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Unformatted text preview: 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 ):
The structure ( combined action of superstructure and raft ) may be
considered as flexible ( relative stiffness factor K > 03, see
Appendix C ).
Variation in adjacent column load does not exceed 20 percent of the
22.214.171.124 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
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.
Nom - 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 ot IS : 4561978*,
the foundation being considered as an inverted beam or slab.
*Code of practice for plain and reinforced concrete ( r&f rev&ion).
9 IIS: 2950 ( Part I ) - 1981 APPENDIX A [ CZause 3.1( f) ]
AND POISSON’S RATIO ( p )
A-l. ( Es ) DETERMJNATION (E, ) OF MODULUS OF ELASTICITY A-l.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 func- tion 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
dimcult. 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
TS : 1888-1982”. where
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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.
- Spring '13