Lecture 8 raft foundation

INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
‫وا‬ ‫م‬ ‫ا‬ ‫و‬ ‫ا‬ ‫ا‬
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303421: Foundation Engineering
Raft Foundation
Dr. Abdulmannan Orabi
Lecture
8

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.
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

Types of Raft Foundation
1) Flat plate
Dr. Abdulmannan Orabi IUST 4

2) Plate with thickened under columns

3) Plate with pedestal

4) Waffle slab

5)Slab with basement
walls as a part of the
mat. The walls act as
stiffeners for the mat.
Section
Plan

The net pressure applied on a foundation may be
expressed as
R
= ℎ
ℎ
Net Pressure Caused by a Raft Foundation
Definition of net pressure on soil caused by a mat foundation
= − ≤ ( )

Net Pressure Caused by a Raft Foundation
The net pressure applied on a foundation may be
expressed as
= − ≤ ( )
where
= ℎ ! ℎ "
= ℎ

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.

Conventional Rigid Method
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
= #$ +#& + #' + ⋯ = ) #*
*+$

Step 2. Determine the
pressure on the soil, q,
below the mat at points
A, B, C, D…. by using
the equation
Figure 7
= ∓
-.
/.
0 ∓
-1
/1
2

B
L
B1
B1
B1
B1B1
B1B1
N1 N2 N3 N4
N5 N6 N7
N8
N9 N10 N11 N12
ey
ex
X
yY1
X1
A B C D
E
FGHI
J
= ∓
-.
/.
0 ∓
-1
/1
2
ℎ :
= 4 5 /. =
678
$&
/1 =
54'
12
-. = ∗ 1 -1 = ∗ .
X1

The load eccentricities, ex and ey , in the x and y
directions can be determined by using x1, y1
coordinates:
2 =
∑ #* 2*
, 2̅ = 2 +
?
2
. =
4
2
− 2̅
Similarly
0 =
∑ #* 0*
, 0@ = 0 +
2
1 = 0@ −
5
2

Step 4. Divide the mat into several strips in the x
and y directions. (See Figure 7). Let the width of
any strip be B1 .
Step 3. Compare the values of the soil pressures
determined in Step 2 with the net allowable soil
pressure to determine whether A . ≤ ( )

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
B =
C + D
2
ℎ C D = E E / F (G E 2)

The total soil reaction is equal to qavB1B.
Now obtain the total column load on the strip as
N1 + N2 +N3 +N4 .
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$ B

For this reason, the soil reaction and the column
loads need to be adjusted, or
" =
44$ B + #$ + #& + #' + #H
2
B(AIJ* * J) = B
"
44$ B
Now, the modified average soil reaction becomes
and the column load modification factor is
F =
" 5
#$ + #& + #' + #H

So the modified column loads are FN1 , FN2
FN3 , and FN4 . This modified loading on the
strip under consideration is shown in Figure below.
FN1 FN2
FN3 FN4
B
I H G F
4$ B(AIJ* * J)
ℎ

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).
21

/2
/2
/2
+ /2
?+
?L = 2 + ? + 2
M N
?L = 2 + ? +
/2
/2
+ /2
?+/2
O N
The critical sections for punching shear are
/2
/2
/2
+
?+
?L = 2 + 2? + 4
/ O N
/2
22

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.

Step 8. Determine the area of steel per unit width for
positive and negative reinforcement in the x and y
directions.

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

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

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.
Check the shear stress. Depth of slab should
be constant but the steel may vary. Design
each beam.

Lecture 8 raft foundation

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