What are the various types of Foundations/footings commonly used for three to four storey buildings

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Foundations are broadly classified in two categories:-

  • Shallow foundation
  • Deep foundation

(i) Shallow foundations:

They are the foundations commonly used in building industry. These foundations are practicable up to the depth of 3 m to 4 m, and is generally convenient above the water table. As these foundations are construct at reasonable shallow depth so these are called as shallow foundations. They are also called as open foundations.

There are various types of shallow foundations as follows:-

(a) Wall footing
(b) Isolated footing or column footing
(c) Combined footing

(d) Strap footing or cantilever footing
(e) Continuous footing

(f) Inverted arch footing
(g) Raft foundation

(h) Grillage footing

(a) Wall Footing: These footings can be either simple or stepped Simple footings are used to carry light loads, whereas stepped footings are used to carry heavy loads, Simple footing has only one projection on either side of the wall whereas stepped footings have numbers of projection on either side of the wall.
The base course of these footings can be concrete or entirely of one material The width of concrete base should be atleast equal to twice the width of wall. The depth of concrete bed should be atleast equal to the projection. Generally, the projection provided in the footing is kept as 15 cm, on either sides.

(b) Isolated Footing or Column Footing: They are used to support individual columns. They can be either of stepped type or have projections in the concrete base. In case of heavy loaded columns, steel reinforcement is provided in both the directions in concrete bed. Generally, 15 cm offset is provided on all sides of concrete bed. In case of brick masonry columns, an offset of 5 cm is provided on all the four sides in regular layers. The footing of concrete columns may be a slab, stepped or sloped type.

(c) Combined Footing: A combined footing supports two or more columns in a row. The combined footing can be rectangular in shape ifboth the columns carry equal loads, or can be trapezoidal if there are space limitations and they carry unequal loads . Generally, they are constructed of reinforced concrete. In the design of footings, the footing is assumed to be rigid and resting on a homogeneous soil. The location of centre of gravity of column loads and the centroid of the footing should coincide. The maximum bending moment is taken as the design value for the reinforced concrete footing and should be checked for maximum shear and bend, etc.

(d) Strap or Cantilever Footing: Strap footing consists of two or more individual footings connected by a beam called a strap. It is also sometimes called as cantilever footing or pump handle foundation.
*Strap Footing with Non-uniform Strap
*Cantilever Footing with Uniform Strap

This type of footing may be used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow with high bending moments. The strap beam does not remain in contact with the soil; so a strap does not transfer any pressure to the soil. However, because the strap is infinitely stiff, hence it serves to transfer column loads on to the soil with equal and uniform soil pressure under both footings.

(e) Continuous Footing: In this single continuous R.C.C. slab is provided as foundation for more than two columns in row. This type of foundation is suitable in earthquake prone area and also prevents differential settlement.

(f) The inverted arch footing is used in places where the bearing capacity of the soil is very poor. An inverted arch or invert is a civil engineering structure in the form of an inverted archinverted in comparison to the usual arch bridge. 

(g) Raft foundations ( also known as Mat Foundations): are a large concrete slab which can support a number of columns and walls. A raft foundation is often used when the soil is weak, as it distributes the weight of the building over the entire area of the building. This reduces the stress on the soil. The more complex types of raft foundation can require sizeable quantities of both concrete and steel reinforcement, making them potentially more expensive than other alternatives particularly when the soil quality is good.

(h) Grillage Foundation definition – The grillage foundation consists of two or more tiers (layers) of beams laid at right angles to distribute the load over a large area. This type of foundation is generally used for heavy structure columns piers and stanchions.  It is used at the base of columns. These tiers are encased in concrete and are at right angles to each other. It is particularly useful for structure carrying heavy concentrated loads and hence are largely adopted for the foundation of building like workshops, factories, or town- halls, theatres etc.

(ii) Deep Foundations:

Deep foundations are foundations which transmit the load of a structure to strong soil beds or rock beds available at great depth. When the soil at or near the ground surface is not capable of supporting a structure, deep foundations are required to transfer the loads to deeper strata.

Deep foundations are of following types —
(a) Pile foundation (b) Pier foundation
(c) Caisson or well foundation. 

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Arbaj Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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