Building foundation

Foundations

Foundation is the basic and the lowermost part of a structure. It provides an uniform base for the super structure and transfers the load of the same down to the soil.

Purpose of foundation

it should not be misunderstood that foundation is provided to support the super structure but should be noted that it is an element which just transfers the load of structure on to the hard bed upon which it lies. foundation is provided for the following main purpose:

1. To provide a level and uniform base to the super-structure.
2. To distribute the load of the structure on a large area of hard pan and also top ensure uniform distribution of load over the soil beneath
3. To provide good anchorage imparting stability to the super-structure  to prevent overturning

Site Investigation

1. To determine the safe bearing capacity of the soil
2. To determine the depth of hard bed upto which the foundation are to be taken to
3. To decide the design suitable, safe and economical foundations.
4. To determine the level of ground water table.
5. To predict the likely settlement of sub soil under loads and to make suitable allowance for the same in the foundationdesign

This can be detremined by simply digging a trial pit of suitable size and depth upto hard bed.

Types of foundations:

Foundations can be classified as 

Shallow Foundations: - Ordinary type buildings

Deep Foundations:- Heavy Loaded foundations

Spread footing foundation

As the name implies the spread footing foundation consists of widened fottings at base of the foundation to transmit the load of the structure uniformly on a greater area. In general all type of foundation have spread footings in one form or other . However from point of view of the material used in the construction its design and the construction technique they are further divided into following categories.

1. Spread footing for walls.
2. Reinforced concrete fottings. 
3. Footings for columns.
4. Grillage foundation.
5. Raft foundation.  

Spread footing for walls

In this type of foundation the base of the wall is widened by means of suitable offsets at foundation level. These may be brick or stone masonry usually a bed of lean concrete is provided below the course of brick or stone masonry footings to a afford a level and strong base. This course of concrete rectifies the inequalities of excavation and also bridges over soft patches in the soil below it. The grade of concrete used for such footings is usually 1:3:6 or 1:4:8 (cement:sand:aggregate) or lime concrete comprising 40mm size stone or over burnt brick aggregates with 40% mortar of 1:2 (lime:sand) or 1:1:1(lime:surkhi:sand)

The principle adopted for footings are:

1. The thickness of the concrete bed should not be more than 15 cm. It generally varies from 20 to 30 cm. But it should not be more than the thickness.
2.  The course of footings are generally 10 to 30 cm in depth the offsets provided are usually 5 cm for brick masonry and 8 cm for stone masonry
3.   general thumb rule followed are

• ·         Width of super-structure wall                           : T 
• ·         Width of foundation concrete                           : 2T+30CM
• ·         Width of bottom most course of footings       : 2T
• ·         Depth of concrete block                                      : (2/3) T

These type of footings are most commonly used ones and are economical as compared to other types of foundations for ordinary types of buildings. Structures in soils with moderate bearing capacity can be safely constructed with this type of foundations.

Reinforced concrete footings

These types of foundations are used in case of soil with low bearing capacity. In these type of foundations, reinforced concrete bed will be provided in place of a lean concrete bed. As per rules the base of the foundation are to be made wide enough to load the soil within its bearing capacity. Lean concrete bed is to be made much deeper to obtain the required width and this is totally uneconomical. Reinforced cement concrete can be provided in such cases since they are thin and economical as compared to lean concrete beds. A 7 to 8 cm thick lean concrete bed is usually provided below RCC bed to give a level surface .

Footings for columns

This is an independent footing and is provided under a column for distributing the concentrated load of the column over a large area uniformly. The shape of the footing may be either square, rectangular or circular in plan. The base area of the footing is obtained by dividing teh load of the column and its self weight with the safe bearing capacity of the soil . The requried base area is obtained by means of sitable offsets symmertical around the column.