Construction Methods - Chapter16.pdf - po ang - Januszek66

PARTTHREE

Construction

Management

PlanningandScheduling

16–1INTRODUCTION

PlanningandScheduling

Asyoualreadyknow,someplanningmustbedoneinordertoperformanyfunctionwitha

minimumofwastedtimeandeffort.Thisistruewhetherthefunctionisgettingtoworkon

timeorconstructingamultimilliondollarbuilding.Ascheduleisnothingmorethanatime-

phasedplan.Schedulesareusedasguidesduringtheperformanceofanoperationinorder

tocontrolthepaceofactivitiesandtopermitcompletionoftheoperationatthedesiredor

requiredtime.

Schedulingisutilizedformanydifferentphasesoftheconstructionprocess,from

masterplanningthroughfacilityconstructiontofacilityoperationandmaintenance.Inthe

constructionphaseitself,schedulesareusefulforanumberofpurposesbeforestartinga

projectandaftercompletionoftheprojectaswellasduringtheactualconductofcon-

structionwork.Someoftheprincipalusesforschedulesduringeachofthesephasesofcon-

structionarelistedbelow.

BeforeStarting

1.Providesanestimateofthetimerequiredforeachportionoftheprojectaswellasfor

thetotalproject.

2.Establishestheplannedrateofprogress.

3.Formsthebasisformanagerstoissueinstructionstosubordinates.

4.Establishestheplannedsequencefortheuseofpersonnel,materials,machines,and

money.

DuringConstruction

1.Enablesthemanagertoprepareachecklistofkeydates,activities,resources,andsoon.

2.Providesameansforevaluatingtheeffectofchangesanddelays.

453

454

CHAPTER16

3.Servesasthebasisforevaluatingprogress.

4.Aidsinthecoordinationofresources.

AfterCompletionofConstruction

1.Permitsareviewandanalysisoftheprojectasactuallycarriedout.

2.Provideshistoricaldataforimprovingfutureplanningandestimating.

SchedulingPrinciples

Thereareanumberofdifferentformsofschedulesthatmaybeused,includingwritten

schedules,bargraphschedules,networkschedules,andothers.Inthischapter,wewillcon-

sideronlybargraph,network,andlinearschedulingmethods.Regardlessoftheschedul-

ingmethodemployed,thefollowinggeneralprinciplesofschedulingshouldbeobserved.

1.Establishalogicalsequenceofoperations.

2.Donotexceedthecapabilitiesoftheresourcesthatareavailable.

3.Provideforcontinuityofoperations.

4.Startprojectcontrolling(orcritical)activitiesearly.

Itmustberecognizedthattheaccuracyorvalidityofschedulingdependsontheva-

lidityoftheworkquantityandproductivityestimatesused.Theaccuracyofanestimateof

thetimeitwilltaketoperformaconstructionoperationisafunctionofthekindofwork

involvedandpriorexperienceindoingthatsortofwork.Forexample,oneexpectsamore

accurateestimateofthetimerequiredtoinstallawastewaterlineinaresidentialstructure

thanofthetimerequiredtoinstallacoolinglineinanuclearpowerplant.Methodsfordeal-

ingwiththeuncertaintyassociatedwithactivity-timeestimateswillbediscussedlaterin

thischapter.

Inadditiontovalidtimeestimatesforactivities,theplannermusthaveathoroughun-

derstandingofthenatureoftheworktobeperformedandtherelationshipsbetweenthevar-

iousworkitemsmakinguptheproject.Oneofthemajordeficienciesofthebargraph

scheduledescribedinthefollowingsectionisthefactthatthebargraphfailstoshowrela-

tionshipsbetweenworkitems.Thatis,whatactivitiesmustbestartedorcompletedbefore

otheractivitiescanbestartedorcompleted?

16–2BARGRAPHMETHOD

TheBarGraphSchedule

The bargraph or barchartschedule isagraphicalschedulerelatingprogressofitemsof

worktoatimeschedule.ThebarscheduletracesitsorigintoachartdevelopedbyHenry

L.Gantt,apioneerintheapplicationofscientificmanagementmethodstoindustrialpro-

duction.Thesecharts,referredtoas Ganttcharts ,tookseveraldifferentforms,depending

455

PLANNINGANDSCHEDULING

ontheirapplication.Becauseoftheirorigin,allformsofbargraphschedulesaresometimes

calledGanttcharts.Inspiteoftheadventofnetworkplanningmethods,thebargraph

scheduleisstillthemostwidelyusedscheduleformfoundinconstructionwork.Itscon-

tinuedpopularityinthefaceofthesignificantdeficienciesthataredescribedinthenextsec-

tionisundoubtedlyduetoitsverygraphicandeasilyunderstoodformat.Asimplebargraph

scheduleforaconstructionprojectisshowninFigure16–1.Themajorworkitems,orac-

tivities,makinguptheprojectarelistedontheleftsideoftheschedulewithatimescale

acrossthetop.Thecolumnheaded“Hours”indicatestheestimatednumberoflabor-hours

requiredforeachactivity.Thecolumnheaded“Weight”indicatestheportionofthetotal

projecteffortaccountedforbyeachactivity.Forexample,“ClearingandStripping”re-

quires750labor-hoursofwork,whichrepresents4.7%ofthe15,900labor-hoursrequired

fortheentireproject.Whileaweightingcolumnisnotalwayspresentonabarchart,its

presenceisveryusefulwhencalculatingcumulativeprojectprogress.Activitiesmaybe

weightedonanydesiredbasis.However,dollarvalueandlabor-hoursaremostfrequently

usedasaweightingbasis.

Noticethattwohorizontalblocksareprovidedoppositeeachactivity.Theupper

block(SCH)representsscheduledprogressandthelowerblock(ACT)isusedtorecordac-

tualprogressasworkproceeds.Foreachblock,abarextendsfromstartingtoendingtimes.

Thenumbersaboveeachbarindicatepercentageofactivitycompletionateachmajortime

division.Againsuchasystemgreatlysimplifiescalculationofscheduledcumulative

progressanditscomparisonwithactualprogress.Toaidintheevaluationofprogress,itis

suggestedthattheactualprogressofeachactivitybeinsertedattheendofeachmajortime

period,asshowninFigure16–1.

CumulativeProjectProgress

Figure16–2showsacumulativeprogress-versus-timecurveforthebargraphofFigure16–1.

Theverticalscalerepresentscumulativeprojectprogressinpercentandthehorizontalscale

indicatestime.Oncethebargraphschedulehasbeenpreparedandweightingfactorscal-

culatedforeachactivity,scheduledcumulativeprogresscanbecalculatedandplottedas

shownonthefigure.Actualcumulativeprogressiscalculatedandplottedasworkpro-

gresses.Toconstructthescheduledcumulativeprogresscurve,scheduledcumulative

progressmustbecalculatedandplottedforasufficientnumberofpointstoenableasmooth

curvetobedrawn.InFigure16–2scheduledcumulativeprogressforthebargraphsched-

uleofFigure16–1hasbeencalculatedandplottedattheendofeachweek.Cumulative

progressmaybecalculatedasfollows:

Cumulativeprogress %

1 Activityprogress 2

1 Weight 2

(16–1)

Examplecalculationsforthescheduledcumulativeprogressforthefirst3weeksofthe

projectwhosebargraphscheduleappearsinFigure16–1follow.

EndofFirstWeek

[Activity1]

Progress % (0.20 $ 4.7) % 0.9%

Construction Methods - Chapter16.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: