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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.
5.2.1.1 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 : 4561978*, 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 )
A1. DETERMINATION OF MODULUS OF ELASTICITY ( Es )
A1.1 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 (A2). Where
a properly equipped laboratory and sampling facility are available, Es
may be determined in the laboratory ( see A3 ).
A2. FIELD DETERMINATION
A2.1 The value of Es shall be determined from plate loan test given in
IS : 18881982*. 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. A2.1.1 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.
A2.1.2 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.
 Spring '13
 PROF.DEEPANKARCHOUDHURY

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