Project_Management_for_Construction(3)项目管理之路3(doc_

SAP问答：培训：，属于我们的sappub!11.AdvancedSchedulingTechniques11.1UseofAdvancedSchedulingTechniquesConstructionprojectschedulingisatopicthathasreceivedextensiveresearchoveranumberofdecades.Thepreviouschapterdescribedthefundamentalschedulingtechniqueswidelyusedandsupportedbynumerouscommercialschedulingsystems.Avarietyofspecialtechniqueshavealsobeendevelopedtoaddressspecificcircumstancesorproblems.Withtheavailabilityofmorepowerfulcomputersandsoftware,theuseofadvancedschedulingtechniquesisbecomingeasierandofgreaterrelevancetopractice.Inthischapter,wesurveysomeofthetechniquesthatcanbeemployedinthisregard.Thesetechniquesaddresssomeimportantpracticalproblems,suchas:schedulinginthefaceofuncertainestimatesonactivitydurations,integratedplanningofschedulingandresourceallocation,schedulinginunstructuredorpoorlyformulatedcircumstances.Afinalsectioninthechapterdescribessomepossibleimprovementsintheprojectschedulingprocess.InChapter14,weconsiderissuesofcomputerbasedimplementationofschedulingprocedures,particularlyinthecontextofintegratingschedulingwithotherprojectmanagementprocedures.Backtotop11.2SchedulingwithUncertainDurationsSection10.3describedtheapplicationofcriticalpathschedulingforthesituationinwhichactivitydurationsarefixedandknown.Unfortunately,activitydurationsareestimatesoftheactualtimerequired,andthereisliabletobeasignificantamountofuncertaintyassociatedwiththeactualdurations.Duringthepreliminaryplanningstagesforaproject,theuncertaintyinactivitydurationsisparticularlylargesincethescopeandobstaclestotheprojectareSAP问答：培训：，属于我们的sappub!stillundefined.Activitiesthatareoutsideofthecontroloftheownerarelikelytobemoreuncertain.Forexample,thetimerequiredtogainregulatoryapprovalforprojectsmayvarytremendously.Otherexternaleventssuchasadverseweather,trenchcollapses,orlaborstrikesmakedurationestimatesparticularlyuncertain.Twosimpleapproachestodealingwiththeuncertaintyinactivitydurationswarrantsomediscussionbeforeintroducingmoreformalschedulingprocedurestodealwithuncertainty.First,theuncertaintyinactivitydurationsmaysimplybeignoredandschedulingdoneusingtheexpectedormostlikelytimedurationforeachactivity.Sinceonlyonedurationestimateneedstobemadeforeachactivity,thisapproachreducestherequiredworkinsettinguptheoriginalschedule.Formalmethodsofintroducinguncertaintyintotheschedulingprocessrequiremoreworkandassumptions.Whilethissimpleapproachmightbedefended,ithastwodrawbacks.First,theuseofexpectedactivitydurationstypicallyresultsinoverlyoptimisticschedulesforcompletion;anumericalexampleofthisoptimismappearsbelow.Second,theuseofsingleactivitydurationsoftenproducesarigid,inflexiblemindsetonthepartofschedulers.Asfieldmanagersappreciate,activitydurationsvaryconsiderableandcanbeinfluencedbygoodleadershipandcloseattention.Asaresult,fieldmanagersmaylooseconfidenceintherealismofaschedulebaseduponfixedactivitydurations.Clearly,theuseoffixedactivitydurationsinsettingupaschedulemakesacontinualprocessofmonitoringandupdatingthescheduleinlightofactualexperienceimperative.Otherwise,theprojectscheduleisrapidlyoutdated.Asecondsimpleapproachtoincorporationuncertaintyalsodeservesmention.Manymanagersrecognizethattheuseofexpecteddurationsmayresultinoverlyoptimisticschedules,sotheyincludeacontingencyallowanceintheirestimateofactivitydurations.Forexample,anactivitywithanexpecteddurationoftwodaysmightbescheduledforaperiodof2.2days,includingatenpercentcontingency.Systematicapplicationofthiscontingencywouldresultinatenpercentincreaseintheexpectedtimetocompletetheproject.SAP问答：培训：，属于我们的sappub!Whiletheuseofthisrule-of-thumborheuristiccontingencyfactorcanresultinmoreaccurateschedules,itislikelythatformalschedulingmethodsthatincorporateuncertaintymoreformallyareusefulasameansofobtaininggreateraccuracyorinunderstandingtheeffectsofactivitydelays.Themostcommonformalapproachtoincorporateuncertaintyintheschedulingprocessistoapplythecriticalpathschedulingprocess(asdescribedinSection10.3)andthenanalyzetheresultsfromaprobabilisticperspective.ThisprocessisusuallyreferredtoasthePERTschedulingorevaluationmethod.[1]Asnotedearlier,thedurationofthecriticalpathrepresentstheminimumtimerequiredtocompletetheproject.Usingexpectedactivitydurationsandcriticalpathscheduling,acriticalpathofactivitiescanbeidentified.Thiscriticalpathisthenusedtoanalyzethedurationoftheprojectincorporatingtheuncertaintyoftheactivitydurationsalongthecriticalpath.Theexpectedprojectdurationisequaltothesumoftheexpecteddurationsoftheactivitiesalongthecriticalpath.Assumingthatactivitydurationsareindependentrandomvariables,thevarianceorvariationinthedurationofthiscriticalpathiscalculatedasthesumofthevariancesalongthecriticalpath.Withthemeanandvarianceoftheidentifiedcriticalpathknown,thedistributionofactivitydurationscanalsobecomputed.Themeanandvarianceforeachactivitydurationaretypicallycomputedfromestimatesofoptimistic(ai,j),mostlikely(mi,j),andpessimistic(