Foundations are the need of every building however small buildings need shallow foundations whereas mega building needs deep foundations. Foundation is the substructure part of the building that is in direct contact with the subsoil. It is one of the most important parts that transmits all the building load to the subsoil. A properly designed foundation will transfer the loads to the subsoil, in such a way that the soil should not fail and the settlement should be within the permissible limits. Depending upon the type of soil and the type of building that is to be constructed, one may choose from various types of foundations.
Building Foundation consists of two parts :
(I) Substructure or Foundation.
(II) Superstructure.
Foundations or Substructure:
Substructure or foundations is that part of the building, which is below the ground level and is in direct contact with the soil however, It also names a foundation, and its function is to transfer the load of the building safely to the ground.
Superstructure:
The superstructure is that part of the building which is above ground level.
PURPOSE OF FOUNDATION:
Foundations are a very important part of any structure that should fulfill its function properly otherwise the structure may collapse or become abandoned. It is provided for the following purposes:
The foundations distribute the weight of the supers structure over a larger area such that the pressure due to loads at the base of the footing does not exceed and the safe bearing capacity of the sub-soil.
It transfers the load of the superstructure evenly or uniformly to the sub-soil, thus preventing the unequal or differential settlement which can cause cracking, tilt or collapse of the building
.It provides a level, even, and hard surface for the construction of the superstructure.
They act as anchors for the superstructure. They anchor the superstructure into the deep ground, to give lateral stability against overturning caused due to horizontal forces like wind, rain, earthquake, etc.
It also provides safety against movement of moisture (in weak and poor soils) and undermining or scouring due to burrowing animals and flood water, etc.
TYPES OF FOUNDATIONS:
Foundations may be broadly classified into two types:
Shallow Foundation.
Deep Foundation.
The definition is given by Terzaghi:
As defined by Terzaghi's, shallow foundations is that foundation in which the depth is equal to or less than its width. And a deep foundation is that in which the depth is greater than its width".
The foundation constructed at a shallow depth, at the lowermost of the superstructure, is termed as the shallow foundation, and a foundation constructed at deeper depths is called a deep foundation.
A spread foundation is built by providing a spread under the base of the wall or column such that to increase the distribution area for the load.
Thus, spread distributes the load on a wider area such that the safe bearing capacity of the soil is not exceeded, therefore; the spread given at the base is called footing and is called a spread foundation.
The various types of footings which come under this category are as follows:
Wall Footing. • Stepped footing • Strip footing
Masonry Pillar Footing.
Concrete Column Footing.
Inverted Arch Footing.
(1) Building Wall Footing:
The spread footing which is built under a wall is called wall footing and is of two types:
(i) Stepped footing
(ii) Strip footing
Spread foundation
(I) Stepped foundations or footings:
Spread foundation for 400mm thick wall
Fig 2.1 Cross-Section view of Spread Foundation.
The stepped footing consists of several layers of brickwork, laid by 50 mm offset or projection on both sides of the wall, however, the thickness of each step is kept 100 mm. It may increase to 200 mm. The bricks used in foundations should be first-class bricks in 1:3 or 1:4 cement mortar.
Construction of Stepped Foundations:
The subsoil surface is level and compacted with rammers.
A layer of sand and gravel lays over the prepared surface which is compacted well. As it provides a cushion to the footing.
A layer of lean concrete lays over this surface and compacted well with the help of rammers.
The thickness of the lean concrete (1: 3:6) or (1:4: 8) bed must not be less than 150 mm or its projection beyond the bottom-most course of the brickwork. This concrete bed provides a plain and even surface for the construction of footing.
On the concrete bed, a course of brickwork is lain in the form of steps as showing in Fig. 2.1,
By giving a 50 mm offset on both sides of the wall. Stepped footings for a 400 mm thick and a 300 mm thick wall showing Fig. 2.1.
Fig 2.2 Stepped Foundation On slope.
USE:
This is the simplest type of footing and is laid under the walls of buildings. The step footing is also good on sloppy sites in the form of steps of concrete.
(ii) Strip Footing:
Fig. 2.3 Sloped strip footing for the wall
A strip footing consists of a continuous strip under the wall. This strip is made of plain concrete or reinforced cement concrete depending upon the structural load. The use of concrete results in a reduced number of steps.
Reinforces strip footing is an economical option for the wall that carries the heavy load to poor soils because in that case, the bricks masonry footing is much larger.
Figure 2.3 shows a sloped R.C.C. strip footing provided under a wall.
(2) Masonry Pillar Foundations or Footings ;
This type of footing is used to support the individual pillars or columns made up of brick masonry. Following is the stepped footing in Fig. 2.4.
Spread foundation with brick massonry
Conclusions:
To summarize this, One must have to check the other post. However, shallow foundations have a variety of options to transfer the load to the subsoil. For listening this article in Mp3 click the download button.
