These cover such types of foundation in which load transfer takes place primarily through shear resistance of the bearing strata; and, such foundations are normally up to a depth of 3 m. The various types of shallow foundations used in building construction are as follows :
- Pad or spread and skip footing,
- Grillage foundation, and
- Raft foundation.
The basic purpose of this foundation is to spread the load over a larger area so that the soil is able to withstand the stress, and the safe bearing pressure is not exceeded. In such types of foundations, if the resultant of the load deviates from the centre line by more than 116 of its least dimension at the base of the footing, it should be suitably reinforced.
Wall Footings
In the case of brick walls, the width of section is increased by 114 brick (5 cm) offset on either side. The base rests on a plain concrete footing which projects 10 to 15 cm beyond the last brick offset as shown in Figure 4.7. The width at the base shall not be less. than the width of the supported wall plus 30 cm.
The depth of each course can be one brick or multiples of brick thicknesses. In the case of stone masonry walls, the offsets could be 15 cm with the heights of the course as 30 cm. The depth of the concrete which is generally of 1 : 4 : 8 (1 Cement : 4 Fine aggregate : 8 Coarse aggregate) or 1 : 5 : 10 ( 1 Cement : 5 Fine aggregate : 10 Coarse aggregate) mix should not be less than 15 cm. The angular spread of load from the wall should not be more than 1 vertical to 112 horizontal in masonry and 1 vertical to 1 horizontal for cement concrete.
 |
| Wall Footing |
If the load on the wall is heavy or the soil is of low bearing capacity, reinforced concrete strip footing can be provided. The thickness of the strip can be reduced towards the edge to effect economy.
 |
| Strip Footing |
RCC Column Footings
These are generally square or rectangular. They are reinforced in both directions and are designed to withstand the upwards soil pressure as shown. The reinforcement of the column is taken right upto the bottom of the footing. The RCC footings are laid over a bed of lean concrete of about 75 mm thickness.
 |
| RCC Column Footings |
As the bending moment decreases towards the edge, the thickness can also be reduced accordingly. The minimum thickness at the edge shall, however, be not less than 15 cm. Stepped footings should be avoided as sudden changes in stress conditions are likely to be produced. The top of the footings can be sloped towards the edge in the shape of a pyramid. Steep slopes exceeding 1 in 2 would require form work and hence, normally the slope is of the order of 1 in 4. In a foundation, it is important to ensure that a minimum cover of 40 to 50 mm is provided to prevent corrosion of reinforcement.
When two column footings are near to each other or the foundations overlap, a combined footing can be provided. Such footings can be rectangular or trapezoidal in plan and its position is so adjusted that its centre of gravity coincides with that of the loads from the columns.
 |
| combined Footing |
Cantilever or Strap Footings
At times, there are restrictions in space for the external columns due to adjacent buildings etc., and it would not be possible to place the footings centrally with respect to the columns. In such cases, the column is connected with the interior column by a beam so that the loads are shared as shown.
 |
| Strap Footing |
No comments:
Post a Comment