Raft Foundation.ppt 2z3m6c

Raft Foundation

Introduction A raft or mat foundation is a large concrete slab used to interface columns in several lines with the base soil. It may occupy the entire foundation area or only a part of it. A raft foundation may be used where the base soil has a low bearing capacity and/or the column loads are so large that more than 50 percent of the area is covered by conventional spread footings

Method of Analysis of Raft Rigid beam analysis.

Non-Rigid or Elastic method

On what basis method of analysis are done: A simple parameter used to test the behavior of a raft as rigid or flexible is the parameter , called “characteristic length parameter”. In general the rigid analysis gives us more conservative values of bending moment than flexible analysis. Thus., rigid analysis results in bigger sections and more steel.

Rigid Beam Analysis It can be used when settlements are small.  Assumptions.  Raft is infinitely rigid with negligible flexural deflection.  Soil bearing pressure to be uniform or varying linearly.

 Limitation It doesn’t model mat foundation . Mat foundation are not truly rigid, so the settlement beneath the column will be greater than that beneath unloaded areas. These differential settlements will cause variations in the soil bearing pressure and corresponding changes in the mat.

Non–Rigid/ Elastic method This method involves : • Plates or beams on elastic foundation. • Plates or beams on elastic half space. • Discrete element methods.(where mat is treated on the basis of elemental analysis)

Plates or beams on elastic foundation Two approaches are popularly used:

 By considering whole raft as a slab or beam, which is then modelled for analysis by computer programs.(SAP computer program).  The raft is cut into a series of structural elements (Beams) into which we can divide the system . The beam elements are then analyzed as beams on elastic foundation by Winkler Theory.

Plates or Beams on Elastic Half Space • In this method we assume raft to be ed on an elastic half space. It is the most accurate method for analyzing the RAFT. • For using this method , the vertical displacement of the raft and elastic half space at various common points across the interface are equated. • The general method of raft analysis uses the finite element method called the SURFACE ELEMENT METHOD, which is based on the principle of coupling of a finite element raft structure to a ground continuum by a basic technique developed by Cheung and Zienkiewicz.

Discrete element method • This method includes: • Finite difference method. • Finite element method. • Finite grid method. Finite element method is the most accurate way of analyzing the RAFT , in which it is consider raft to be on an elastic foundation.

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