Design of Raft Foundation

DESIGN OF RAFT FOUNDATION FOUNDATION Design Considerations: If the loads transmitted by the ol!mns in a str!t!re are are so hea"y or the allo#able soil $ress!re so small that indi"id!al footings #o!ld o"er more than abo!t one%half of the area& it may be better to $ro"ide a ontin!o!s footing !nder all ol!mns and #alls' S!h a footing is alled a raft or mat fo!ndation' Raft fo!ndations are also !sed to red!e the settlement of str!t!res str!t!res loated abo"e highly om$ressible de$osits' Sine rafts are a re !s!ally at some de$th in the gro!nd& a large "ol!me of e(a"ation may be re)!ired' re)!ired' If #eight of the e(a"ated soil is e)!al to the #eight of the str!t!re and that of the raft& and the entres of gra"ity of e(a"ation and str!t!re oinide& settlement #o!ld be negligible' *here om$lete om$ensation is not feasible& a shallo#er raft may be ae$ted if the net inrease in loads in small eno!gh to lead to tolerable settlement' A raft fo!ndation may be retang!lar or ir!lar and may be #ith or #itho!t an o$en as sho#n in +g', and +g'-'

Fig',: Solid Raft Slab

Fig'-' Cir!lar Raft Fo!ndation If the ol!mns are e)!ally s$aed and loads are not "ery hea"y& a raft may be designed as ha"ing !niform thi.ness' The on"entional design of s!h a raft onsists of establishing its dimensions& from #hih the soil $ress!re at "ario!s loations beneath the slab may be om$!ted' The raft is di"ided into a series of ontin!o!s stri$s entered on the a$$ro$riate ol!mns and ro#s in both diretions as sho#n in +g'/' The shear and bending moment

diagrams may be dra#n !sing ontin!o!s beam analysis or oe0ients for eah stri$' The de$th is seleted to satisfy shear re)!irements'

Fig'/: Raft fo!ndation #ith stri$ entered on ol!mns  The steel re)!irements #ill "ary from stri$ to stri$' This method gi"es a onser"ati"e design sine the interation of ad1aent stri$ is negleted' If the ol!mns are e)!ally s$aed and their loads are e)!al& the $ress!re on the soil #ill be !niform& other#ise moments of the loads may be ta.en abo!t entre of the base and $ress!re distrib!tion determined' Sine the e)!ations for this are !s!ally deri"ed for a rigid member and a raft in general is not a rigid member& the $ress!re and res!lting internal stresses may be serio!sly in error if the eentriity is "ery large' The #eight of the raft is not onsidered in the str!t!ral design bea!se it is ass!med to be arried diretly by the s!bsoil' Raft may be ribbed #here the ol!mn s$aing is irreg!lar or for eonomy in !sing a relati"ely thin slab o"er most of the area as sho#n in +g'2' Alternati"ely& rafts may be thi.ened at the ol!mn loations for eonomy and de$th sho!ld be made s!0ient to resist shear' A ribbed raft fo!ndation onsists of a slab ated !$on by !$#ard soil $ress!re at its !nderside and s!$$orted by beams from ol!mn at its to$ #hih balane the !$#ard $ress!re #ith do#n#ard ol!mn loads' It is a similar to a 3oor slab resting on a system of beams and ol!mns' The $ortion bet#een beams is designed as a on"entional one #ay or t#o #ay slab' If the beams are dee$& they sho!ld be designed as dee$ beams'

Fig'2: Raft fo!ndation #ith #alls as sti4ener