PROJECT REPORT ON DESIGN AND CONSTRUCTION OF COMBINED FOOTING

INTRODUCTION

2.1   Foundation:

                   A foundation is a part of a building or structure which is in direct contact with the ground and which transfers the loads imposed upon it to subsoil beneath.

                   Foundation may be broadly classified into shallow foundation and deep foundation based on ratio of depth and width.

       A foundation consists of:

• Soil strata, with the earth giving support. 
• Footing, the widened part of the structure resting upon the bed. .
• The size of the footing depends on the load-carrying capacity of the soil and the weight of the building and its contents.

               The choice of foundation will depend on site condition, ground water conditions  and bearing capacity of the soil. These are normally taken to be:

2.2   FOOTING:         

■ A footing is a structural element that transfer force from a structure to soil.

■ The soil has much lower strengths than do the structural materials, so the force needs to be distributed over a large area in order to get the normal stresses to be lower than what the soil is capable of supporting.

Combined footing:

              Whenever two or more columns in a straight line are carried on a single spread footing, it is called a combined footing.

3. CHOICE OF FOOTING TYPE

1. The type of footing depends upon the load carried by the column and bearing capacity of the supporting soil. It may be noted that the earth under the foundation is susceptible to large variations. Even under one small building the soil may vary from soft clay to hard murum.
2. It is necessary to conduct the survey in the area where the proposed structure is to be constructed to determine the soil properties. Drill holes and trail pits should be taken and in situ plate load test may beperformed and samples of soil tested in the laboratory to determine the bearing capacity of soil and other properties.
3. If columns are very closely spaced and bearing capacity of the soil is low, raft foundation can be an alternative solution. For column on the boundary line, a combined footing or a strap footing may be provided.

4. SOIL BEARING PRESSURE AT BASE OF FOOTING

1. The distribution of soil bearing pressure on the footing depends on the manner in which the column loads are transmitted to the footing slab and the degree of rigidity of the footing.
2. The soil under the footing is assumed to be homogeneous elastic material. The soil bearing pressure can be considered uniformly distributed, if the load acts through the axis of the footing slab area.
3. The loads pass from their point of application into the super structure, then to the foundation, and then into the under lying supporting material.
4. The foundation is considered to be the entire most supporting part of the structure.
5. Normally a footing is last, structural element of the foundation through which the loads pass.

     4.1   Factors influencing Bearing Capacity:

     Bearing capacity of soil depends on many factors. The following are some important ones:

1. Type of soil
2. Unit weight of soil
3. Surcharge load
4. Depth of foundation
5. Mode of failure
6. Size of footing
7. Shape of footing

8 .Depth of water table

9. Eccentric loads on footing

           10 .Inclination of load on footing

           11 .Inclination of ground level

The bearing capacity is fixed by:

1. Presumption depending on soil
2. Calculation depending on laboratory tests
3. Actual field tests:

• By plate load test
• Standard penetration test

             The load bearing capacity of the soil is 450KN/m2.As bearing capacity of soil is high, combined footings at shallow foundation is suitable to carry the superstructure loads.

2.3   FACTOR OF SAFETY:

                Sub soil is a mixture of solid particles, air and water. with any other material it has to be a certain strength to resist to compressive force, but it will fall if it is overloaded.

               In order to provide a factor of safety, it is usual to design a foundation so that the bearing pressure on the ground, imposed by the structure through the foundation, is about one third of the bearing pressure of the subsoil. As per IS code the partial safety factor is taken as 1.5.