the pad footing.
Design of Pad Foundation for Punching Shear
Punching shear is also critical as pad foundations are essentially slabs which support concentrated loads. The punching shear is checked at basic control perimeter which is located at a distance of 2d away from the column surface. However, unlike in flat slabs the load within the control perimeter complements the resistance to punching and should be subtracted so that only the area outside the control perimeter contributes to punching shear stress. Whenever punching shear stress is greater than the punching shear capacity of the concrete without reinforcement, the thickness of the pad is increased until the capacity of the concrete becomes sufficient without providing shear reinforcement. This makes punching shear resistance to determine the thickness of pad foundations.
Additionally, punching shear is also checked at column face. Should the punching shear at the column face be greater than the maximum shear resistance (VRdmax) then the thickness of the foundation has to be increased until it becomes sufficient.
Steps in Pad Foundation Design
- Calculate the plan dimension of the foundation using the safe or allowable bearing pressure base on serviceability limit load.
- Calculate the bearing pressure base on ultimate limit load
- Assume a thickness for the foundation
- Determine the bending moment at the face of the column due to the ultimate bearing pressure
- Check for punching shear at the face of the column
- Check for direct shear at 1.0d from the column face
- Check for punching shear at a distance of 2d (according to EN-1992-1-1-2004) or 1.5d (according to BS 8110-1-1997) from the face of the column
Reinforcement Detailing for pad footing
- A mesh of reinforcement of two layers with each rebars in each layer spanning either of the orthogonal length of the footing
- The reinforcement along the longer length is placed at a greater depth
- Dowel/starter bars are provided in the footing so as to provide continuity for main bars in the supported column
- For rectangular shape footing, reinforcements in the short direction are placed at a closer spacing under and near the column. Two-third of the total reinforcement required at each layer should be placed within a width of c + 3d under the column. (where c is the dimension of the column along the dimension, and d is the effective depth of the footing)
