Project Management for Construction Chapter 11

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Project Management for Construction Chapter 1111. Advanced Scheduling Techniques11.1 Use of Advanced Scheduling TechniquesConstruction project scheduling is a topic that has received extensive research over a number ofdecades. The previous chapter described the fundamental scheduling techniques widely used andsupported by numerous commercial scheduling systems. A variety of special techniques have alsobeen developed to address specific circumstances or problems. With the availability of more powerfulcomputers and software, the use of advanced scheduling techniques is becoming easier and of greaterrelevance to practice. In this chapter, we survey some of the techniques that can be employed in thisregard. These techniques address some important practical problems, such as: • scheduling in the face of uncertain estimates on activity durations, • integrated planning of scheduling and resource allocation, • scheduling in unstructured or poorly formulated circumstances.A final section in the chapter describes some possible improvements in the project scheduling process.In Chapter 14, we consider issues of computer based implementation of scheduling procedures,particularly in the context of integrating scheduling with other project management procedures.Back to top11.2 Scheduling with Uncertain DurationsSection 10.3 described the application of critical path scheduling for the situation in which activitydurations are fixed and known. Unfortunately, activity durations are estimates of the actual timerequired, and there is liable to be a significant amount of uncertainty associated with the actualdurations. During the preliminary planning stages for a project, the uncertainty in activity durations isparticularly large since the scope and obstacles to the project are still undefined. Activities that areoutside of the control of the owner are likely to be more uncertain. For example, the time required togain regulatory approval for projects may vary tremendously. Other external events such as adverseweather, trench collapses, or labor strikes make duration estimates particularly uncertain.Two simple approaches to dealing with the uncertainty in activity durations warrant some discussionbefore introducing more formal scheduling procedures to deal with uncertainty. First, the uncertaintyin activity durations may simply be ignored and scheduling done using the expected or most likelytime duration for each activity. Since only one duration estimate needs to be made for each activity,this approach reduces the required work in setting up the original schedule. Formal methods ofintroducing uncertainty into the scheduling process require more work and assumptions. While thissimple approach might be defended, it has two drawbacks. First, the use of expected activity durationstypically results in overly optimistic schedules for completion; a numerical example of this optimismappears below. Second, the use of single activity durations often produces a rigid, inflexible mindseton the part of schedulers. As field managers appreciate, activity durations vary considerable and canbe influenced by good leadership and close attention. As a result, field managers may loose confidence 354in the realism of a schedule based upon fixed activity durations. Clearly, the use of fixed activitydurations in setting up a schedule makes a continual process of monitoring and updating the schedulein light of actual experience imperative. Otherwise, the project schedule is rapidly outdated.A second simple approach to incorporation uncertainty also deserves mention. Many managersrecognize that the use of expected durations may result in overly optimistic schedules, so they includea contingency allowance in their estimate of activity durations. For example, an activity with anexpected duration of two days might be scheduled for a period of 2.2 days, including a ten percentcontingency. Systematic application of this contingency would result in a ten percent increase in theexpected time to complete the project. While the use of this rule-of-thumb or heuristic contingencyfactor can result in more accurate schedules, it is likely that formal scheduling methods thatincorporate uncertainty more formally are useful as a means of obtaining greater accuracy or inunderstanding the effects of activity delays.The most common formal approach to incorporate uncertainty in the scheduling process is to apply thecritical path scheduling process (as described in Section 10.3) and then analyze the results from aprobabilistic perspective. This process is usually referred to as the PERT scheduling or evaluationmethod. [1] As noted earlier, the duration of the critical path represents the minimum time required tocomplete the project. Using expected activity durati ...