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Design of Concrete Structure I

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Title: Design of Concrete Structure I


1

2
Design of Concrete Structure I
University of Palestine
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Chapter 9
Instructor
Eng. Mazen Alshorafa
3
Design of Concrete Structure I
University of Palestine
Footing
Introduction
Footings are structural elements used to support
columns and walls and transmit their loads to the
underlying soil without exceeding its safe
bearing capacity below the structure.
Instructor
Eng. Mazen Alshorafa
4
Design of Concrete Structure I
University of Palestine
Footing
Introduction
The design of footings calls for the combined
efforts of geotechnical and structural engineers.
The geotechnical engineer, on one hand, conducts
the site investigation and on the light of his
findings, recommends the most suitable type of
foundation and the allowable bearing capacity of
the soil at the suggested foundation level. The
structural engineer, on the other hand,
determines the concrete dimensions and
reinforcement details of the approved foundation

Instructor
Eng. Mazen Alshorafa
5
Design of Concrete Structure I
University of Palestine
Types of Footing
Wall Footings
Wall footing are used to support structural walls
that carry loads for other floors or to support
nonstructural walls.
W kN/m
W kN/m
Wall
Secondary reinft
Footing
Main reinft.
Instructor
Eng. Mazen Alshorafa
6
Design of Concrete Structure I
University of Palestine
Types of Footing
Isolated Footings
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.

Instructor
Eng. Mazen Alshorafa
7
Design of Concrete Structure I
University of Palestine
Types of Footing
Combined Footings
Combined footings are used when two columns are
so close that single footings cannot be used or
when one column is located at or near a property
line.
P2
P1
P2 kN
P1 kN
L
B
C2
C2
C1
C1
L2
L1
L2
Instructor
Eng. Mazen Alshorafa
8
Design of Concrete Structure I
University of Palestine
Types of Footing
Continuous Footings
Continuous footings support a row of three or
more columns
P4 kN
P3 kN
P2 kN
L
P1 kN
B
Instructor
Eng. Mazen Alshorafa
9
Design of Concrete Structure I
University of Palestine
Types of Footing
Strap (Cantilever ) footings
Strap footings consists of two separate footings,
one under each column, connected together by a
beam called strap beam. The purpose of the
strap beam is to prevent overturning of the
eccentrically loaded footing.
P2 kN
P2
P1
Strap Beam
property line
P1 kN
L1
L2
C2
B1
B2
C2
C1
C1
Instructor
Eng. Mazen Alshorafa
10
Design of Concrete Structure I
University of Palestine
Types of Footing
Mat (Raft) Footings
Mat Footings consists of one footing usually
placed under the entire building area. They are
used, when soil bearing capacity is low, column
loads are heavy and differential settlement for
single footings are very large.
B
L
Instructor
Eng. Mazen Alshorafa
11
Design of Concrete Structure I
University of Palestine
Types of Footing
Pile caps
Pile caps are thick slabs used to tie a group of
piles together to support and transmit column
loads to the piles.
P
B
L
Instructor
Eng. Mazen Alshorafa
12
Design of Concrete Structure I
University of Palestine
Footing
Distribution of Soil Pressure
The distribution of soil pressure under a footing
is a function of the type of soil, the relative
rigidity of the soil and the footing, and the
depth of foundation at level of contact between
footing and soil For design purposes, it
is common to assume the soil pressures are
linearly distributed. The pressure distribution
will be uniform if the centroid of the footing
coincides with the resultant of the applied loads
Centroidal axis
Footing on sand
Footing on clay
Equivalent uniform distribution
Instructor
Eng. Mazen Alshorafa
13
Design of Concrete Structure I
University of Palestine
Footing
Concentrically loaded Footings
If the resultant of the loads acting at the base
of the footing coincides with the centroid of the
footing area, the footing is concentrically
loaded and a uniform distribution of soil
pressure is assumed in design, as shown in
Figure
Instructor
Eng. Mazen Alshorafa
14
Design of Concrete Structure I
University of Palestine
Footing
Eccentrically Loaded Footings
Footings are often designed for both axial load
and moment. Moment may be caused by lateral
forces due to wind or earthquake, and by lateral
soil pressures. Footing is eccentrically Loaded
if the supported column is not concentric with
the footing area or if the column transmits at
its juncture with the footing not only a vertical
load but also a bending moment.
P
e
Centroidal axis
y
L
P/A
My/I
Instructor
Eng. Mazen Alshorafa
15
Design of Concrete Structure I
University of Palestine
Footing
Pressure Distribution Below Footings
The maximum intensity of loading at the base of a
foundation which causes shear failure of soil is
called ultimate bearing capacity of soil, denoted
by qu. The allowable bearing capacity of soil is
obtained by dividing the ultimate bearing
capacity of soil by a factor of safety on the
order of 2.50 to 3.0. The allowable soil
pressure for soil may be either gross or net
pressure permitted on the soil directly under the
base of the footing. The gross pressure
represents the total stress in the soil
created by all the loads above the base of the
footing. a net soil pressure is used instead of
the gross pressure value
Instructor
Eng. Mazen Alshorafa
16
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 1- Select a
trial footing depth. According to ACI Code, depth
of footing above reinforcement is not to be less
than 15 cm for footings on soil. Noting that 7.5
cm of clear concrete cover is required if
concrete is cast against soil, a practical
minimum depth is taken as 25 cm. 2- Establish
the required base area of the footing. qall (net)
qall (gross) - ?c hc - ?s (Df - hc)
Where hc is assumed footing depth, df is distance
from ground surface to the contact surface
between footing base and soil, ?c is weight
density of concrete, and ?s is weight density of
soil on top of footing.
P
Df
hs
qall(net)
Instructor
Eng. Mazen Alshorafa
17
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. Based on ACI
Code, base area of footing is determined from
unfactored forces transmitted by footing to soil
and the allowable soil pressure evaluated through
principles of soil mechanics. where PD and PL
are column service dead and live loads
respectively. Select appropriate L, and B values,
if possible, use a square footing to achieve
greatest economy. 3- Evaluate the net factored
soil pressure. Evaluate the net factored soil
pressure by dividing the factored column loads by
the chosen footing area, or
Instructor
Eng. Mazen Alshorafa
18
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 4- Check footing
thickness for punching shear. When loads are
applied over small areas to slabs and footings
with no beams, punching failure may occur. The
sloping failure surface takes the shape of a
truncated pyramid in case of rectangular columns,
and a truncated cone in case of circular
columns. The ACI Code assumes that failure
takes place on vertical planes located at
distance d/2 from faces of column.
Instructor
Eng. Mazen Alshorafa
19
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 4- Check footing
thickness for punching shear contd. The depth
of the footing must be set so that the shear
capacity of the concrete equals or exceeds the
critical shear forces produced by factored
loads The critical punching shear force Vu can
be evaluated as follows
Instructor
Eng. Mazen Alshorafa
20
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 4- Check footing
thickness for punching shear contd. Punching
shear force resisted by concrete Vc is given as
the smallest of ßc long side/short side of
column, and as 40 for interior, 30 for side,
and 20 for corner columns. bo length of critical
perimeter around the column 2(C1d)(C2d)
Instructor
Eng. Mazen Alshorafa
21
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 5- Check footing
thickness for beam shear in each direction. If Vu
FVc, thickness will be adequate for resisting
beam shear. The critical section for beam shear
is located at distance d from column faces. In
the short direction The factored shear force is
given by The factored shearing force resisted
by concrete is given as
Instructor
Eng. Mazen Alshorafa
22
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 5- Check footing
thickness for beam shear in each direction
contd. In the long direction The factored
shear force is given by The factored shearing
force resisted by concrete is given as Increase
footing thickness if necessary until the
condition Vu FVc is satisfied.
Instructor
Eng. Mazen Alshorafa
23
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 6-Compute the
area of flexural reinforcement in each
direction. The critical section for bending is
located at face of column, for footings
supporting a concrete column. Reinforcement in
the short direction Reinforcement in the long
direction The reinforcement ratio is
calculated based on rectangular section design,
where the minimum reinforcement ratio ?min is not
to be less than 0.0018.
Instructor
Eng. Mazen Alshorafa
24
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 6-Compute the
area of flexural reinforcement in each
direction. For rectangular footings, the
reinforcement in the long direction is uniformly
distributed while the reinforcement in the short
direction is concentrated in a band centered on
centerline of column and with a width equals to
the short dimension of the footing as shown in
Figure where
Band width
B
B
L
Instructor
Eng. Mazen Alshorafa
25
Design of Concrete Structure I
University of Palestine
Footing
Design of Isolated Footings
Design of isolated rectangular footings is
detailed in the following steps. 7-Check for
anchorage of the reinforcement. 8-Prepare
neat design drawings showing footing dimensions
and provided reinforcement.
Instructor
Eng. Mazen Alshorafa
26
Design of Concrete Structure I
University of Palestine
Footing
Example 1
Design an isolated square footing to support an
interior column 4040cm in cross section and
carries a dead load of 80 tons and a live load of
60 tons. Use fc 250 kg/cm2 and fy 4200
kg/cm2, qall (gross) 20 t/m2, ?soil 1.7
t/m3
PD 80 tons PL 60 tons
Df1.0
40
40
Instructor
Eng. Mazen Alshorafa
27
Design of Concrete Structure I
University of Palestine
Footing
Solution
1- Select a trial footing depth Assume that the
footing is 50 cm thick. 2- Establish the required
base area of the footing Use 280x280x50
cm footing 3- Evaluate the net factored soil
pressure
40
40
280
280
Instructor
Eng. Mazen Alshorafa
28
Design of Concrete Structure I
University of Palestine
Footing
Solution
4- Check footing thickness for punching
shear Increase footing thickness
to 55 cm, and repeat punching shear check.
Instructor
Eng. Mazen Alshorafa
29
Design of Concrete Structure I
University of Palestine
Footing
Solution
4- Check footing thickness for punching
shear i.e. footing thickness is
adequate for resisting punching shear.
Instructor
Eng. Mazen Alshorafa
30
Design of Concrete Structure I
University of Palestine
Footing
Solution
5- Check footing thickness for beam shear in each
direction In short direction Maximum factored
shear force Vu is located at distance d from
faces of column FVc80.77 t gt Vu50.8 t
OK In long direction Same as short
direction FVc80.77 t gt Vu50.8 t OK
280
45.9
280
Instructor
Eng. Mazen Alshorafa
31
Design of Concrete Structure I
University of Palestine
Footing
Solution
6- Compute the area of flexural reinforcement in
each direction
1.20
280
1.20
280
24.49 x 2.8
Instructor
Eng. Mazen Alshorafa
32
Design of Concrete Structure I
University of Palestine
Footing
Solution
7- Check for anchorage of the reinforcement Bottom
longitudinal reinforcement (F14mm) a1.0 for
bottom bars, ß1.0 for uncoated bars a ß
1.0 lt1.7 OK ?0.8 for F14mm, ?1.0
for normal weight concrete C the smallest of
7.50.78.3 cm
280-2(7.5)-1.4/(18)(2)7.32 cm i.e., C is taken
as 7.32 cm Available length 120-7.5112.5 gt
34 cm
Instructor
Eng. Mazen Alshorafa
33
Design of Concrete Structure I
University of Palestine
Footing
Solution
7- Check for anchorage of the reinforcement Dowel
reinforcement (F14mm) Available length
55-7.5-1.4-1.444.7 cm gt 27 cm Column
reinforcement splices
Instructor
Eng. Mazen Alshorafa
34
Design of Concrete Structure I
University of Palestine
Footing
Solution
8- Prepare neat design drawings showing footing
dimensions and provided reinforcement

Instructor
Eng. Mazen Alshorafa
35
Design of Concrete Structure I
University of Palestine
Footing
Example 1
Design an isolated rectangular footing to support
an interior column 4040cm in cross section and
carries a dead load of 80 tons and a live load of
60 tons. Use fc 250 kg/cm2 and fy 4200
kg/cm2, qall (gross) 20 t/m2, ?soil 1.7
t/m3
Instructor
Eng. Mazen Alshorafa
36
Design of Concrete Structure I
University of Palestine
Footing
Solution
1- Select a trial footing depth Assume that the
footing is 55 cm thick. 2- Establish the required
base area of the footing Use 320x245x55
cm footing 3- Evaluate the net factored soil
pressure
Instructor
Eng. Mazen Alshorafa
37
Design of Concrete Structure I
University of Palestine
Footing
Solution
4- Check footing thickness for punching
shear i.e. footing thickness is
adequate for resisting punching shear.
Instructor
Eng. Mazen Alshorafa
38
Design of Concrete Structure I
University of Palestine
Footing
Solution
5- Check footing thickness for beam shear in each
direction In short direction Vu is located at
distance d from faces of column FVc 70.7 t gt
Vu 56.5 t OK In long direction Vu is
located at distance d from faces of
column FVc 92.3 t gt Vu 44.4 t OK
Instructor
Eng. Mazen Alshorafa
39
Design of Concrete Structure I
University of Palestine
Footing
Solution
6- Compute the area of flexural reinforcement in
each direction a- Reinforcement in long
direction The critical section for bending is
shown in Figure
Instructor
Eng. Mazen Alshorafa
40
Design of Concrete Structure I
University of Palestine
Footing
Solution
6- Compute the area of flexural reinforcement in
each direction b- Reinforcement in short
direction The critical section for bending is
shown in Figure
Instructor
Eng. Mazen Alshorafa
41
Design of Concrete Structure I
University of Palestine
Footing
Solution
6- Compute the area of flexural reinforcement in
each direction b- Reinforcement in short
direction The critical section for bending is
shown in Figure
Instructor
Eng. Mazen Alshorafa
42
Design of Concrete Structure I
University of Palestine
Footing
Solution
7- Check for anchorage of the reinforcement Bottom
longitudinal reinforcement (F14mm) in long
direction a1.0 for bottom bars, ß1.0
for uncoated bars a ß 1.0 lt1.7 OK ?0.8 for
F14mm, ?1.0 for normal weight
concrete C the smallest of 7.50.78.3 cm
245-2(7.5)-1.4/(22)(2)5
.2 cm i.e., C is taken as 5.2 cm Available
length 140-7.5132.5 gt 34 cm
Instructor
Eng. Mazen Alshorafa
43
Design of Concrete Structure I
University of Palestine
Footing
Solution
7- Check for anchorage of the reinforcement Dowel
reinforcement (F14mm) Available length
55-7.5-1.4-1.444.7 cm gt 27 cm Column
reinforcement splices
Instructor
Eng. Mazen Alshorafa
44
Design of Concrete Structure I
University of Palestine
Footing
Solution
8- Prepare neat design drawings showing footing
dimensions and provided reinforcement

45cm
55 cm
245 (18F14)
3.20 m
2F14 B
2F14 B
18F14 B
2.45 m
23F14 B
Width band 245
42.5
42.5
Instructor
Eng. Mazen Alshorafa
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