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B2. FIELD DETERMINATION
B2.1 In cases where the depth of the soil affected by the width of the
footing may be considered as isotropic, the value of k may be
determined in accordance with IS : 92141979*. The test shall be
carried out with a plate of size not less than 30 cm.
B2.2 The average value of k shall be based on a number of plate load
tests carried out over the area, the number and location of the tests
depending upon the extent and importance of the structure.
B3. LABORATORY DETERMINATION
B3.1 For stratified deposits or deposits with lenses of different
materials, evaluation of k from plate load test will be unrealistic and
its determination shall be based on laboratory tests [ see IS : 2720
(Part XI)1971† and IS : 2720 (Part XII)1981‡ ].
B3.2 In carrying out the test, the continuing cell pressure may be so
selected as to be representative of the depth of average stress influence
zone ( about 0.5 B to B ).
B3.3 The value of k shall be determined from the following
relationship: where
Es
E
µ
I =
=
=
= Modulus of elasticity of soil ( see Appendix A ),
Young’s modulus of foundation material,
Poisson’s ratio of soil ( see Appendix A ), and
Moment of inertia of structure if determined or of the
foundation. B3.4 In the absence of laboratory test data, appropriate values of Es
and µ may be determined in accordance with Appendix A and used
in B3.2 for evaluation of k.
*Method of determination of subgrade reaction ( k value ) of soils in the field.
†Methods of test for soils: Part XI Determination of shear strength parameters of
specimen tested in unconsolidated undrained triaxial compression without the
measurement of pore water pressure.
‡Methods of test for soils: Part XII Determination of shear strength parameters of
soil from consolidated undrained triaxial compression test with measurement of pore
water pressure ( first revision ). 13 IS : 2950 (Part I)  1981
B4. CALCULATIONS
B4.1 When the structure is rigid ( see Appendix C ), the average
modulus of subgrade reaction may also be determined as follows:
Average contact pressure
ks = Average settlement of the raft APPENDIX C
( Clauses 5.1.1, 5.2.1 and B4.1 )
RIGIDITY OF SUPERSTRUCTURE AND FOUNDATION
C1. DETERMINATION OF THE RIGIDITY OF THE
STRUCTURE
C1.1 The flexural rigidity EI of the structure of any section may be
estimated according to the relation given below ( see also Fig. 2 ): where
El = modulus of elasticity of the infilling material (wall
material) in kg/cm2,
Il = moment of inertia of the infilling in cm4, b = length or breadth of the structure in the direction of
bending, H = total height of the infilling in cm,
E2 = modulus of elasticity of frame material in kg/cm2,
Ib = moment of inertia of the beam in cm4,
Iu
I´u =  ,
hu
Il
I´l =  ,
h1
14 IS : 2950 (Part I)  1981
Ib
I´b =  ,
l
= spacing of the columns in cm, l hu = length of the upper column in cm,
hl = length of the lower column in cm,
If
I´f =  ,
l
Iu = moment of inertia of the upper column in cm4,
Il = moment of inertia of the lower column in cm4, and If = moment of inertia of the foundation beam or raft in cm4. NOTE — The summation is to be done over all the storeys, including the foundation
beam of raft. In the case of the foundation, I´f replaces I´b and Il becomes zero,
whereas for the topmost beam, I´u becomes zero. FIG. 2 DETERMINATION OF RIGIDITY OF A STRUCTURE C2. RELATIVE STIFFNESS FACTOR K
C2.1 Whether a structure behaves as rigid or flexible depends on the
relative stiffness of the structure and the foundation soil. This relati...
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 Spring '13
 PROF.DEEPANKARCHOUDHURY

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