Footings can be of the following types.
- stretched or isolated or pad footing
- strap footing
- joint basis
- strip or continuous footing
- rafter footing
What is spread footing or isolated or pad footing?
It is a circular, square or rectangular slab of uniform thickness. Sometimes, it is pushed forward to spread the load over a larger area. When footing is provided to support an individual column, it is referred to as “separate footing”.
What is Strap Footing?
It consists of two separate footings connected by a structural strap or lever, as shown in the figure below. The leash connects the footings in such a way that they behave as a unit. The strap simply acts as a connecting beam. When the allowable soil pressure is relatively high and the distance between the pillars is large, a strap footing is more economical than a combined footing.
What is Joint Footing?
It supports two pillars as shown in the figure below. It is used when two columns are so close to each other that their separate legs overlap. A combined footing is also provided when the property line is so close to a column that a spread footing will load eccentrically when placed entirely within the property line. By connecting it with an internal column, the load is distributed more evenly. A combine footing may be rectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one column is greater than that on the other column.
What is Strip Footing or Continuous Footing?
Strip footing is another type of spread footing provided for a load bearing wall. A strip footing may also be provided for a row of columns that are so closely spaced that their spanning legs overlap or nearly touch each other. In such cases, it is more economical to provide strip footing than to provide multiple footing footings in a row. Strip footing is also known as continuous footing.
What is mat or rough footing?
It is a large slab supporting a number of pillars and walls under the whole structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the column and walls are so close that the individual feet overlap or nearly touch each other. Mat foundations are useful in reducing inter-settlements on non-homogeneous soils or where there is a large variation in load on individual pillars.