Combined footing 1-1-18

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

*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.

*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.

*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.

*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.

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

*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

*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)

*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 .

*
*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.

*
*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).

*
* 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.

*
*Longitudinal reinforcement
*Shear reinforcement
*Transverse reinforcement
*Reinforcement details

*Design of Slab- Beam Type
Combined Footing

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

Combined footing 1-1-18

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Combined footing 1-1-18