This document discusses various types of shallow foundations, including spread footings, combined footings, strap footings, and mat foundations. It provides details on: 1) Combined footings, which support two or more columns, can be rectangular or trapezoidal shape depending on whether the columns carry equal or unequal loads. 2) Strap footings connect the independent footings of two columns with a beam to transfer the column loads equally to the soil. 3) Mat or raft foundations are used when soil pressure is low or building loads are high, and cover the entire building area as a solid concrete slab supported by beams.

1. BEG 116 AR
DATE: 12/1/2022

2. 1. Spread footings
a. Single footing
b. Stepped footing
c. Sloped footing
d. Wall footing without step
e. Stepped footing for wall
f. Grillage foundation
2.Combined footings
a. Rectangular combined footing
b. Trapezoidal combined footing
c. Combined column-wall footings
3. Strap footings
4. Mat foundation
a. Solid slab system
b. Beam slab system
c. Cellular system
TYPES OF SHALLOW FOUNDATION:……….

3. 2.Combined footings:
•A spread footing which supports two or more
columns is termed as combined footing
Combined footings are provided when:
•Columns are very near to each other and footings of
two columns overlap
•Bearing capacity of soil is very less and require
more area under individual footing
•When a end column is near property line and its
footing has no area to spread.

5. 2.Combined footings:………….
Types:
a. Rectangular combined footing
b. Trapezoidal combined footing
c. Combined column-wall footings

6. 2.Combined footings:………….

7. 2.Combined footings:………….
•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 C.G of
column loads coincide with the centroid of the footing
area.
• If the columns carry unequal loads, the footing is of
trapezoidal shape
•It may be required to provide a combined footings
for columns and a wall too

8. 2.Combined footings:………….
•Fig a when the
columns carry equal
loads
•Fig b when the
columns carry
unequal loads

9. 3. Strap footing:
•If the independent footings of two columns are
connected by a beam, it is called a strap footing

10. 3. Strap footing:

11. 3. 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
•In that case, each column is provided with its
independent footings and a beam is used to connect
the two footings
•The strap beam does not remain in contact with soil
and thus does not transfer any pressure to the soil

12. 3. Strap footing:…………………..
•The strap, assumed to be infinitely stiff,
•It serves to transfer the column loads on to the soil
with equal and uniform soil pressure under both
footings

13. •mat is a combined
footing that covers the
entire area beneath a
structure and supports all
the walls and columns
4. Mat or Raft foundation:
•When the allowable soil pressure is low, or the
building loads are heavy, the use of spread footings
(with large base) would cover more than one half
the area

14. 4. Mat or Raft foundation:
•So that it is better to adopt mat foundation and this
may prove to be more economical to use mat or raft
foundation
•Raft foundation is also used to reduce unsettlement
above highly compressible soils
•This is done by making the weight of structure and
raft approximately equal to the weight of the soil
excavated

15. 4. Mat or Raft foundation:
•based on their design and construction mat
foundation may be:
(i) Solid slab system
(ii) Beam slab system
(iii) Cellular system
•Raft or mat is a flat concrete slab with uniform
thickness
•The thickness of the slab and the size of beams
depends upon the spacing and loading of the column
and degree of rigidity required in the raft.

16. Methods of construction
The area to be excavated is marked, excavated,
compacted and rammed
•One layer brick soling is done(2” thick)
•3” to 6” thick PCC(1:2:4)
• then after reinforcement is laid which consists of
closely spaced bars at right angles to each other
•Then cement concrete is laid at the ratio of 1:1.5:3
or 1: 2:4 and Curing is done
•When loads are excessive, beams are also
provided to tie the column

17. 4. Mat or Raft foundation:…………..
Suitability:
•Used for public buildings like office building and in
some case residential building where the ground
condition is very poor, bearing capacity of soil is so
low that the spread footing is not suitable

18. ECENTRICALLY LOADED FOOTING
•When there is no area to spread for the footing in
one or two sides, eccentrically loaded footing is
provided
•Normally the footings are so designed and
proportioned that the C.G of the superimposed load
coincide with the C.G of the base area

19. Assignment 1
• Discuss the various functions served by
foundation.
• What are the requirements of good
foundation?
• Explain with the help of sketches, various type
of shallow foundation
• Differentiate between rectangular and
trapezoidal combined footings.
• What are the cause of failure of foundation?
What measures can be used to overcome it?

21. THANK YOU