52 flexible foundation 521 simplified method in this

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Unformatted text preview: ntional method are commonly justified. 5.2 Flexible Foundation 5.2.1 Simplified Method — In this method, it is assumed that the subgrade consists of an infinite array of individual elastic springs each of which is not affected by others. The spring constant is equal to the modulus of subgrade reaction ( k ). The contact pressure at any point under the raft is, therefore, linearly proportional to the settlement at the point. This method may be used when the following conditions are satisfied ( see Appendix E ): a) The structure (combined action of superstructure and raft) may be considered as flexible (relative stiffness factor K < 0.5, see Appendix C). b) Variation in adjacent column load does not exceed 20 percent of the higher value. General method — For the general case of a flexible foundation not satisfying the requirements of 5.2.1, the method based on closed form solution of elastic plate theory may be used. This method is based on the theory of plates on winkler foundation which takes into account the restraint on deflection of a point provided by continuity of the foundation in orthogonal foundation. The distribution of deflection and contact pressure on the raft due to a column load is determined by the plate theory. Since the effect of a column load on an elastic foundation is damped out rapidly, it is possible to determine the total effect at a point of all column loads within the zone of influence by the method of super imposition. The computation of the effect at any point may be restricted to columns of two adjoining bays in all directions. The procedure is outlined in Appendix F. NOTE — One of the recent general methods based on the above mentioned theory is numerical analysis by either finite difference method or finite element method. This method is used for accurate analysis of the raft foundation. The details of this method could be covered at a later stage. 6. STRUCTURAL DESIGN 6.1 The general design for loads, shrinkage, creep and temperature effects and provision of reinforcement and detailing shall conform to IS : 456-1978*, the foundation being considered as an inverted beam or slab. *Code of practice for plain and reinforced concrete ( third revision ). 9 IS : 2950 (Part I) - 1981 APPENDIX A [ Clause 3.1(f) ] DETERMINATION OF MODULUS OF ELASTICITY ( Es ) AND POISSON’S RATIO ( µ ) A-1. DETERMINATION OF MODULUS OF ELASTICITY ( Es ) A-1.1 The modulus of elasticity is a function of the composition of the soil, its void ratio, stress history and loading rate. In granular soils it is a function of the depth of the strata, while in cohesive soils it is markedly influenced by the moisture content. Due to its great sensitivity to sampling disturbance accurate evaluation of the modulus in the laboratory is extremely difficult. For general cases, therefore, determination of the modulus may be based on field tests (A-2). Where a properly equipped laboratory and sampling facility are available, Es may be determined in the laboratory ( see A-3 ). A-2. FIELD DETERMINATION A-2.1 The value of Es shall be determined from plate loan test given in IS : 1888-1982*. where q B s µ Iw = = = = = = intensity of contact pressure, least lateral dimension of test plate, settlement, Poisson’s ratio, Influence factor, and 0.82 for a square plate. A-2.1.1 The average value of Es 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. A-2.1.2 Effect of Size — In granular soils, the value of Es corresponding to the size of the raft shall be determined as follows: B f ( B f + B p )2 E s = E p ------ --------------------------2 Bf...
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This note was uploaded on 03/14/2014 for the course CE 684 taught by Professor Prof.deepankarchoudhury during the Spring '13 term at IIT Bombay.

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