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B-2. FIELD DETERMINATION
B-2.1 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 : 9214-1979*. The test shall be
carried out with a plate of size not less than 30 cm.
B-2.2 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.
B-3. LABORATORY DETERMINATION
B-3.1 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‡ ].
B-3.2 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 ).
B-3.3 The value of k shall be determined from the following
= 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. B-3.4 In the absence of laboratory test data, appropriate values of Es
and µ may be determined in accordance with Appendix A and used
in B-3.2 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
B-4.1 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 B-4.1 )
RIGIDITY OF SUPERSTRUCTURE AND FOUNDATION
C-1. DETERMINATION OF THE RIGIDITY OF THE
C-1.1 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,
I´u = ------ ,
I´l = ----- ,
14 IS : 2950 (Part I) - 1981
I´b = ---- ,
= spacing of the columns in cm, l hu = length of the upper column in cm,
hl = length of the lower column in cm,
I´f = --- ,
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 C-2. RELATIVE STIFFNESS FACTOR K
C-2.1 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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