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What is Pad Foundation?The pad foundations are shallow foundation that take and spread point loads to the soil safely. The pad foundation are preferred if the soil at the site have sufficient strength and is not too deep to construct. The thickness of pad foundations are generally uniform. In some situations the upper surface can be stepped or sloped.The design of pad foundation is simple and straight which makes them an economic solution.
Design Principle of Pad FoundationAs defined, the pad foundation is supposed to spread the concentrated load safely to the bearing stratum. Hence, pad foundation must be designed stiff so that uniform spreading of the load to the soil must happen without making the foundation pressure exceed the permissible bearing stress. This requirement is achieved by either reinforcing the pad or making the pad deep. Both the methods helps to spread the force in a predefined angle. The angle of spreading is determined based on te bearing capacity of the underlying soil and the concrete strength. The size of the pad must be such that tension need to be prevented within the concrete so that no cracking is caused, which will result in failure. The ability to resist the punching shear is the governing criteria that determines the depth of reinforced concrete pad foundations. Another important factor governing the depth of pad is the ability to resist the bending. The pad foundation arrangement is mainly dependent on the load bearing capacity of the soil, the structure to be supported, available space and the imposed loads. Based on these factors the "pads" can be arranged as:
- Separate discrete pads
- Base pads that are balanced. This will supposed more than one point loads
- Continuous pads
- Series of pads supporting a continuous beam
Types of Pad FoundationsPad foundations are generally rectangular or square foundation to transfer load from structure to the ground. These are provided at shallow depth and are shallow foundations. Read More : Foundations The major types of foundations are:
- Plain Concrete
- Reinforced Concrete
- Continuous Pad foundation
- Combined Pad Foundation
- Pad and Ground Beam Foundation
1. Plain Concrete Pad FoundationPlain concrete is non- reinforced concrete pad foundation which is an economic option where the loading is relatively light as T must equal P ( As shown in figure-1 below) otherwise excessively thick pads are needed which is not economic. The selected depth of the pad must be equal to the distance between the vertical element face and the edge of the pad on both the sides.
2. Reinforced Concrete Pad Foundations
Reinforced concrete enables relatively wide but shallow foundations, often designed to be square plan area to make the reinforcing cage easier to construct and place. Rectangular pads are used for eccentric/inclined loading (longer dimension parallel to direction of inclination/eccentricity). The thickness demanded by these type of foundation will be less.
3. Combined Pad Foundations
Combined pad foundations are adopted close to a site boundary to enable the balancing effect of an internal column to be incorporated.The existence of a service or a utility may restrict the extension of the pad foundation. As shown in the figure above, the pad of the external column will be connected to the internal column pad. This will help to counterbalance the loads coming.
4. Continuous Pad Foundations
Continuous pad exists when pads and the columns they support are fairly closely spaced. Extending the reinforcing between pads ensures longitudinal stiffness. This technique helps in resisting the differential settlement.
5. Pad and Ground Beam
Pad and ground beam comprises smaller isolated pads are connected by ground beams to provide structural rigidity.
The technique helps in improving the integrity along with balancing the eccentric loads.
Selection of Type of Pad Foundation
Among different pad foundations explained, the selection is based on the following factors:
- The safe bearing capacity of the soil
- The load to be supported
- The Column arrangement
- Site conditions
- Subsurface conditions
- The depth of water table
- Cost Conditions