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Unformatted text preview: ed to evaluate the validity of the contact
pressure distribution.
4.6 Heavy Vibratory Loads  Foundations
loads should preferably be isolated.
7 subjected to heavy vibratory IS : 2950 ( Part I )  1981
4.7 Expansion Joints  In case the structure supported by the raft consists
of several parts with varying heights and loads, it is advisable to provide
expansion joints between these parts. Joints may also be provided wherever
there is a change in the direction of the raft.
5. METHODS OF ANALYSIS 5.0 The essential task in the analysis of a raft foundation is the determination of the distribution of contact pressure underneath the raft which is a
complex function of the rigidity of the superstructure, raft itself and the
supporting soil, and cannot except in very simple cases, be determined with
exactitude. This necessitates a number of simplifying assumptions to make
the problem amenable to analysis. Once the distribution of contact pressure
is determined, design bending moments and shears can be computed based
on statics. The following methods of analysis are suggested which are
distinguished by the assumptions involved. Choice of a particular method
should be governed by the validity of the assumptions in the particular case.
5.1 Rigid Foundation ( Conventional Method )  This is based on the
assumptions of linear distribution of contact pressure. The basic assumptions of this method are:
a) The foundation is rigid relative to the supporting soil and the compressible soil layer is relatively shallow.
b) The contact pressure variation is assumed as planar, such that the
centroid of the contact pressure coincides with the line of action of
the resultant force of all loads acting on the foundation.
5.1.S This method may be used when either of the following conditions is
satisfied:
a) The structure behaves as rigid ( due to the combined action of the
superstructure and the foundation ) with a relative stiffness factor
K > 0.5 ( for evaluation of K, see Appendix C ).
b) The column spacing is less than 1*75/X( see Appendix C ).
5.1.2 The raft is analysed as a whole in each of the two perpendicular
directions. The contact pressure distribution is determined by the procedure
outlined in Appendix D. Further analysis is also based on statics.
5.1.3 In cases of uniform conditions when the variations in adjacent
column loads and column spacings do not exceed 20 percent of the higher
value. the raft may be divided into perpendicular strips of widths equal to
the distance between midspans and each strip may be analysed as an independent beam with known column loads and known contact pressures.
8 IS : 2950 ( Part I )  1981
Such beams will not normally satisfy statics due to shear transfer between
adjacent strips and the design may be based on suitable moment coefficients, or on moment distribution.
Nore  On soft soils, for example, normally consolidated clays, peat, muck, organic
silts, etc. the assumptions involved in the conventional method are commonly justilied. 5.2 Flexible Foundation
53.1 Simplijed Method ...
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 Spring '13
 PROF.DEEPANKARCHOUDHURY

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