FOUNDATION
Every structure consists of the following two parts:
(i) Foundations; and
(ii) superstructures .
The lowest artificially prepared parts of the structures which are in direct contact with the ground and which transmit the loads of the structures to the ground are known as the foundations or substructures. the solid ground on which the foundations rest is called the foundation bed or foundation soil and it ultimately bears the load and interact with the foundation of building . The lowermost portion of the foundation which is in direct contact with the sub-soil is called the footing.
 |
| FOUNDATION |
The term superstructure is used to mean that part of the structure which is above ground level. A part of superstructure located between the ground level and the floor level is known as the plinth . Thus the term plinth is defined as the portion of the structure between the surface of the surrounding ground and level of the floor immediately above the ground .
for any structure, its foundations forms the most important part and as it remains below the ground , it is not available for inspection. Hence the failures of foundation are not noticed till the structure is seriously affected by such failure. It becomes therefore necessary to exercise extreme care in the construction and design of foundation to avoid their failure in future.
OBJECTS OF FOUNDATIONS
The foundations are provided for the following purposes:
(i) to distribute the total load coming on the structure on a large area so as to bring down the intensity of load at its base below the safe bearing capacity of sub-soil:
(ii) to support the structure;
(iii) to give enough lateral stability to the structures against various disturbing horizontal forces such as wind , rain, earthquake ,etc.;
(iv) to prepare a level and hard surface for concreting and masonry work;
(v) to transmit the super- imposed loads through side friction and end bearing in case of deep foundation;
(vi) to distribute the non-uniform loads superstructure evenly to the sub-soil
(vii) to provide the structural safety against undermining of scouring due to animals ,flood water,etc.;
(vii) to prevent or minimize cracks due to movement of moisture in case of weak or poor soils;ec.;
ESSENTIAL REQUIREMENTS OF A GOOD FOUNDATION
Following are the three requirements to be fulfilled by a foundation to be satisfactory:
(1) Location: The foundation structure should be so located that it is able to resist any unexpected future influences which may adversely affect its performance. This aspect requires careful engineering judgement.
(2) Stability : The foundation structure should be stable or safe against any possible failure. The foundation base should be rigid enough to bring down the differential settlements to a minimum extent specially when the superimposed loads are unevenly distributed.
(3) Settlements: The foundation structure should not settle or deflect to such an extent so as to impair its usefulness or the stability of buildings or the adjoining structures. It is however difficult the objectionable amount of settlement or deflection.
SHALLOW FOUNDATION
The foundation can broadly be classified into two categories : shallow and deep.If it is possible to construct foundation of a building at reasonable shallow depth, the foundations are termed as the shallow foundations. In such cases, a spread is given under the base of a wall or a column. This spread is known as the footing and the foundation is known as the spread footing.
 |
| SHALLOW FOUNDATION |
COMBINED FOOTING
 |
| COMBINED FOUNDATION |
Following points are to be noted in case of combined footings:
(i) The shape of the combined footing should be so selected that the center of gravity of the column loads and of soil reaction remain in the same vertical line. Usually a rectangular or a trapezoidal shape is selected.
(ii) The area of the combined footing should be equal to or greater than the ratio of the combined footing is treated as an inverted floor supported by columns and loaded by earth reactions.
CANTILEVER FOOTING
A cantilever footing consists in an eccentric footing for the exterior column,a concentric footing for the interior column and a strap or a cantilever beam joining these two footings as shown in fig
 |
| CANTILEVER FOOTING |
CONTINUOUS FOOTING
In this type of construction , the footings of two or three adjacent columns are made continuous by providing beams the successive footings as shown in fig.This type of construction is cheaper than raft foundations and it is adopted to avoid differential or uneven settlement of the structure and to make the structure safe from earthquake disturbances.
 |
| CONTINUOUS FOOTING |
STEPPED FOUNDATION
When the ground is sloping, it becomes uneconomical to provide foundations at the same level,. In such cases, the correct levels of the sloping ground on which the building is to be constructed are taken ans a longitudinal section is prepared accordingly. The stepped foundations are then provided as shown in fig.
 |
| STEPPED FOUNDATION |
Following points should be noted:
(i) The overlap between two layers of foundation concrete should be equal to the depth of foundation concrete or twice the height of the step, whichever is greater.
(ii) A minimum depth of 800 mm should be provided at all points so as to protect the foundations from weathering effects.
(iii) The depth of foundation concrete should be in even number of the masonry courses.
(iv) The distances of the sloping surface from the lower edge of the footing should not be less than 600 mm for rock and 1 m for soils.
(iv) When footings are heavily loaded, it becomes necessary to carry out a slope stability analysis.
0 Comments
Post a Comment