Presentation on theme: "Project Scheduling: Lagging, Crashing and Activity Networks"— Presentation transcript:
1 Project Scheduling: Lagging, Crashing and Activity Networks Chapter 10
2 Learning Goals Apply lag relationships to project activities. Construct and comprehend Gantt charts.Understand the trade-offs required in the decision to crash project activities.Develop activity networks using Activity- on-Arrow techniques.Understand the differences in AON and AOA and recognize the advantages and disadvantages of each technique.
3 Lags in Precedence Relationships The logical relationship between the start and finish of one activity and the start and finish of another activity.Four logical relationships between tasksFinish to StartFinish to FinishStart to StartStart to Finish
4 Finish-to-Start (FS) Lag Most common type of sequencingShown on the line joining the nodesAdded during forward passSubtracted during backward passThis lag is not the same as activity slackASpec Design6BDesign Check5CBlueprinting7Lag 4 days
5 Finish-to-Finish (FF) Lag Two activities share a similar completion pointThe interior construction cannot happen until wiring, plumbing, and HVAC installation are completeSPlumbing2THVAC3UInterior Const.6RWiringLag 3 days
6 Start-to-Start (SS) Lag Often two or more activities must start at the same timeSPlumbing2THVAC3UInspection6RWiringLag 3Logic must be maintained by both forward and backward pass
7 Start-to-Finish (SF) Lag Least common type of lag relationshipSuccessor’s finish dependent on predecessor’s startRWiring6Lag 3SPlumbing6THVAC5UInspection1
8 Gantt ChartsA graphical representation of the project schedule that illustrates how work flows over timeShows activity start and end dates and durationsLinks project activities to a project schedule baselineCan be used as a tracking toolBenefits of Gantt chartsEasy to create, read, and comprehendIdentify the project network and schedule baselineAllows for updating and controlUseful to identify resource needs and assigning resources to tasks
9 ExampleTaskTimePredecessorZ8--Y5XW4Y,XVU3T6S7U,TR9Create a Gantt chart based on the activities listed in the table.Task ES EF LS LFZYXWVUTSR
10 Gantt Chart Example from MSExcel TaskStartDurationZ8Y5XW164V20U3T256S317R389
11 Sample Tracking Gantt Chart With Critical Path A-C-D-F-H Critical path in red
12 Principal options for crashing Crashing ProjectsAccelerating a project by committing more resources.Principal options for crashingImproving existing resources’ productivityChanging work methods usedIncreasing the quantity of resources
13 Crashing Projects – Under What Conditions? The initial schedule may be too optimistic.Market needs change and the project is in demand earlier than anticipated.The project has slipped considerably behind schedule.The contractual situation provides even more incentive to avoid schedule slippage.
14 Steps in Project Crashing Compute the crash cost per time period. If crash costs are linear over time:Crash costper period=(Crash cost – Normal cost)(Normal time – Crash time)Using current activity times, find the critical path and identify the critical activities.
15 Steps in Project Crashing If there is only one critical path, then select the activity on this critical path thatcan still be crashed, andhas the smallest crash cost per period.If there is more than one critical path, then select one activity from each critical path such thateach selected activity can still be crashed, andthe total crash cost of all selected activities is the smallest.Note: the same activity may be common to more than one critical path.
16 Steps in Project Crashing Update all activity times.If the desired due date has been reached, stop. If not, return to Step 2.
17 Time-Cost Tradeoffs for Crashing Activities | | |1 2 3 Time (Weeks)$34,000 —$33,000 —$32,000 —$31,000 —$30,000 ——Activity CostCrash PointNormal PointCrash Cost/Wk =Crash Cost – Normal CostNormal Time – Crash Time=$34,000 – $30,0003 – 1= = $2,000/Wk$4,0002 WksCrash TimeNormal TimeCrash CostNormal CostHow far do you need to speedup the process?
18 Crashing Activities – An Example Formula Slope = crash cost – normal cost normal time – crash time Example – calculating the cost of crashing Suppose: Normal activity duration = 8 weeks Normal cost = $14,000 Crashed activity duration = 5 weeks Crashed cost = $23,000 The activity cost slope = 23,000 – 14,000 or, $9,000 = $3,000 per week 8 – 5 3
19 Managerial Considerations Determine activity fixed and variable costsThe crash point is the fully expedited activityOptimize time-cost tradeoffsShorten activities on the critical pathCease crashing whenthe target completion time is reachedthe crash cost exceeds the penalty cost
20 Crash Example Activity Pred Normal Time Min Time Normal Cost Crash CostA--1495001500B5210001600C10820002900DB, C2500E61900F3000GE, F746001800H15113600Initial early and late start and finish timesTask ES EF LS LF SlackABCDEFGHProject 63Reduce A by 5 weeks at $200/week = $1000 Project length is now 58 weeksReduce G by 3 weeks at $400/week = $1200 Project length is now 55 weeksReduce C by 2 weeks at $450/week = $900 Project length is now 53 weeksTotal cost to finish project in 53 weeks is $3100 extra
21 Crash ExampleWhat is the lowest cost to complete this project in 53 weeks? Times are in weeks and costs in dollars.
22 Activity on Arrow (AOA) Networks Activities represented by arrowsEvent nodes easy to flagForward and backward pass logic similar to AONTwo activities may not begin and end at common nodesDummy activities may be requiredEarliest Event Time (ES)Event labelLatest Event Time (LF)Task DescriptionDuration
24 Activity on Arrow (AOA) Network BHAFCDKGTask ES EF LS LFABCDEFGHK
25 A Comparison of AON and AOA Network Conventions Activity on Activity Activity onNode (AON) Meaning Arrow (AOA)A comes before B, which comes before C(a)ABCA and B must both be completed before C can start(b)ACBB and C cannot begin until A is completed(c)BAC
26 A Comparison of AON and AOA Network Conventions Activity on Activity Activity onNode (AON) Meaning Arrow (AOA)C and D cannot begin until both A and B are completed(d)ABCDC cannot begin until both A and B are completed; D cannot begin until B is completed. A dummy activity is introduced in AOA(e)CABDDummy activity
27 A Comparison of AON and AOA Network Conventions Activity on Activity Activity onNode (AON) Meaning Arrow (AOA)B and C cannot begin until A is completed. D cannot begin until both B and C are completed. A dummy activity is again introduced in AOA.(f)ACDBDummy activity
28 Discussion QuestionsPlease give examples of circumstances in which a project would employ lag relationships between activities using:Finish to startFinish to finishStart to startStart to finishThe advantage of Gantt Charts lies in their linkage to the project schedule baseline. Explain this concept.What are the advantages in the use of Gantt charts over PERT diagrams? In what ways might PERT diagrams be advantageous?Under what circumstances might you wish to crash a project?
29 Discussion QuestionsIn crashing a project, we routinely focus on those activities that lie on the critical path, not activities with slack time. Explain why this is the case.What are some of the advantages in the use of AOA notation as opposed to AON? Under what circumstances does it seem better to apply AON methodology in network development? Explain the concept of a “dummy variable.” Why are they employed in AOA notation? Why is there no need to use dummy variables in an AON network?Explain the concept of a “dummy variable.” Why are they employed in AOA notation? Why is there no need to use dummy variables in an AON network?
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