Different types of footing are most commonly used in building construction, it can be deep or shallow footing depending upon their depth.
Basically footing means the enlargement of the base of a column or wall for the purpose of distributing the load on the supporting soil.
Here we learn about footing, types of footings & much more.
Introduction to footing:
Footings are the structural members used to support columns and walls.
Transmitting and distributing its load to the soil in such a way that the load carrying capacity of the soil is not high, differential settlement and adequate protection against overturning or slipping is maintained.
General requirements for footing design:
- A site investigation is essential for the determination of chemical and physical properties of the soil.
- Determine the magnitude and distribution of loads form the superstructure.
- Also, decide the most suitable and economical type of foundation.
- Determine the depth of the footings below the ground level and method of excavation.
- Establish the permissible bearing pressure used in the design.
Different types of footing used in buildings:
Wall footing is of two types i.e., simple and step wall footing.
For light structures or temporary structures, simple footings are provided.
They have a solid base under the walls with no steps on the wall.
In the case of load-bearing structures, the wall area is stretched step by step until the level of the foundation is reached with a layer of concrete 15 cm to 20 cm thick also provided under the wall.
Simple Wall Footing:
When a certain load carried by a column, is supported on a subsoil whose bearing strength is estimated at some definite figure.
Stepped Wall Footing or Spanning Wall Footing:
The area of the wall is increased so as to transmit the load over a large area of soil.
The footing depth is limited to 1 meter but in the case of black cotton soil, the footing should be moved below the area without cracks.
The simple footing is used under compound walls and light structures while stepping footing is used under load-bearing structures.
2.Column Footing or Separate Footings or Pad Footings:
These types of footing are used to support individual columns, they can either of phased type or approximate on a solid basis.
Generally, 15 cm offset is provided on all sides of the concrete bed, used to support individual columns.
In frame structures, it is customary to have separate legs for each column, it can be square, rectangular, circular or sloping, depending on the distribution of load required.
Isolated sloped column footing is used in RCC buildings.
Sometimes two columns are located very close to each other in a structure.
If separate footings are provided under these columns, they may interfere with each other.
Therefore, it is necessary to provide a combined footing made of reinforced concrete.
The essential condition in this type of footing is the centre of gravity of the column load and the centroid of the footing should match.
Types of Combined footings:
Rectangular footings: This footing are provided under two columns where the column is equal load.
Trapezoidal footings: This footing are provided when the two columns are unevenly loaded.
4.Inverted Arch Footing:
This types of footing are old foundation suitable for building bridges, tanks, underground sewers and reservoirs.
It can be used for structures where loads are concentrated on columns or where a load is distributed over a large area or where the bearing capacity of the soil is poor.
An inverted sectional arch is constructed below the columns, the rise of the arch is from 1 / 10th to 115th of the period.
5.Cantilever Footing or Strap Footing:
This types of footing are also called strap footing, used when a column is near the plot boundary and column footing is not allowed to extend beyond the plot boundary.
In this footing, the load of the column near the boundary is supported on the strap beam, it originates from a column which is in the interior of the plot.
Grillage foundations are used to convey heavy loads from steel columns to low bearing capacity soils.
Such foundations are lighter and economical as they avoid deep digging.
Footing consists of steel beams in one or two levels.
Beams in each tier are held in place by spacer bars with a diameter of 20 to 25 mm.
The space between the beams is filled with concrete to protect the steel beam from corrosion.
7.Raft or Mat Foundation:
The types of footing consist of thick reinforced concrete slabs that cover the entire area below the structure on sub-soil.
The slabs are reinforced with right-sided straps at each other and the stiffness of the slab bridges over loose patches in the soil.
It is suitable where the ground is soft, soils or marshes with low bearing capacity and sub-water conditions are uncertain.
In the built-up ground, softer soils or marshy sites have a lower value of its bearing strength, heavier concentrated structure loads are usually supported by providing a raft foundation.
This main purpose for footing constructed is to distributing the load over a large area.
In cold areas, the footing is placed below the frost line to prevent movement.