Types of Shallow Foundation and uses
A shallow foundation is that foundation whose which width is greater than its depth. There are placed just below the lowest part column or wall which they support. The footing is the structural member which are made of brickwork, masonry or concrete. To transfer the load of the column or wall, footings are used so that the load can be distributed over a large area. In fact, a footing is an enlargement of the base of the wall or column it supports.
Different types of Shallow Foundation
- Spread or isolated footing
- Strip footing
- Combined footing
- Strap or cantilever footing
- Mat or raft foundation
Spread or isolated footing
A spread footing is used to provide support for an individual column. A spread footing is rectangular, square or circular slab of uniform thickness. Sometimes, it is haunched or stepped to spread the load over a large area.
Strip footing
For a load bearing wall, a strop footing is provided. This footing is also given for a row of columns which are so closely spaced that their spread footings nearly touch or overlap each other. In such case, it is more economical a strip footing than to provide a number of spread footings in one line. A strip footing is also called continuous footings.
Combined Footing
Two columns are supported by the combined footing when the two columns are so close to each other that their individual footings would overlap then this type footing are used. It is also provided when the property line is so close to one column that a spread footing would be eccentrically loaded when keep entirely within the property by combining it with that of an interior column. Combined footing may be trapezoidal or rectangular in the plan.
Strap or cantilever footing
A Strap footing consists of two isolated footings connected with the structural strap or a lever. They behave as one unit when the strap connects the two footings. The strap does not take any soil reaction because it simply acts as a connecting beam. The Strap footing is designed as a rigid beam.
The individual footings are designed in such a way that their combined line of action passes through the resultant of the total load. When the Distance between the columns is large and the allowable soil pressure is relatively high then the strap footing is more economical than a combined footing.
Raft Foundation
A mat or raft foundation is a large slab which is used to support a number of walls and columns under the large portion of the structure or an entire structure. A mat is needed when the allowable soil pressure is low or where column and wall are so close that individual footings would nearly touch or overlap each other.