The footing is an important part of the structure which transfers the load of the structure to the foundation soil. The footing distributes the load over a large area. So that pressure on the soil does not exceed its allowable bearing capacity and restricts the settlement of the structure within the permissible limits. Foundation increases the stability of the structure. The settlement of the structure should be as uniform as possible and it should be within the tolerable limits. (footings)

Main functions of footing

1-Distribution of loads

2- Stability against sliding & overturning

3- Minimize differential settlement

4- Safe against undermining

5- Provide level surface

6- Minimize distress against soil movement

Depending on Soil bearing capacity of a particular location. Different Types of Footing are constructed.

Footings can be of the following types

1- Spread or isolated or pad footing

2- Strap footing

3- Combined footing

4- Strip or continuous footing

5- Mat or raft footing

1- Spread or isolated or pad footing

It is circular, square or rectangular slab of uniform thickness. Sometimes, it is stepped to spread the load over a larger area. When footing is provided to support an individual column, it is called “isolated footing”.

2- Strap footing

It consists of two isolated footings connected with a structural strap or a lever, as shown in figure below. The strap connects the footing such that they behave as one unit. The strap simply acts as a connecting beam. A strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and distance between the columns is large.

3- Combined footing

It supports two columns as shown in the figure below. It is used when the two-column are so close to each other that their individual footings would overlap. A combined footing is also provided when the property line is so close to one column that a spread footing would be eccentrically loaded when kept entirely within the property line. By combining it with that of an interior column, the load is evenly distributed. A combined footing may be rectangular or trapezoidal in plan. The trapezoidal footing is provided when the load on one of the column is larger than the other column.

4- Strip or continuous footing

A strip footing is another type of spread footing which is provided for a load-bearing wall. A strip footing can also be provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other. In such cases, it is more economical to provide a strip footing than to provide a number of spread footings in one line. A strip footing is also known as continuous footing.

5- Mat or raft footing

It is a large slab supporting a number of columns and walls under an entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is large variation in the loads on the individual columns.

Deep foundatin

pile foundations

A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground so that structures can be supported on top of it.

Pile foundations are used in the following situations:

1- When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer.

2- When a building has very heavy, concentrated loads, such as in a high rise structure.

Pile foundations are capable of taking higher loads than spread footings.

There are two types of pile foundations, each of which works in its own way.

End Bearing Piles

In end bearing piles, the bottom end of the pile rests on a layer of, especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts as a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load, therefore, bypasses the weak layer and is safely transferred to the strong layer.

Friction Piles

Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil. To visualise how this works, imagine you are pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream. Once you have pushed it in, it is strong enough to support some load. The greater the embedment depth in the ice cream, the more load it can support. This is very similar to how a friction pile works. In a friction pile, the amount of load a pile can support is directly proportionate to its length.

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