lecture 8 raft foundation
TRANSCRIPT
INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
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CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303421: Foundation Engineering
Raft Foundation
Dr. Abdulmannan Orabi
Lecture
8
Raft Foundation
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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
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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.
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Types of Raft Foundation
1) Flat plate
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Types of Raft Foundation
2) Plate with thickened under columns
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Types of Raft Foundation
3) Plate with pedestal
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Types of Raft Foundation
4) Waffle slab
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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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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
� = �� − ��� ≤ ����(���)
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Net Pressure Caused by a Raft Foundation
The net pressure applied on a foundation may be expressed as
� = �� − ��� ≤ ����(���)
where
� = �����������ℎ��� �!�� �����ℎ���"������ = � �����ℎ� �������������
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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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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
� = #$ +#& +#' +⋯= )#*�
*+$
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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
� = �� ∓-.
/. 0 ∓ -1/1 2
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B
LB1
B1
B1
B1B1B1B1
N1 N2 N3 N4
N5 N6 N7 N8
N9 N10 N11 N12
ey
ex
X
yY1
X1
A B C D
E
FGHI
J
Structural Design of Mat Foundations
� = �� ∓-.
/. 0 ∓ -1/1 2
ℎ� �:� = 45 /. = 678
$&/1 = 54'
12-. = � ∗ �1���-1 = � ∗ �.
X1
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The load eccentricities, ex and ey , in the x and y directions can be determined by using x1, y1
coordinates:
Structural Design of Mat Foundations
Conventional Rigid Method
2 = ∑#*2*� , 2̅ = 2 + ?
2 ����. =42 − 2̅Similarly
0 = ∑#*0*� , 0@ = 0 + �
2 ����1 = 0@ − 52
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Step 4. Divide the mat into several strips in the x and y directions. (See Figure 7). Let the width of any strip be B1 .
Structural Design of Mat Foundations
Conventional Rigid Method
Step 3. Compare the values of the soil pressures determined in Step 2 with the net allowable soil pressure to determine whether �A�. ≤ ����(���)
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Structural Design of Mat Foundations
Conventional Rigid Method
Step 5. Draw the shear, V, and the moment, M, diagrams for each individual strip (in the x and y directions). For example, the average soil pressure of the bottom strip in the x direction, x1, of Figure 7 is
��B = �C + �D 2
ℎ� ��C����D = ����E ���� ���E�����/���F(G��E2)Dr. Abdulmannan Orabi IUST 17
The total soil reaction is equal to qavB1B. Now obtain the total column load on the strip as N1 + N2 +N3 +N4 .
Structural Design of Mat Foundations
Conventional Rigid Method
The sum of the column loads on the strip will not equal qavB1B, because the shear between the adjacent strips has not been taken into account.
4$��B
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Structural Design of Mat Foundations
Conventional Rigid Method
For this reason, the soil reaction and the column loads need to be adjusted, or
�"� ������� = 44$��B +#$ + #& + #' + #H2
��B(AIJ*�*�J) = ��B�"� �������
44$��B
Now, the modified average soil reaction becomes
and the column load modification factor is
F = �"� ���5���#$ + #& + #' + #H
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Structural Design of Mat Foundations
Conventional Rigid Method
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
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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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�/2
�/2�/2
� + �/2
?+�
?L = 2� + ? + 2�
M�����N��
?L = 2 + ? + �
�/2�/2
� + �/2
?+�/2
O� �� ��N��
Structural Design of Mat Foundations
Conventional Rigid Method
The critical sections for punching shear are
�/2
�/2�/2
� + �
?+�
?L = 2� + 2? + 4�
/��� ���O���N�
�/2
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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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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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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.
Design of Beam-Slab Raft Foundation
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.
Design of Beam-Slab Raft Foundation
Check the shear stress. Depth of slab should
be constant but the steel may vary. Design
each beam.