Types of pad foundation
Pad foundations are a form of spread foundation formed by rectangular, square, or sometimes circular concrete ‘pads’ that support localised single-point loads such as structural columns, groups of columns or framed structures. This load is then spread by the pad to the bearing layer of soil or rock below. Pad foundations can also be used to support ground beams.
There are several different types of pad foundation:
Plain concrete pad foundations that do not use reinforcement are an economical solution but only where the applied load will be relatively light. These can also be referred to as footings. The general rule is that the depth of the pad should be equal to the distance from the face of the vertical element to the edge of the pad on both sides. If this is not the case then the pad will often be excessively thick and less cost-effective.
The addition of reinforcement allows for relatively wide but shallow pad foundations. In order to make the reinforcing cage easier to construct and place, the pads tend to be designed as a square plan area. The reinforced concrete base is designed to span in one direction, with the main bars longitudinal in the bottom.
Where the width of the base is restricted or where there is eccentric/inclined loading, rectangular pads can be designed as there is a longer dimension parallel to the direction of the inclination/eccentricity.
 Combined column foundations
These are where two pad foundations are combined into a longer one and can be used where the outer column is close to a site boundary or existing wall. The purpose is so that the balancing effect of the internal column can be incorporated. The plan shape is usually a rectangle.
 Continuous pad
This is where the pad foundations are combined together as a single long structural element. This is often the case where the pads and the columns they support are closely spaced. By extending the reinforcement between the pads, differential settlement can be resisted and longitudinal stiffness can be improved.
 Pad and ground beam
This type is similar to a continuous pad but differs in that smaller isolated pads are instead connected by ground beams. This helps to improve structural rigidity.
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