Foundations which carry and spread concentrated loads to the soil from superstructures is called pad foundation. They are usually placed to transfer point loads from the column or framed structures and consists of a concrete block or concrete pad. The pads are usually placed at a shallow depth, but they can also be used as deep foundation depending on the loads to be transferred and condition of the subsoil.
Pad foundations may be square, rectangular or circular in shape. If the pad is subjected to a heavy loaded structure, the pad footing may be stepped. The loads from the structure are simply distributed by the pad to the bearing layer of soil. Pad foundations are also used to support ground beams.
Isolated Pad Foundation is Selected When-
- There is high bearing capacity at a shallow depth.
- There is no chance of differential settlement.
- Loads from superstructure are relatively low.
- For cost-effective solution.
Factors Affecting Pad Foundations
- Condition of subsurface
- The magnitude of the load to be transferred
- The proximity of other structures
- Water table depth
Types of Pad foundation
Followings are the types of pad foundation used in construction
- Plain Concrete Pad Foundation
- Reinforced Concrete Pad Foundation
- Combined Pad Foundation
- Continuous Pad Foundation
- Pad Foundations with Ground Beam
Different types of pad foundations are briefly discussed below-
Plain Concrete Pad Foundation
Pad foundations can be reinforced or non-reinforced. Plain concrete pad foundations are a non-reinforced foundation. For non-reinforced pad foundation, the thickness of the foundation should be equal or more than the required width of the foundation to safely distribute the load. This type of foundation can be used when the loads from the superstructure are low.
Reinforced Concrete Pad Foundation
Reinforcement in pad foundation reduces the thickness required for the foundation. For the ease of construction, they are usually designed to be square plan area. Rectangular pads are also used, especially when there is eccentric or inclined loading.
Combined Pad Foundation
Combined pad foundations are usually used for the columns placed near to the property boundary or in the case where the pad cannot be extended to a particular direction due to other reasons like the existence of service or utility lines. In this type of foundation, the pad of external column is connected with the pad of an internal column to counterbalance the loads.
Continuous Pad Foundation
This type of pad foundations is used when the columns are closely placed and the individual pad foundations overlap each other. The reinforcement in continuous foundation ensures longitudinal stiffness. This type of foundation can also be used to resists differential settlement.
Pad Foundations with Ground Beam
This type of foundation and its purpose of use is similar to combined pad foundation. In this case, instead of continuous foundation, isolated pads are connected by ground beams. This ground beams provides integrity and counterbalance the eccentric loads.
Pad foundations are one of the most common and economic foundation systems when loads from superstructure are relatively low. The size of the pad depends on the safe bearing capacity of the soil and the load from the structure. The shape of the foundation depends on the arrangements of columns and other site conditions such as available space for construction.