Types of Footings.ppt

Footing
Definition
Footings are structural members used to support
columns and walls and to transmit and distribute
their loads to the soil in such a way that the load
bearing capacity of the soil is not exceeded,
excessive settlement, differential settlement,or
rotation are prevented and adequate safety
against overturning or sliding is maintained.

Types of Footing
Wall footings are used to
support structural walls that
carry loads for other floors
or to support nonstructural
walls.

Types of Footing
Isolated or single footings
are used to support single
columns. This is one of the
most economical types of
footings and is used when
columns are spaced at
relatively long distances.

Types of Footing
Combined footings usually
support two columns, or
three columns not in a row.
Combined footings are used
when tow columns are so
close that single footings
cannot be used or when one
column is located at or near
a property line.

Types of Footing
Cantilever or strap footings
consist of two single
footings connected with a
beam or a strap and support
two single columns. This
type replaces a combined
footing and is more
economical.

Types of Footing
Continuous footings
support a row of three or
more columns. They have
limited width and continue
under all columns.

Types of Footing
Rafted or mat foundation
consists of one footing
usually placed under the
entire building area. They
are used, when soil bearing
capacity is low, column
loads are heavy single
footings cannot be used,
piles are not used and
differential settlement must
be reduced.

Types of Footing
Pile caps are thick slabs
used to tie a group of piles
together to support and
transmit column loads to the
piles.

Distribution of Soil Pressure
When the column load P is
applied on the centroid of the
footing, a uniform pressure is
assumed to develop on the soil
surface below the footing area.
However the actual distribution of
the soil is not uniform, but
depends on may factors especially
the composition of the soil and
degree of flexibility of the footing.

Distribution of Soil Pressure
Soil pressure distribution in
cohesionless soil.
Soil pressure distribution in
cohesive soil.

Design Considerations
Footings must be designed to carry the column loads
and transmit them to the soil safely while satisfying
code limitations.
The area of the footing based on the allowable
bearing soil capacity
Two-way shear or punching shear.
One-way bearing
Bending moment and steel reinforcement required
*
*
*
*

Design Considerations
Footings must be designed to carry the column loads
and transmit them to the soil safely while satisfying
code limitations.
Bearing capacity of columns at their base
Dowel requirements
Development length of bars
Differential settlement
*
*
*
*

Size of Footing
The area of footing can be determined from the
actual external loads such that the allowable soil
pressure is not exceeded.
 
pressure
soil
allowable
weight
-
self
including
load
Total
footing
of
Area 
footing
of
area
u
u
P
q 
Strength design requirements

Two-Way Shear (Punching Shear)
For two-way shear in slabs (& footings) Vc is smallest of
long side/short side of column
concentrated load or reaction area<2
length of critical perimeter around the
column
where, bc =
b0 =
ACI 11-35
d
b
f
V 0
c
c
c
4
2 









b
When b >2 the allowable Vc is reduced.

Design of two-way shear
Assume d.
Determine b0:
b0 = 4(c+d) for square columns
where one side = c
b0 = 2(c1+d) +2(c2+d) for
rectangular columns of sides c1
and c2.
1
2

The shear force Vu acts at a
section that has a length
b0 = 4(c+d) or 2(c1+d) +2(c2+d)
and a depth d; the section is
subjected to a vertical downward
load Pu and vertical upward
pressure qu.
3
 
   columns
r
rectangula
for
columns
square
for
2
1
u
u
u
2
u
u
u
d
c
d
c
q
P
V
d
c
q
P
V








Allowable
Let Vu=fVc
4
d
b
f
V 0
c
c 4f
f 
0
c
u
4 b
f
V
d
f

If d is not close to the assumed d,
revise your assumptions

Design of one-way shear
For footings with bending action
in one direction the critical
section is located a distance d
from face of column
d
b
f
V 0
c
c 2f
f 

The ultimate shearing force at
section m-m can be calculated










 d
c
L
b
q
V
2
2
u
u
If no shear reinforcement is to
be used, then d can be checked

b
f
V
d
2 c
u
f

If no shear reinforcement is to
be used, then d can be checked,
assuming Vu = fVc

Flexural Strength and Footing
reinforcement
2
y
u
s










a
d
f
M
A
f
The bending moment in each
direction of the footing must be
checked and the appropriate
reinforcement must be
provided.

reinforcement
b
f
A
f
a
85
.
0 c
s
y

Another approach is to
calculated Ru = Mu / bd2 and
determine the steel percentage
required r . Determine As then
check if assumed a is close to
calculated a

reinforcement
The minimum steel percentage
required in flexural members is
200/fy with minimum area and
maximum spacing of steel bars
in the direction of bending
shall be as required for
shrinkage temperature
reinforcement.

reinforcement
The reinforcement in one-way
footings and two-way footings
must be distributed across the
entire width of the footing.
1
2
direction
short
in
ent
reinforcem
Total
width
band
in
ent
Reinforcem


b
footing
of
side
short
footing
of
side
long

b
where

Bearing Capacity of Column at Base
The loads from the column act on the footing at the base
of the column, on an area equal to area of the column
cross-section. Compressive forces are transferred to the
footing directly by bearing on the concrete. Tensile
forces must be resisted by reinforcement, neglecting any
contribution by concrete.

Force acting on the concrete at the base of the column
must not exceed the bearing strength of the concrete
 
1
c
1 85
.
0 A
f
N f

where f = 0.7 and
A1 =bearing area of column

The value of the bearing strength may be multiplied by a
factor for bearing on footing when the
supporting surface is wider on all sides than the loaded
area.
0
.
2
/ 1
2 
A
A
The modified bearing
strength
 
 
1
c
2
1
2
1
c
2
85
.
0
2
/
85
.
0
A
f
N
A
A
A
f
N
f
f



Dowels in Footings
A minimum steel ratio r = 0.005 of the column section
as compared to r = 0.01 as minimum reinforcement for
the column itself. The number of dowel bars needed is
four these may be placed at the four corners of the
column. The dowel bars are usually extended into the
footing, bent at the ends, and tied to the main footing
reinforcement. The dowel diameter shall not =exceed
the diameter of the longitudinal bars in the column by
more than 0.15 in.

Development length of the Reinforcing Bars
The development length for compression bars was
given
but not less than
Dowel bars must be checked for proper
development length.
c
b
y
d /
02
.
0 f
d
f
l 
in.
8
003
.
0 b
y 
d
f

Differential Settlement
Footing usually support the following loads
Dead loads from the substructure and superstructure
Live load resulting from material or occupancy
Weight of material used in backfilling
Wind loads

General Requirements for Footing Design
A site investigation is required to determine the
chemical and physical properties of the soil.
Determine the magnitude and distribution of
loads form the superstructure.
Establish the criteria and the tolerance for the
total and differential settlements of the structure.
1
2
3

Determine the most suitable and economic type
of foundation.
Determine the depth of the footings below the
ground level and the method of excavation.
Establish the allowable bearing pressure to be
used in design.
4
5
6

Determine the pressure distribution beneath the
footing based on its width
Perform a settlement analysis.
7
8

Types of Footings.ppt

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Types of Footings.ppt