Design calculations of raft foundation

Design calculations of raft foundation

1. DESIGN OF RAFT FOUNDATION
Name of Project : Design Example 18.5, Page 561, Reinf concrete A.K.Jain
Given Data:
1Grade of Steel Fe 415
2Grade of Concrete M 15
3Size of Column(X x Y) 300mmX 300mm
4Safe Bearing Capacity of Soil 65.00
5Diameter of Bars(Along X-direction) 20mm
6Diameter of Bars(Along Y-direction) 20mm
1 2 Y 3 4
C C
B B
X
A A
O
1 2 3 4
Details of Columns
Considering column A-1 as origin (O)Moment about X-axisMxclockwise +ve
Moment about Y-axisMyAnticlock -ve
Column X-cord Y-cord Load Moment MyMoment Mx
(in m) (in m) (KN) (KN-m) (KN-m)
A-1 0 0 550 0 0
A-2 7 0 1200 0 0
A-3 14 0 1200 0 0
A-4 21 0 550 0 0
B-1 0 6 600 0 0
B-2 7 6 2000 0 0
B-3 14 6 2000 0 0
B-4 21 6 1200 0 0
C-1 0 12 500 0 0
C-2 7 12 1500 0 0
C-3 14 12 1500 0 0
C-4 21 12 500 0 0
Left 0.3mTop 0.3m
Right 0.3mBottom 0.3m
KN/m
2

Cantilever length from centre line of columns
To Run the Program Press < Ctrl+w >

3. P= 13300 KN
Eccentricity along x-direction
Taking moment of column forces about the grid 1-1
x=10.974 m
= 10.974 - 10.5
= 0.474 m
Eccentricity along y-direction
Taking moment of column forces about the grid A-A
y=6.226 m
= 6.226 - 6
= 0.226 m
= 21.6X 12.63
12
= 3600.68
= 12.6X 21.63
12
= 10581.58
A= 12.6 X 21.6
= 272.16
=3000.00
=6300.00
P/A = 48.87
Soil pressure at different points is as follows
s =
P
+ .x + .y
A
Corner C-4
= 48.87+
6300.00
10.8 +
3000.00
6.3
10581.58 3600.68
= 48.87+ 6.430+5.249
=60.547
Corner A-4
= 48.87+ 6.430 -5.249
=50.049
Corner C-1
= 48.87 -6.430+5.249
=47.687
Total Vetical Column Load
e
x

e
y

I
x

m
4

I
y

m
4

m
2

M
xx
=P.e
y
KNm
M
yy
=P.e
x
KNm
KN/m
2

M
yy
M
xx

I
y
I
x

s
C-4

KN/m
2

s
A-4

KN/m
2

s
C-1

KN/m
2

4. Corner A-1
= 48.87 -6.430 -5.249
=37.189
Grid B-4
= 48.87+ 6.430 -0.000
=55.298
Grid B-1
= 48.87 -6.430 -0.000
=42.438
Maximum Soil Pressure = 60.547 <65.00
Hence OK
In the X-direction, the raft is divided in three strips :-
(i)Strip C-C
Width = 3.3m
Soil Pressure =60.547
Span = 7.00m
Maximum moment = 60.547X
7 2
10
=296.68
(ii)Strip B-B
Width = 6 m
Soil Pressure = 57.923
Span = 7.00m
Maximum moment = 57.923X
7 2
10
=283.82
(iii)Strip A-A
Width = 3.3m
Soil Pressure = 52.674
Span = 7.00m
Maximum moment = 52.674X
7 2
10
=258.10
Cantilever Moment along X-direction
Soil Pressure = 60.547
Span = 0.30m
Maximum moment = 60.547X
0.32
2
=2.72
(iii)Strip4-4
Maximum Soil Pressure = 60.547 <65.00
Span = 6 m
Maximum moment = 60.547X
6 2
8
=272.46
s
A-1

KN/m
2

s
B-4

KN/m
2

s
B-1

KN/m
2

KN/m
2
KN/m
2

KN/m
2

KNm/m
KN/m
2

KNm/m
KN/m
2

KNm/m
KN/m
2

KNm/m
KN/m
2
KN/m
2

KNm/m

5. Cantilever Moment along X-direction
Soil Pressure = 60.547
Span = 0.30m
Maximum moment = 60.547X
0.32
2
=2.72
Therefore,
Maximum Factored Bending Moment = 445.02
Limiting Moment of Resistance= 0.138
Therefore depth required d= 470mm
Check for Punching Shear :
Let depth required= 819mm
Shear Strength of Concrete=
Where = 0.5+
=
Short dimension of column
=1
Long dimension of column
Therefore = 1
= 0.25
= 0.97
Hence,
Shear Strength of Concrete= 0.97
For Corner Column
Perimeter = 859 + 859 + 0 + 0
= 1719 mm
Nominal Shear Stress = =
825000
1407109
= 0.59
For Side Column
Perimeter = 1119 + 859 + 859 + 0
= 2837 mm
Nominal Shear Stress = =
2250000
2322996
= 0.97
- = 0.00
Hence Effective depth is O.K.
Therefore effective depth required= 819 mm
Adopt effective depth = 820 mm
Overall depth = 860 mm
KN/m
2

KNm/m
KNm/m
s
ckbd
2

k
s
t
c

k
s
b
c

b
c

k
s
t
c
Ös
ck
N/mm
2

N/mm
2

N/mm
2

b
o

t
v

V
u

b
o
d
N/mm
2

b
o

t
v

V
u

b
o
d
N/mm
2

t
v
t
c
N/mm
2

6. Development Length :
=
where, 1.6x 1.0= 1.6
Therefore,
= 56f
For20mm bars (along X-direction)
= 1120mm
For20mm bars (along Y-direction)
= 1120mm
Reinforcement in long direction
M =
4.45E+08= 361.05 820 -0.027667
9.98905 - 296061 + 4.45E+08= 0
= 1588
Minimum reinforcement required= 0.12%
= 1032
Therefore area of steel required= 1588
Provide20 200mm C/Cin long direction
(At top and bottom)
Reinforcement in short direction
M =
4.09E+08= 361.05 820 -0.027667
9.98905 - 296061 + 4.09E+08= 0
= 1452
Minimum reinforcement required= 0.12%
= 1032
Therefore area of steel required= 1452
Provide20 220mm C/Cin short direction
(At top and bottom)
Development length L
d
0.87 s
y
f
4 t
bd

t
bd
=
L
d

L
d

L
d

0.87 s
y
A
st
(d - s
y
A
st
/s
ck
b)
A
st
( A
st
)
A
st
2

A
st

A
st
mm
2
/m
mm
2
/m
mm
2
/m
mm f bars @
0.87 s
y
A
st
(d - s
y
A
st
/s
ck
b)
A
st
( A
st
)
A
st
2

A
st

A
st
mm
2
/m
mm
2
/m
mm
2
/m
mm f bars @

7. DRAWING
21.60
1
2
.
6
0
20 200mm c/c
(top & bottom)
20 220mm c/c
(top & bottom)
20 200mm c/c
0
.
8
6
0
20 220mm c/c
Note : (1) All dimension in m where not specified
(2) Drawing not to scale
(Reinforcement Details)
mm f bars@
mm f bars @
mm f bars@
mm f bars@

8. DRAWING
21.60
1
2
.
6
0
20 200mm c/c
(top & bottom)
20 220mm c/c
(top & bottom)
20 200mm c/c
0
.
8
6
0
20 220mm c/c
Note : (1) All dimension in m where not specified
(2) Drawing not to scale
(Reinforcement Details)
mm f bars@
mm f bars @
mm f bars@
mm f bars@