12 is 2950 part i 1981 b 2 field determination b 21

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Unformatted text preview: 81 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 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. 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. CALCULATIONS 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 STRUCTURE 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, 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 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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