Shallow foundation & there types with detail

Support civil September 08, 2018 Building material , Soil mechanics No comments

Foundations may be broadly classified under two heads
Shallow Foundations :A foundation is shallow if its depth is equal to or less than its width.
Deep Foundations : In case of deep foundations, the depth is equal to or greater than its width.
SHALLOW FOUNDATIONS:
From the point of view of design, shallow foundations may be of the following types:
1 Spread footings
2 Combined footings
3 Strap footings
4 Mat foundation
1.Spread footings - Spread footings are those which spread the super-imposed load of wall or column over a larger area. Spread footings support either a column or wall.
Spread footings may be of the following kinds :
Single footing for a column
Stepped footing for a column
Sloped footing for a column
Wall footing without step
Stepped footing for wall

picture credit :BuildingHow

2 Combined footings -Combined Footings a spread footing which supports two or more columns is termed as combined footing. Combined footings are invariably 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 centroid of the footing area.
If the columns carry unequal loads, the footing is of trapezoidal shape.
Sometimes, It may be required to provide a combined footing for columns and a wall.
The combined footings may be of the following kinds:
Rectangular combined footing
Trapezoidal combined footing
Combined column-wall footings

picture credit : Builder's Engineer

3 Strap footings - If the independent footings of two columns are connected by a beam, it is called a strap footing. A strap footing may be used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow, with high bending moments. In that case, each column is provided with its independent footings and a beam is used to connect the two footings. The strap serves to transfer the column loads on to the soil with equal and uniform soil pressure under both footings.

picture credit : Wikipedia

4 Mat or Raft Foundation -A raft or mat is a combined footing that covers the entire 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 footings would cover more than one half the area and it may prove more economical to use mat or raft foundation. They are also used where the soil mass contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult to control.
Raft foundation is also used to reduce settlement above highly compressible soils, by making the weight of structure and raft approximately equal to the weight of the soil excavated.