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Crack Width

The document provides information about soil shear strength, modulus of subgrade reaction, and rigidity of members resting on soil for cohesive and cohesionless soils. It includes equations to calculate the shear strength, unconfined compressive strength, and modulus of subgrade reaction based on parameters like number of blows from standard penetration test and modulus of elasticity. It also provides criteria to determine if a member resting on soil is rigid, intermediate, or flexible based on the relationship between rigidity parameter and footing width.

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Wasim Kazi
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571 views3 pages

Crack Width

The document provides information about soil shear strength, modulus of subgrade reaction, and rigidity of members resting on soil for cohesive and cohesionless soils. It includes equations to calculate the shear strength, unconfined compressive strength, and modulus of subgrade reaction based on parameters like number of blows from standard penetration test and modulus of elasticity. It also provides criteria to determine if a member resting on soil is rigid, intermediate, or flexible based on the relationship between rigidity parameter and footing width.

Uploaded by

Wasim Kazi
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as XLS, PDF, TXT or read online on Scribd
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Basic data regarding soil shear strength

a. For cohesive soils.

qu = N / 7.50 kg/cm2

c = qu / 2 kg/cm2
or
c = N / 15 kg/cm2

Where
c = Shear strength of soil
N = No of blows from SPT test
qu = Unconfined compressive strength

b. For cohesionless soils .

For cohesionless soil corrections in N values shall be use


as per graph given in IS:2131-1981

Modulus of subgrade reaction (ks) in kn/m3

ksf = 0.65 x Es Es x B4 .. Vesic equation


1- M2 Eb x I

Where
Eb = Modulus of elasticity of footing material in kn/cum
Es = Modulus of elsticity of soil in kn/cum
I = Moment of inertia of footing in m4
M = Poisson's ratio for soil

Rigidity of members resting on soil

a. Member is rigid if, L < / 4


b. Member is intermidiate if , / 4 < L<
c. Member is flexible if , L<

1/4
where , = ksf x B
4Eb x I

B = Width of footing
Data
Width of section = 3000
Depth of section = 500
Total design moment = 30.7
Grade of concrete = 25
Grade of reinforcement steel = 415
Permissible stress in concrete, bending compression = 8.5
Clear cover at tension face = 50
Clear cover at compression face = 50
Effective depth for tension side = 442
Effective depth for compression face = 442

Reinforcement calculation
Dia-1 No Dia-2 No Area of steel
Compression 16 4 12 2 1029.92
Tension 16 15 12 0 3014.4

Section parameters
Modular ratio (m) m = 280
3 cbc
= 18.667

Modulus of elasticity of steel Es = 200000

Compression face
58 50

500

192.27
acr
58 50
Tension face
3000

Depth of neutral axis (under reinforced section)


1/2

Neutral axis x = (m x Ast) 2


+ 2xmxAstxd - m x Ast
(b)2 b b

1/2

= 351.7975 + 16580.5397 - 18.756


= 130.1243 - 18.756
= 111.3680 mm

Lever arm z = d- x/3


= 404.88 mm

Stress in steel at level of reinforcement bar

st = M
z x Ast
= 251.544 N/mm2

Strain in steel
s = st
Es
= 0.001257720

Strain in steel,calculated ignoring the stiffness of concrete in tension zone

1 = D-x x s
d-x

= 0.0014783516

Average strain at the level where the cracking is being considered

m = 1 - b (D-x) 2
3x EsxAstx(d-x)

= 0.0014783516 - 0.00075771
= 0.0007206446

Minimum cover to tension steel

Cm = 50

Distance from the point considered to the surface of the nearest longitudinal bar

acr = (d1)2 +
(d2)2 -
8

= 36966.471111 +
2500 - 8

= 190.66170016 mm

Design surface crack width


3 x acr x m
W max = 1+ 2(acr-cm)
(h-x)

= 0.4121979618
1.7238812941
= 0.2391104093 mm safe

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