types of foundation/-shallow foundation

                                              //  TYPES OF FOUNDATION//
**/Foundation may be broadly classified under two heads :-

(a)Shallow foundation               (b)Deep foundations

=>According to Terzaghi,a foundation is shallow if its depth is equal to or less than its width.In case of deep foundations,the depth is equal to or greater than its width.

(A) SHALLOW FOUNDATIONS:-
From the point of view of design,shallow foiundations may be of the following types:
1.Spread footings                               2.Combined footings
3.Strap footings                                 4.Mat foundations

Various types of shallow foundations describe below :
Details about the design requirements etc........ are described

1.##Spread footings:-Spread footing are those which spread the super-imposed load of wall or column over a large area.Spread footings support either a column or a wall.Spread footings may be of the following kinds :-

(a)Single footing for a column  or(simple footing)   
(b)Stepped footing for a column
(c)Slopped footing for a column
(d)Wall footing without step
(e)Stepped footing for a wall
(f)Grillage foundations

FIG.2.2  SPREAD FOOTINGS FOR COLUMNS

FIG.2.2/(a)Shows single footing for a column,in which the loaded area (b*b)of the column has been spread to the size B*B through a single spread.The base is generally made of concrete.FIG.2.2/(b) shows the stepped footing for a heavily loadedcolumn,which requires greater spread.the base of column made of concrete.FIG.2.2(c) shows the case in which the concrete base does not have uniform thickness but its made slopped,with greater thickness at its junction with the column and smaller thickness at the ends.......
##/SPREAD FOOTING FOR WALLS:STRIP FOOTING--

          FIG.2.3  SPREAD FOOTING FOR WALLS:STRIP FOOTING
FIG.2.3 (a)Shows the spread footing for a wall,consisting of concrete base without any steps.Usually,masonry walls have stepped footing as shoen in FIG.2.3(b),with a concrete base

##/GRILLAGE FOUNDATION:-

FIG.Steel Grillage Foundations for a steel stanchion carying heavy load.It is special type of isolated footing generally provided for heavily loaded steel stanchion and used in those locations where bearing capacity of soil is poor.The depth of such a foundation is limited to 1 to 1.5m .The load of the stanchion is distributed or spread to a very large area by means of two or more tiers of rolled steel joints,each layer bearing laid at right angles to the layer below it.Both the tier if the joints are thgan embeded in cement concrete to keep the joints in position and to prevent their corrosion.

(2)##COMBINED FOOTINGS:-
A spread footing which supports two or more columns is termed as combined footing.The combined footings may be of the following kinds:-
(A)Rectangular combined footing
(B)Trapezodial combined footing
(C)Combined column wall footings

Combined footings are in variable constructed of reinforced concrete .
The combined footing for columns will be rectangular in shape if they carry equal loads.The design of rigid rectangular combined footing should be done in such a way that centre of gravity of column loads coincide with the centroide of the footing area.If the columns carry unequal loads,the footing is of trapezodial shape.
##COMBINED FOOTINGS FOR COLUMNS AND WALL:-

Sometimes it may be required to provide a combined footing for columns and a wall.Such combined footings are shown in fig....[When the columns carry equal loads ]
and (b)[When the columns carry unequal loads].The design principles of these footings have been discussed later....

3.## STRAP FOOTING:-If the independent footings of two columns are connected by a beam,it is called strap footing.A strap footing may be used where the distance between the columns is so great that a combined trapezodial footings becomes quite narrow,with high bending moments.

FIG.2.7/-shows the strap footing for two columns A and B.Column A is so near to an existing wall that the footing of the wall does not permit the independent footing of column A spread out towards the wall,though it has freedom in other directions.

4.##MAT FOUNDATIONS:-A raft or mat is a combined footing that covers the entier area beneath a structure and supports all the walls and columns.When the allowable soil pressure is low,or the building loads are heavy,the use of spread footing would cover more than one half the area and it may prove more economical to use mat or raft foundation

Rafts may be divided into three types based on their design and construction:-//
(A)Solid slab system                             (B)Beam slab system
(C)Cellular system
All the three types are basically the same,consisting of a large,generally unbroken area of slab covering the whole or the large part of the structures.The thickness of the slab and the size of beams (if any) will be governed by the specing and loading of the column and degree of rigidity required in the raft.