Foundation of a structure is like the roots of a tree without which the tree cannot stand. The construction of any structure, be it a residence or a skyscraper; starts with the laying of foundations.
Before designing the foundation, the type of soil is determined. Depending on whether the soil is hard soil or soft soil, a specific type of foundation is adopted.
Foundations are made in various materials… They could be reinforced cement concrete foundations or brick foundations or stone rubble masonry foundations etc. The choice of material to be used in the construction of foundations also depends on the weight of the structure on the ground.
The bearing capacity of soil plays a major role in deciding the type of foundation. The safe bearing capacity of soil should be 180N/mm2 to 200N/mm2.
Foundations are broadly classified into shallow foundations and deep foundations. The depth of the foundation means the difference of level between the ground surface and the base of the foundation. If the depth of the foundation is greater than its width the foundation is classified as a deep foundation.
Shallow foundations are commonly used in smaller structures such as residences and small buildings whose floor height is limited to 10m whereas Deep Foundations are used in Skyscrapers…. Piles are the most commonly used Deep Foundations used in skyscrapers…
Types of Shallow foundations
Footings are structural members used to support columns and walls and to transmit their load to the underlying soils.
Mats or rafts
Combined footings, strap and strip footings
- In this type of foundation the base of the column is sufficiently enlarged to act as the individual support. The widened base not only provides stability but is useful in distributing the load on sufficient area of the soil.
- Column footings are usually used in the foundations of residences and buildings where the soil is hard enough has has sufficient bearing capacity.
Pressure distribution Under a Foundation
- The law of distribution of pressure under a foundation depends on the homogeneity of the soil and flexibility of the base. If really the soil is homogeneous and the base of the foundation is flexible, the pressure distribution under the foundation will be uniform. On the contrary if the foundation base is absolutely rigid, the pressure distribution will not be uniform but may follow such pattern.
- In our designs it is usual to assume a flexible base and hence to regard the pressure distribution to be uniform. This can be achieved by gradually decreasing the thickness of the base towards the edges so that the base is only as much thick as it is regarded to resist the induced moments and shears.
General rules of Foundation Design
While designing a foundation the following points must be borne in mind.
- When a soil is yielding soil, a certain amount of settlement must be reduced as much as possible by bringing down the pressure intensities.
- It is necessary that a foundation shall be designed so that if at all a settlement should occur, it will be uniform. In other words, the settlement of all the footings must be more or less the same.
- This is a very important point in reinforced concrete structures due to the rigid connection between the different components of the structure.
In our next article, we will discuss the procedure of designing an isolated foundation and also justify the foundation design rules mentioned above.