lecture 8 raft foundation

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


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

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

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


� = �� ∓-.

/. 0 ∓ -1/1 2

ℎ� �:� = 45 /. = 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 ��. =42 − 2̅Similarly

0 = ∑#*0*� , 0@ = 0 + �

2 ��1 = 0@ − 52


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��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 .



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$��B +#$ + #& + #' + #H2

��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).

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�/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

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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.