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:
1 Grade of Steel Fe 415
2 Grade of Concrete M 15
3 Size of Column (X x Y) 300 mm X 300 mm
4 Safe Bearing Capacity of Soil 65.00
5 Diameter of Bars (Along X-direction) 20 mm
6 Diameter of Bars (Along Y-direction) 20 mm
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-axis Mx clockwise +ve
Moment about Y-axis My Anticlock -ve
Column X-cord Y-cord Load Moment My Moment 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 0.3 m Top 0.3 m
Right 0.3 m Bottom 0.3 m
KN/m2
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
= 21.6 X 12.6 3
12
= 3600.68
= 12.6 X 21.6 3
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
ex
ey
Ix
m4
Iy
m4
m2
Mxx
= P.ey KNm
Myy
= P.ex KNm
KN/m2
Myy
Mxx
Iy
Ix
sC-4
KN/m2
sA-4
KN/m2
sC-1
KN/m2

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 = 3.3 m
Soil Pressure = 60.547
Span = 7.00 m
Maximum moment = 60.547 X
7 2
10
= 296.68
(ii) Strip B-B
Width = 6 m
Soil Pressure = 57.923
Span = 7.00 m
Maximum moment = 57.923 X
7 2
10
= 283.82
(iii) Strip A-A
Width = 3.3 m
Soil Pressure = 52.674
Span = 7.00 m
Maximum moment = 52.674 X
7 2
10
= 258.10
Cantilever Moment along X-direction
Soil Pressure = 60.547
Span = 0.30 m
Maximum moment = 60.547 X
0.3 2
2
= 2.72
(iii) Strip 4-4
Maximum Soil Pressure = 60.547 < 65.00
Span = 6 m
Maximum moment = 60.547 X
6 2
8
= 272.46
sA-1
KN/m2
sB-4
KN/m2
sB-1
KN/m2
KN/m2
KN/m2
KN/m2
KNm/m
KN/m2
KNm/m
KN/m2
KNm/m
KN/m2
KNm/m
KN/m2
KN/m2
KNm/m

5. Cantilever Moment along X-direction
Soil Pressure = 60.547
Span = 0.30 m
Maximum moment = 60.547 X
0.3 2
2
= 2.72
Therefore,
Maximum Factored Bending Moment = 445.02
Limiting Moment of Resistance = 0.138
Therefore depth required d = 470 mm
Check for Punching Shear :
Let depth required = 819 mm
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/m2
KNm/m
KNm/m
sck bd2
ks
tc
ks
bc
bc
ks
tc
Ösck N/mm2
N/mm2
N/mm2
bo
tv
Vu
bo
d
N/mm2
bo
tv
Vu
bo
d
N/mm2
tv
tc N/mm2

6. Development Length :
=
where, 1.6 x 1.0 = 1.6
Therefore,
= 56 f
For 20 mm bars (along X-direction)
= 1120 mm
For 20 mm bars (along Y-direction)
= 1120 mm
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
Provide 20 200 mm C/C in 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
Provide 20 220 mm C/C in short direction
(At top and bottom)
Development length Ld
0.87 sy
f
4 tbd
tbd
=
Ld
Ld
Ld
0.87 sy
Ast
(d - sy
Ast
/sck
b)
Ast
( Ast
)
Ast
2
Ast
Ast mm2
/m
mm2
/m
mm2
/m
mm f bars @
0.87 sy
Ast
(d - sy
Ast
/sck
b)
Ast
( Ast
)
Ast
2
Ast
Ast mm2
/m
mm2
/m
mm2
/m
mm f bars @

7. DRAWING
21.60
12.60
20 200 mm c/c
(top & bottom)
20 220 mm c/c
(top & bottom)
20 200 mm c/c
0.860
20 220 mm 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
12.60
20 200 mm c/c
(top & bottom)
20 220 mm c/c
(top & bottom)
20 200 mm c/c
0.860
20 220 mm 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@