What is Spread Footing 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”.
What is 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.
What is Combined Footing?
It supports two columns as shown in 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 combine footing may be rectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one of the column is larger than the other column.
What is Strip Footing 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.
What is Mat or Raft Footing?
It is a large slab supporting a number of columns and walls under 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.