This document discusses the design of combined footings. It defines a combined footing as one that supports two or more adjacent columns to provide uniform bearing pressure and minimize differential settlement. It describes the different types of combined footings based on connectivity (slab, slab-beam, strap-beam) and shape (rectangular, trapezoidal). The key steps of the design process are outlined, including determining the footing size based on load and soil capacity, performing structural analysis to calculate moments and shear, and designing the longitudinal, shear, and transverse reinforcement.

1. Presented by
Dr. Subhash V. Patankar
Professor
Department of Civil Engineering,
Sanjivani college of engineering,
kopargaon, SPPU, Pune.
*DESIGN OF COMBINED
FOOTING

2. *Foundation
*Foundation is the lowest part
of structure which is in direct contact
with soil and transfers the load to the soil beneath.
*Footings transfer the vertical loads, horizontal loads,
moments and other forces to the soil.
*Footing has to be designed to safely transmit the load of
the structure on to a sufficient area of the soil.
*If a soil is overstressed, it may lead to shear failure
resulting in sliding of the soil along a plane of rupture
and thus result in collapse of the structure.

3. *Combined Footing
*A Combined footing is used to
support the load of two or more
adjacent columns.
*To have uniform bearing pressure
and to minimize differential
settlement between columns.

4. *When Combined Footings provided
*When the columns are nearer to each other so
that their isolated footings overlaps.
*when the bearing capacity of soil is lower and
required more area under individual footing.
*When one column is close to a property line or
the Centre of gravity of column will not coincide
with footing or dimensions of one side of footing
are restricted to some lower value.

5. *Types of Combined Footings
On the basis of connectivity
Slab type combined footing
*It supports two or more column with bottom slab only.
Slab-beam type combined footing
*It supports two or more column with bottom slab and
beam.
*Strap-beam type combined footing
Strap footings are normally used when one of the
columns is subjected to large eccentric loadings or when
two columns are far apart, a strap is designed to transfer
eccentric moment between two columns.

6. *Other Combined footings
*Raft foundation is used when soil have low
bearing capacity, large variation in soil
behaviour, Sloping ground, etc.

7. *Types of Combined Footings
On the basis of shape
Rectangular
* If width of footing is restricted.
*Load on adjacent column is same.
Trapezoidal
* Heavy load on one column near to property line
adjacent column.
*If length of footing is restricted

8. *Data required for Design
*Size of column (b X D)
*Load on each column (WA & WB)
*Center-to-center distance between two columns
*Safe bearing capacity of soil (q0)
*Grade of concrete (Fck)
*Grade of steel (Fy)

9. *Design Steps
*Determine the size of combined footing.
*Determine moment and shear in various
section of the footing (SFD & BMD).
*Check the d for punching shear & direct
shear (one way shear).
*Design longitudinal reinforcements.
*Design transverse reinforcements.
*Reinforcement details .

10. *
*Area of footing is calculated as total working
load on columns + self weight of footing which
is considered 10% of total load on columns / Safe
bearing capacity of soil.
*The size of the footing shall be determined to
have uniform bearing pressure under the footing
so that differential settlement is minimized.
* The resultant of bearing pressures needs to
coincide with the resultant of column loads.

11. *
*Structural analysis of a combined footing is the
same as analyzing an invert simply support beam
supported by two columns with factored soil
pressure as loading.
The procedures are as follows:
*Calculate factored footing pressure.
*Calculate maximum shear at an effective depth
from the face of column.
*Calculate maximum positive and negative
moment in the footing. (Maximum positive moment
occurs at face of column. Maximum negative moment
occurs between two columns at zero-shear).

12. *
* The critical section of punching shear is taken at
½ d (effective depth) from face of column.
*For column at the edge of footing the critical
section of punching shear only has three sides
along the column.
*The critical section of direct shear is taken at d
(effective depth) from face of column.

13. *
*Longitudinal reinforcement
*Shear reinforcement
*Transverse reinforcement
*Reinforcement details

14. *Design of Slab- Beam Type
Combined Footing

36. *Now You Can Start to
stable our StructuresThank You
for your support