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uses and proportioning-- floating foundation.

**FOOTINGS AND RAFTS**

P_{1}=800kN

P_{2}
=1000kN

q_{o} =20
t/m^{2},M15, f_{y}=415kN/m^{2}

Fig. 4.51 Loading
on combined footing

Column size:
400x400mm.

See Fig 4.54 for
details of footing. Column design

Let pt=0.8%

A_{x}
=.008A; A_{x}=0.992A

Clause.39.3 of IS 456-2000

A=146763.8mm^{2}

A_{x}
=1174.11 mm^{2}, =145589.746mm^{2}

Provide footing of
400x400size for both columns.

Using 8-16 j as main
reinforcement and 8 j @250c/c as lateral tie

Design of Footing

Fig. 4.52 Forces
acting on the footing

Resultant of Column
Load

R =1800 kN acting
3.08m from the boundary.

Area of the footing
:

Taking length L=6m,
Depth of footing D_{f}=0.9m,

Width of footing, =1.549m.

Therefore, provide
footing of dimension 6m x 1.6m

Soil Pressure q = 180/
6 x 1.6 =18.75 t/m^{2}< 20 t/m^{2} OK.

q_{u}=28.125
t/m^{2}

Soil pressure
intensity acting along the length =B x
=1.6x28.125 =45t/m.

R_{B}
=119.88kN, R_{C} =150.12kN.

**Thickness of Footing**

i. Wide beam shear:

Maximum shear force
is on footing C,SF=115.02KN

for percentage reinforcement P_{t}
=0.2%

0.32 x d x 1.6=45
[2.556-0.2-d]

d=1.1m

for percentage reinforcement =0.6% 0.6 x d x 1.6=45 [2.556-0.2-d]
d=0.847m.D=900mm.OK.

ii.Two way Shear **Thickness of Footing**

i. Wide beam shear:

Maximum shear force
is on footing C,SF=115.02KN

for percentage
reinforcement P_{t}=0.2%

0.32 x d x 1.6=45
[2.556-0.2-d]

d=1.1m

for percentage reinforcement P_{t}
=0.6%

0.6 x d x 1.6=45
[2.556-0.2-d]

d=0.847m.D=900mm.OK.

ii.Two way Shear

Column B

d=0.415m.

Column A

2d[(0.4+d)+(0.42+d/2)]
x 96.8=120-28.125[(0.4+d)(0.42+d/2)] d=0.3906m

d_{reqd}
=0.85mm

D_{provided}
=900mm, d_{reqd} =850mm.OK.

** Flexural reinforcement**

Along Length
Direction

_{u}/bd^{2}=1.15N/mm^{2}

P_{t} =0.354%

P_{tprovided}=0.6%

A_{strequired}=5100
mm^{2}/mm

Provide 28 j @120mmc/c at top
and bottom of the footing

Along width
direction

Raft Footing Design the raft footing for the
given loads on the columns and spacing between the columns as shown below.

Fig 4.57 column
locations and intensity of loads acting on the raft

**a) Column sizes**

Take size of the columns are as: 300*450 mm
for load of less than 115 ton

450*450 mm for a
load of greater than 115 ton

**Thickness of raft**

Two way shear

The shear should be
checked for every column, but in this case because of symmetry property
checking for 115 t, 150 t, and 55 t is enough.

For 150 t column

Fig 4.58 section
for two way shear for 150 t column

IS: 456-1978,

b_{c}=450/450=1.0

4(0.45+d)*d*96.8=150*1.5-5.607(0.45+d)2

Therefore d=0.562 m

For 115 t column

Fig 4.59 section
for two way shear for 115 t column

2(0.45+d+0.15+0.3+d/2)
d*96.8=115*1.5-5.607(0.45+d)(0.3+0.15+0.5d)

Therefore d=0.519 m

For 55 t column

Fig 4.60 section
for two way shear for 55 t column

2(0.45+0.075+0.5d+0.15+0.3+0.5d)
d*96.8=55*1.5-5.607(0.45+0.5d+0.075)(0.3+0.5d+0.15)

Therefore d=0.32 m

The guiding
thickness is 0.562m and code says that the minimum thickness should not be less
than 1.0m.

let provide a
overall depth of 1.1m=D

d_{pdv}
=1100-75-20/2=1015mm.

**To calculate k
& l ****Stiffness factors**

There are two
criterions for checking the rigidity of the footing:

Plate size used is
300*300 mm.

For clays: Mus=0.5,

If column spacing
is less than 1.75/ l, then the footing is said to
be rigid.

Therefore the given
footing is rigid.

One criterion
showing the footing is flexible and another showing that the given footing is
rigid. Both are contradicting each other, so design the footing for both
criterions.

**Reinforcement in width direction**

From SP-16 graphs

P_{t}
=0.102%, but minimum is 0.12%.

A_{dx} =(0.12*1000*1015)/100=1218
mm^{2}

Provide 20 mm
diameter bars @250 c/c along shorter direction in bottom.

**Reinforcement in length direction **

Provide 20 mm
diameter bars @250 c/c in longer direction.

Clause 33.3.1

Provide 20 mm
diameter bars @ 200 c/c in central band and 20 mm diameter bars @300 c/c at
other parts along shorter direction at bottom.

**Shear (wide beam shear criterion)**

In width direction

=0.235 N/mm^{2}
< r(0.27 N/mm^{2})

Therefore no shear
reinforcement is required.

**Along the width
direction**

Fig. 4.63 Shear
Force and Bending Moment Diagrams of strips 1 and 4

In width direction:
Strip1/4:- =141.2tm

Strip2/3

Fig. 4.64 Shear
Force and Bending Moment Diagrams of strips 2 and 3

Strip 2/3

M_{u}
=282.36tm

M/bd^{2 }= 0.364N/mm^{2}

Minimum P_{t}=0.12%has to be provided.

Provide 20 j @200c/c in centre
band and 20 j @300c/c at other parts along the shorter direction.

**1. Shear check**

Along width
direction:-

For strip1/4:

V_{a}=76.35t

For strip 2/3:

V_{a}=159.14
t

Hence no shear
reinforcement is required.

**Development Length**

At the ends, length
of bar provided=150mm.

Extra length to be
provided=1128.3-150-8x20=818.3mm.

Provide a
Development length of 850mm

**3. Transfer of load at the base of the
column:-**

For end column;

A1=2650X2725=7.22125x106mm^{2}

A2=300x450=135000mm^{2 }

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Civil : Foundation Engineering : Footings And Rafts : Footings And Rafts |

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