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1、FootingsTypesandfunctionofsubstructure,orfoundation,isthatpartofastructurewhichisusuallyplacedbelowthesurfaceofthegroundandwhichtransmitstheloadtotheunderlyingsoilorrock.Allsoilscompressnoticeablywhenloadedandcausethesupportedstructuretosettle.Thetwoessentia
2、lrequirementsinthedesignoffoundationsarethatthetotalsettlementofthestructureshallbelimitedtoatolerablysmallamountandthatdifferentialsettlementofthevariouspartsofthestructureshallbeeliminatedasnearlyaspossible.Withrespecttopossiblestructuraldamage,theeliminat
3、ionofdifferentialsettlement,i.e.,differentamountsofsettlementwithinthesamestructure,isevenmoreimportantthanlimitationsonuniformoverallsettlement.Tolimitsettlementasindicated,itisnecessarytotransmittheloadofthestructuretoasoilstratumofsufficientstrengthandtos
4、preadtheloadcoverasufficientlylargeareaofthatstratumtominimizebearingpressure.Ifadequatesoilisnotfoundimmediatelybelowthestructure,itbecomesnecessarytousedeepfoundationssuchaspilesorcaissonstotransmittheloadtodeeper,firmerlayers.Ifsatisfactorysoildirectlyund
5、erliesthestructure,itismerelynecessarytospreadtheload,byfootingsorothermeans.Suchsubstructuresareknownasspreadfoundations,anditismainlythistypewhichwillbediscussed.TypesofspreadfoundationsFootingsgenerallycanbeclassifiedaswallandcolumnfootings.Thehorizontalo
6、utlinesofthemostcommontypesaregiven.Awallfootingissimplyastripofreinforcedconcrete,widerthanthewall,whichdistributespressure,singlecolumnfootingareusuallysquare,sometimesrectangular,andrepresentthesimplestandmosteconomicaltype.Theiruseunderexteriorcolumnsmee
7、tswithdifficultiesifpropertyrightspreventtheuseoffootingprojectingbeyondtheexteriorwalls.Inthiscasecombinedfootingsorstrapfootingsareusedenableonetodesignafootingwhichwillnotprojectbeyondthewallcolumn.Combinedfootingsundercloselyspaced,heavilyloadedinteriorc
8、olumnswheresinglefootings,iftheywereprovided,wouldcompletelyornearlymerge.Suchindividualorcombinedcolumnfootingsarethemostfrequentlyusedtypesofspreadfoundationsonsoilsofreasonablebearingcapacity