Lecture 8 raft foundation

Foundation Engineering

1. INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
INTEREAOLR UVSEYFYRVC&IHR VCNRUGR VCF8I2R
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303421: Foundation Engineering
Raft Foundation
Dr. Abdulmannan Orabi
Lecture
8

2. Raft Foundation
2
Introduction
A raft foundation is a large concrete slab used to
interface one column, or more than one column in
several lines, with the base soil.
A raft foundation may be used to support one-
grade storage tanks or several pieces of industrial
equipment. Rafts are commonly used beneath soil
clusters chimneys and various tower structure.
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3. Dr. Abdulmannan Orabi IUST
Raft Foundation
Introduction
A raft foundation may be used where the base soil
has a low bearing capacity and/ or the column
loads are so large that more 60 percent of the area
is covered by conventional spread footing.
A practical advantage for mat foundation at or
below the ground water table is to provide a water
barrier.
3

4. Types of Raft Foundation
1) Flat plate
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5. Types of Raft Foundation
2) Plate with thickened under columns
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6. Types of Raft Foundation
3) Plate with pedestal
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7. Types of Raft Foundation
4) Waffle slab
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8. 5)Slab with basement
walls as a part of the
mat. The walls act as
stiffeners for the mat.
Section
Plan
Types of Raft Foundation
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9. The net pressure applied on a foundation may be
expressed as
R
I
N
T E RAOL UVOSYL
FC LYV &FOH
Net Pressure Caused by a Raft Foundation
Definition of net pressure on soil caused by a mat founda tion
G E
8
2
DT I
N r G
.bbdulma
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10. Net Pressure Caused by a Raft Foundation
The net pressure applied on a foundation may be
expressed as
G E
8
2
DT I
N r G
.bbdulma
where 8 EIVni HFni FC LYV &L3R4LR3V nAi LYV HO5V HFni
2E23Vn FC LYV 3nCL CFRAinLOFA
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11. Structural Design of Mat Foundations
The structural design of mat foundations can
be carried out by two conventional methods:
the conventional rigid method and the
approximate flexible method. Finite-difference
and finite-element methods can also be used,
but this section covers only the basic concepts
of the first design method.
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11

12. Conventional Rigid Method
Structural Design of Mat Foundations
The conventional rigid method of mat foundation
design can be explained step by step with reference
to Figure 8.10:
Step 1.
Figure 7 shows mat dimensions of L and B
and column loads of N1 , N2 , N3 , … . Calculate
the total column load as
8 E6
$+#
&+ #
'+⋯ =
)
#
*
u
*+$
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12

13. Structural Design of Mat Foundations
Conventional Rigid Method Step 2.
Determine the
pressure on the soil, q,
below the mat at points
A, B, C, D….by using
the equation
Figure 7
GE
8
2
∓
-
.
/
.0 ∓
-
1
/
1 2
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14. B
L
B
1
B
1
B
1
B
1
B
1
B
1
B
1
N
1
N
2
N
3
N
4
N
5
N
6
N
7
N
8
N
9
N
10
N
11
N
12
e
ye
x
X
y
Y
1
X
1
ABCD
E
F G H I
J
Structural Design of Mat Foundations
GE
8
2
∓
-
.
/
.0 ∓
-
1
/
1 2
UYV3V3
2 E 4 5/
.=
67
8
$&
/
1=
54
'
12 -
.E 8;V
1 nAi -
1E 8;V
.

X1
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15. The load eccentricities, ex and ey , in the x and y
directions can be determined by using x 1, y1
coordinates:
Structural Design of Mat Foundations
Conventional Rigid Method
2 =
∑
#
* 2
*
8
, 2
̅
=2+
?
2
nAi V
.=
4
2
−2
̅
Similarly
0=
∑
#
* 0
*
8
, 0
@
=0+
n
2
nAi V
1=0
@
−
5
2
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16. Step 4.
Divide the mat into several strips in the x
and y directions. (See Figure 7). Let the width of
any strip be B 1.
Structural Design of Mat Foundations
Conventional Rigid Method
Step 3.
Compare the values of the soil pressures
determined in Step 2 with the net allowable soil
pressure to determine whether
G
A..rG
.bbdulma
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17. Structural Design of Mat Foundations
Conventional Rigid Method
Step 5.
Draw the shear, V, and the moment, M,
diagrams for each individual strip (in the x and y
directions).
For example, the average soil pressure of the
bottom strip in the x direction, x1, of Figure 7 is
G
.B
=
G
C
9G
D

2
UYV3V G
C nAi G
DE&FOH E3V&&R3V nL EFOAL& / nAi F
dGLVE :a
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18. The total soil reaction is equal to q avB1B.
Now obtain the total column load on the strip as
N1 + N2 +N3 +N4 .
Structural Design of Mat Foundations
Conventional Rigid Method The sum of the column loads on the strip will not
equal qavB1B, because the shear between the
adjacent strips has not been taken into account.
4
$G
.B
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19. Structural Design of Mat Foundations
Conventional Rigid Method For this reason, the soil reaction and the column
loads need to be adjusted, or
25V3nSV HFni E
44
$G
.B+ #
$+#
&+#
'+#
H
2
G
.BdAIJ/N/lJaE G
.B
25V3nSV HFni
44
$G
.B
Now, the modified average soil reaction becomes
and the column load modification factor is
F =
25V3nSV 5Fni
#
$+#
&+#
'+#
H
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20. Structural Design of Mat Foundations
Conventional Rigid Method
So the modified column loads are FN 1 , FN2
FN3 , and FN4 . This modified loading on the
strip under consideration is shown in Figure below.
FN
1FN
2
FN
3FN
4
B
IHG F
4
$G
.BdAIJ/N/lJa
RAOL HVASLY
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21. Dr. Abdulmannan Orabi IUST
Structural Design of Mat Foundations
Conventional Rigid Method
The shear and the moment diagram for the strip can
now be drawn, and the procedure is repeated in the x
and y directions for all strips.
Step 6.
Determine the effective depth d of the mat
by checking for diagonal tension shear near various
columns. (For punching shear).
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22. Dr. Abdulmannan Orabi IUST
iK:
iK:
iK:
n 9 iK:
? 9 i
?
LE :n 9 ? 9 :i
MiSV FC NnL
?
LE :9 ? 9 i
iK:
iK:
n 9 iK:
? 9 i/2
OF3AV3 FC NnL
Structural Design of Mat Foundations
Conventional Rigid Method
The critical sections for punching shear are
iK:
iK:
iK:
n 9 i
? 9 i
?
LE :n 9 :? 9 Pi
/ALV3AnH OFHRNA
iK:
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23. Structural Design of Mat Foundations
Conventional Rigid Method
Step 7.
From the moment diagrams of all strips in
one direction (x or y), obtain the maximum positive
and negative moments per unit width (i.e.,Mu
=M/B1). Since factored column loads are used in
accordance with ACI Code 318-14 (see Step 6), Mu is
the factored moment.
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24. Step 8.
Determine the area of steel per unit width for
positive and negative reinforcement in the x and y
directions.
Structural Design of Mat Foundations
Conventional Rigid Method
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25. The system of the beam slab raft foundation is
exactly the same as an inverted beam -slab roof.
The problem is to find out the raft dimensions
and the pressure distribution under the raft then
the design should follows the same way as done in
a simple beam-slab roof.
Design of Beam-Slab Raft Foundation

26. The system of beam-slab roof consists of slab,
main beams in short and long directions, and
secondary beams may also be used.
Design of Beam-Slab Raft Foundation
Bending moment and shear force diagrams for
each slabs and beams in the short direction.

27. Design slabs, beams in the other directions.
Pressure under the raft might not be uniform .
Consider the average uniform value of pressure
act on each continuous slab.
Design of Beam-Slab Raft Foundation Check the shear stress. Depth of slab should
be constant but the steel may vary. Design
each beam.