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ID:7910046
大小:2.30 MB
页数:40页
时间:2018-03-02
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1、CHAPTER13SHALLOWFOUNDATIONII:SAFEBEARINGPRESSUREANDSETTLEMENTCALCULATION13.1INTRODUCTIONAllowableandSafeBearingPressuresThemethodsofcalculatingtheultimatebearingcapacityofsoilhavebeendiscussedatlengthinChapter12.Thetheoriesusedinthatchapterarebasedonshearfailurecriteri
2、a.Theydonotindicatethesettlementthatafootingmayundergoundertheultimateloadingconditions.Fromtheknownultimatebearingcapacityobtainedfromanyoneofthetheories,theallowablebearingpressurecanbeobtainedbyapplyingasuitablefactorofsafetytotheultimatevalue.Whenwedesignafoundatio
3、n,wemustseethatthestructureissafeontwocounts.Theyare,1.Thesupportingsoilshouldbesafefromshearfailureduetotheloadsimposedonitbythesuperstructure,2.Thesettlementofthefoundationshouldbewithinpermissiblelimits.Hence,wehavetodealwithtwotypesofbearingpressures.Theyare,1.Apre
4、ssurethatissafefromshearfailurecriteria,2.Apressurethatissafefromsettlementcriteria.Forthesakeofconvenience,letuscallthefirsttheallowablebearingpressureandthesecondthesafebearingpressure.Inallourdesign,weuseonlythenetbearingpressureandassuchwecallqnathenetallowablebear
5、ingpressureandqsthenetsafebearingpressure.Indesigningafoundation,weuse545546Chapter13theleastofthetwobearingpressures.InChapter12welearntthatqnaisobtainedbyapplyingasuitablefactorofsafety(normally3)tothenetultimatebearingcapacityofsoil.Inthischapterwewilllearnhowtoobta
6、inqs.Evenwithoutknowingthevaluesofqnaandqs,itispossibletosayfromexperiencewhichofthetwovaluesshouldbeusedindesignbaseduponthecompositionanddensityofsoilandthesizeofthefooting.Thecompositionanddensityofthesoilandthesizeofthefootingdecidetherelativevaluesofqnaandqs.Theul
7、timatebearingcapacityoffootingsonsandincreaseswithanincreaseinthewidth,andinthesamewaythesettlementofthefootingincreaseswithincreasesinthewidth.Inotherwordsforagivensettlement5pthecorrespondingunitsoilpressuredecreaseswithanincreaseinthewidthofthefooting.Itistherefore,
8、essentialtoconsiderthatsettlementwillbethecriterionforthedesignoffootingsinsandbeyondaparticularsize.Experimentalevid
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