The design must be based on the worst conditions 454

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Unformatted text preview: lidation process. The design must be based on the worst conditions. 4.5.4 Rigidity of the Foundation — Rigidity of the foundation tends to iron out uneven deformations and thereby modifies the contact pressure distribution. High order of rigidity is characterized by large moments and relatively small, uniform settlements. A rigid foundation may also generate high secondary stresses in structural members. The effects of rigidity shall be taken into account in the analysis. 4.5.5 Rigidity of the Superstructure — Free response of the foundations to soil deformation is restricted by the rigidity of the superstructure. In the extreme case, a stiff structure may force a flexible foundation to behave as rigid. This aspect shall be considered to evaluate the validity of the contact pressure distribution. 4.6 Heavy Vibratory Loads — Foundations subjected to heavy vibratory loads should preferably be isolated. 7 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. 5.1.1 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/λ ( 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 8 IS : 2950 (Part I) - 1981 known contact pressures. 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. NOTE — On soft soils, for example, normally consolidated clays, peat, muck, organic silts, etc, the assumptions involved in the conve...
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