why buoyancy forces cannot be ignored

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July 28, 2010 1 Comment

ForcesareatWorkBeneaththeFeetthatMustbeReckonedWithBeforeDesigninganUndergroundStructure

ByClaudeGoguen,P.E.,LEEDAPandRonaldThornton,P.E.

EveryoneprobablyrememberstheimagesofUSAirwaysFlight1549driftinginthemiddleoftheHudsonRiverJan.15,2009,whilefrightenedpassengersdisembarkedontothewings.Thecourageousactionsofthepilotsandflightcrewwerecreditedforsavingthelivesofallonboard.Butitwasphysicalforcethatkepttheairplaneafloatforceknownasbuoyancy.BuoyancyisalsoanimportantelementoftheannualNationalConcreteCanoeCompetitionhostedbytheAmericanSocietyofCivilEngineers.Morethan200universityteamscompeteforAmericasCupofCivilEngineering.TheUniversityofBerkeleywonthe22ndNCCCtitlethispastJune.

Why Buoyancy Forces Cannot Be Ignored


5/19/2015http://precast.org/2010/07/why-buoyancy-forces-cannot-be-ignored/

InthecaseofFlight1549,orifyourebuildingconcretecanoe,buoyancyisgoodthing.Forthoseintheundergroundutilitybusiness,however,itcanbepainifitisnotaccountedforinthedesign.Buoyancyisdefinedasthetendencyoffluidtoexertsupportingupwardforceonbodyplacedinfluid.Thefluidcanbeliquid,asinthecaseofboatfloatingonlake,orthefluidcanbegas,asinhelium-filledballoonfloatingintheatmosphere.simpleexampleofbuoyancycanbeseenwhentryingtopushanemptywaterbottledownwardinsinkfullofwater.Whenapplyingdownwardforcetothewaterbottlefromyourhand,thewaterbottlewillstaysuspendedinplace.Butassoonasyouremoveyourhand,thewaterbottlewillfloattothesurface.Thebuoyantforceontheobjectdetermineswhetherornottheobjectwillsinkorfloat.BuoyancywasntofficiallydocumentedandconceptuallygraspeduntilArchimedes(287-212B.C.)establishedthetheoryofflotationanddefinedthebuoyancyprinciple.Herealizedthatsubmergedobjectsalwaysdisplacefluidupward.Thenwiththatobservation,heconcludedthatthisforce(buoyant)mustbeequaltotheweightofthedisplacedfluid.Archimedesthenwentontostatethatsolidobjectwouldfloatifthedensityofthesolidobjectwerelessthanthedensityofthefluidandviceversa.Butwhatisthebasicproceduretofollowinordertodeterminewhetheranundergroundconcretestructurewillresistbuoyantforces?Itcanbedeterminedifanundergroundconcretestructurewillfloatorsinkusingbasicprinciples.Essentiallyconcretestructurewillnotfloatifthesumoftheverticaldownwardforcesisgreaterthantheverticalupwardforce.Whenapplyingthisprincipletostructurebelowgrade,itcanbesaidthatifthebuoyantforce(Fb)isgreaterthanthemassofthestructureandthecombinedmassofsoilsurchargesandobjectscontainedwithinthestructure,thestructurewillfloat.Whyisbuoyancyanimportantfactorinthedesignofanundergroundconcretestructure?Thesimpleansweristhatthebuoyantforcescreatedbywaterneedtoberesistedtopreventthestructurefromfloatingorshiftingupward.Determiningwatertablelevels



Whendesigninganundergroundprecastconcretestructure,itisnecessarytoknowwhatstructuretomakeaswellasitsintendeduse.Typicallycontractorswhoneedprecaststructureswillpresentprecasterswithdetailsonwhattheyneedandgivedesignrequirementsandinformationontheundergroundconditions.Notalways,however,dotheyinformprecastersabouteverydetail,especiallyjobsiteconditionsandproblemsintheconstructionarea.Siteandsubsurfaceconditionsarevitalpiecesofinformationneededforthedesigncalculationstooptimizetheperformanceofthestructureintheinstalledconditionandtopreventflotation.Sohowdoesthedesignengineerdeterminewhentherecouldbepotentialproblemwiththejobsiteconditionsandwithflotation?First,thedesignengineershouldreviewandinvestigatetheplans,specificationsandsoilsreportstogainmoreinsightabouttheprojectandtheundergroundconditions.Afterobtainingtherequirementsandspecificationsforthestructuraldesign,thedesignengineershouldobtainextensiveinformationonthesoilsandsubsurfaceconditions.Oneofthefirstfactorsthatmustbedeterminedwhenanalyzinganareainwhichtheconcretestructurewillbeplacedbelowgradeisthewatertable,orgroundwaterlevel.Obtainingthisinformationwillhelpthedesignersidentifysiteswhereflotationmayormaynotbefactorinthedesign.Howcanonedeterminethewatertablelevelintheprojectarea?Thedesignengineershouldcheckthesoilsreporttoobtainmoreinformationonthearea.Thesoilsreportistypicallythemostreliablesourceofdata,asitsbasedonstudyofthejobsiteconditions.Ifthereisntsoilsreport,coredrillingmaybenecessary.Bycoredrillinginthevicinityoftheproject,thedepthofthewaterlevelfromgradecanbedetermined.Itshouldbenotedthatgroundwaterlevelsidentifiedondrillingreportsareonlysnapshotintimeandmaynotaccountforseasonalvariations.Anotherpossiblesourceofinformationwouldbefromlocalwelldrillers,whotypicallymaintainrecordsofwatertablelevels.Afterthewatertablelevelhasbeendeterminedanditisknownthattherewillmostlikelynotbeproblemwithbuoyancyorflotationissues,thedesignercanfocusonmaximizingthestructurewithouttheconsiderationofbuoyant



forces.Inmostcases,flotationwillnotbeprobleminareasofthecountrywithoutgroundwater(partsofTexas,ArizonaandNevada)andwherethegroundwaterisbelowtheanticipateddepthofthestructure.Thefactthatthebuoyancyforceexistspresupposesthatthewatertableatthesiteisatanelevationabovethelowestpointoftheinstalledstructure.Ifyourstructureistobeplacedabovethegroundwaterlevel(accordingtothesiteswatertable),lessconcernisneeded.Ontheotherhand,areaswhereflotationcausespotentialproblemsaretypicallyatlowelevationwherethewaterlevelisatgrade(valleys,oceanshores)andinareaswheregroundwaterispresentbelowgradeatthetimeofinstallation(beforesoilhasbeencompacted).Beawareofseasonalandregionalvariations

Thewatertableistheupperlevelofanundergroundsurfaceinwhichthesoilissaturatedwithwater.Thewatertablefluctuatesbothwiththeseasonsandfromyeartoyearbecauseitisaffectedbyclimaticvariationsandbytheamountofprecipitationusedbyvegetation.Italsoisaffectedbyexcessiveamountsofwaterwithdrawnfromwellsorbyrechargingthemartificially.Thedesignengineershouldmakecertaintoaccountforseasonalandregionalfluctuationsinthewatertablelevelinthedesignofanundergroundprecastconcretestructure;thiswillensurethattheundergroundstructurewillnotfloatorshiftupwardfromwatertablelevelmiscalculation.Errontheconservativeside

Iftherearenosoilsreportsorpreviouswatertabledataavailableforfluctuations(seasonalandregional),mostengineerswilldesignthestructureontheconservativeside.Thiswillensurethatthestructurewillbeabletowithstandseasonalandregionalfluctuations.Designingontheconservativesidereferstostructurewiththewaterlevelatgrade,eveniffloodinginthatareaisnotcommon.conservativedesignpproachmaycontributetooffsettingunnecessaryandunforeseencostswhensufficientinformationaboutthesoil/siteconditionsisunavailable.Therefore,overdesigningstructureshouldbekepttominimumsincethiswouldaddsubstantialcoststoproduction.



Computingdownward(gravity)forces

Afterthewatertablelevelhasbeenidentified,thedesignengineerneedstolookatcomputingallthedownwardforcesthatwillbeactingonthestructure.Allverticaldownwardforcesarecausedbygravitationaleffects,whichneedtobecalculatedinthedesignofanundergroundstructure.Essentially,theengineerdeterminesifthetotaldownwardforces(gravitational,WT)aregreaterthantheupwardforce(buoyant,Fb).Thetotaldownwardforce(WT)iscalculatedbythesummationofalldownwardverticalforces(W).Dependingonthedesignoftheundergroundstructure,thetotalverticaldownwardforces(WT)mayormaynotbethesameforallapplications.Inconservativeapproach,thedesignofundergroundstructuresassumesthatthewatertableatthespecificsiteisatgrade.Inthiscase,itisessentialtoaccountforallverticaldownwardforces(WT)toensurethatthestructurewillnotfloat(WTFb).Foranundergroundstructure,designedforworst-casescenario,thefollowingverticaldownwardforces(W)needtobeconsidered:

Weightofallwallsandslabs Weightofsoilonslabs Weightofsoilonshelforshelves Weightofequipment(permanent)insidestructure Weightofinvertsinsidestructure Frictionofsoiltosoil Additionalconcreteaddedinsidestructure WeightofreinforcingsteelAsnotedpreviously,notallundergroundstructuresarethesame,andthereforesomeofthelistedverticaldownwardforces(W)abovemaynotbeincludedinthesummationoftotalverticaldownwardforce(WT).



ComputingupwardbuoyantforceAsstatedinArchimedesPrinciple,anobjectisbuoyedupbyforceequaltotheweightofthefluiddisplaced.Mathematically,theprincipleisdefinedbytheequation:FbgfndFbbuoyantforce(lb)gfdensityofwater(62.4lb/ft)nddisplacedvolumeofthefluid(ft)

Whenanalyzingbuoyancy-relatedconcreteapplications,thestructureistypicallybelowgradeandstationary.Assumingtheapplicationisstationaryinfluid,analysisusesthestaticequilibriumequationintheverticaldirection,Fv0.Analyzingbuoyancyrelatedtoundergroundstructuresrequiresuseofthesamestaticequilibriumequation,assumingthestructuretobestationaryandeithersubmergedorpartiallysubmergedinfluid(inthelattercase,thesurroundingsoil/fillmaterialandanyassociatedgroundwater).Safetyfactorguidelines

TheFactorofSafety(FS)considerstherelationshipbetweenresistingforceanddisturbingforce.Inthiscase,itstherelationshipbetweentheweightofthestructureandtheupliftforcecausedbybuoyancy.Failureoccurswhenthatfactorofsafetyislessthan1.0.Generallyspeaking,thegreatertheFSthegreatertheimpacttotheproject/structure.AnoptimaldesignwouldbeanappropriateFSthatisadequatefortheconditionspresentatthatspecificsite.ItisrecommendedthatthedesignerchooseanappropriateFSafterreviewingjobsiteinformation.



AccordingtoACI350,thesafetyfactoragainstflotationisusuallycomputedasthetotaldeadweightofthestructuredividedbythetotalhydrostaticupliftforce.TheFSshouldreflecttheriskassociatedwithhydrostaticloadingconditions.Insituationsoffloodingtothetopofthestructureandusingdead-weightresistanceonly,FSof1.10iscommonlyused.Infloodzoneareas,orwherehighgroundwaterconditionsexist,FSof1.25canbeused.Wheremaximumgroundwaterorfloodlevelsarenotwelldefinedorwheresoilfrictionisincludedintheflotationresistance,higherFSvaluesshouldbeconsidered.Buoyancycountermeasures

Thereareseveralmethodsthatcanbeusedintheindustrytoovercomebuoyancyproblem.Ifthedesignoftheundergroundstructuredoesnotmeettherequiredsafetyfactor,therearewaystofixtheproblem.Herearesomedifferentmethodsusedtoovercomebuoyancy,bothbeforeandaftershiftingorflotation:1. Baseextension(cast-in-placeorprecast).Usingtheadditionalweightofsoilbyaddingshelvesiscommonmethodusedtocounteractbuoyancy.Extendingthebottomslabhorizontallycreatesshelfoutsidethewallsofthestructureandaddsadditionalresistancetothebuoyantforce.Theadditionalverticaldownwardforcecomesfromtheadditionalweightofthesoilactingontheshelves(Wshelf).Thesizeoftheshelfcanbedesignedaslargeandwideasneededsothebuoyantforceisresisted.However,limitsinshippingwidthmustbeconsidered.Inmanycases,thisisthemostcost-effectivemethodusedtoresistthebuoyantforce(Fb).Whenpouringtheshelfinplace,mechanicalconnectionsmustbedesignedtoresisttheverticalshearforces.Ifpossible,itisbesttohavetheshelfmonolithicallypouredwiththestructure.2. Anti-flotationslab.Anothermethodthathasbeenusedinconstructionistoanchorthestructuretolargeconcretemass(shelf)pouredonsiteoruseprecastconcretemanufacturedoffsite.Thestructuresitsdirectlyontopofthislargeconcretemassthathaspreviouslybeenpouredinplaceorcast,curedanddeliveredbyanoff-sitemanufacturer.Thismethodcancauseproblems,however,becausebothbaseslabsmustsitflushontopof



oneother.Ifbaseslabsarenotalignedperfectly,crackingduetopointloadsmayresult.Cast-in-placeconcretecanbeexpensiveandcausedelaysduetostrengthcuringtime.Precastconcretealleviatesalignmentanddelaysforstrengthgain,butthesub-basemustbelevelandsetflush.mortarbedbetweenthetwosurfacesisrecommended.Todesignthemechanicalconnectionbetweentheanti-flotationslabandthestructure,thenetupwardforcemustbecalculated.ThiscalculationcanbeachievedbymultiplyingthebuoyantforcebytheFS,andsubtractingthedownwardforce.3. Increasememberthickness.Onemethodusedtoovercomebuoyancyistoincreasetheconcretemass(m).Thisisaccomplishedbyincreasingmemberthickness(wallsandslabs).Increasingthethicknessofthewallsandslabscanaddsignificantdownwardgravitationalforce,butthismaynotbecosteffective.Increasingconcretemasscanbeanexpensivealternativeduetoincreasedmaterialsandproductioncosts.4. Lowerstructureelevationandfillwithadditionalconcrete.Anothermethodusedtoovercomebuoyancyistosettheprecaststructuredeeperthanrequiredforitsfunctionalpurposes.Thiswilladdadditionalsoilweightontopofthestructuretoopposebuoyantforces.Also,withthestructurebeingdeeperinthesoil,somecontractorsopttopouradditionalconcreteintothebaseoftheinstalledprecastconcretestructure.Thiswilladdmoremasstothestructure,whichhelpsovercomebuoyancy(mFb).

Itisfairlysimpleconcept:downwardgravitationalforcesneedtoexceedupwardbuoyantforces.IgnoringthismayresultinyourstructuresurfacinglikesubmarineintheSouthPacific.Onceprecastvaultisinstalledunderground,youexpectittostayput.Sinceconcreteisabout2.5timesheavierthanwater,onewouldnotexpectflotationtobemuchofanissuewithburiedconcretestructures,butinfactitisseriousconsiderationinareasofhighgroundwater.ClaudeGoguen,P.E.,LEEDAP,isNPCAsdirectorofTechnicalServices.

RonaldThornton,P.E.,isprojectmanagerforDeltaEngineersinBinghamton,N.Y.,withmorethan25yearsofexperienceintheconcreteindustry.Hehas



extensiveexperienceinthedesign,manufactureandinstallationofprecastproductsforuseinstate,municipalandprivateprojects.ThorntoncurrentlyservesontheNPCAUtilityProductCommitteeaswellastheASTMC27Committee.

Filed Under: Archive - 2009-2010, Precast Inc. Magazine, Precast MagazinesTagged With: buoyancy, underground structure

Comments

Bob Grotke saysJune 13, 2012 at 3:29 pm

The problem of bouyancy became an issue at my workplace a while back. Ourdepartment has used open bottom fiberglass manholes for years and the typicalinstallation involved dewatering the excavation as required, placing the fiberglassmanhole in the proper orientation for the sewer pipes, and pouring mass concretearound the base of the manhole, typically at least 1 foot thick and 1 foot wider than themanhole diameter all around. After an initial set, the excavation would be backfilled andcompacted while the dewatering system remained operating. The interior base of themanhole would be finished with additional concrete, formed to provide a flow channelwith built up concrete haunches to direct the flow. After construction was complete, thedewatering system would be removed and the ground water would return to seek itsown level. Design was typically conservative with satuarated ground assumed up tofinished grade. We had never had any problems with this construction, but the engineersmanaging the office one day decided that a 2-way reinforcing steel mat was required inthe concrete base to resist unopposed hydrostatic forces that could cause the curedconcrete to fail. I was assigned to determine what reinforcement was necessary. Istated that none was necessary as the weight of the manhole, concrete base, manholelid and frame and the soil surcharge overcame the bouyant forces, the net force wasdownward and any resulting bending stress in the concrete was within the limitsallowed by the ACI plain concrete building code. This resulted in much heateddiscussion and agravation. I finally told management that if they sincerely believed thereinforcement was necessary, they should just direct the crews to install thereinforcement and they wouldnt get any arguments, however I believed it was not



necessary. The bad feelings resulting from this situation eventually led me to findemplyment elsewhere, but to this day I wonder if their thoughts about unopposedhydrostatic forces had any merit.

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