Precedence Networks - Construction Project Scheduling and Control - page 100

to activity B, while the next 6 days are open-ended or dangling. Such cases are not
detected by the computer software and needs an expert eye to catch.
CPM CALCULATIONS FOR PRECEDENCE DIAGRAMS
The CPM calculations for precedence diagrams differ from those for standard arrow
or node diagrams. However, before we start the CPM calculations, we must establish
an important distinction: Are the activities contiguous (continuous) or interruptible?
In other words, once an activity starts, can it be performed on intermittent basis
(paused, resumed, etc.) or must it proceed continuously until it is finished? In real
life, both scenarios are possible, but the assumption whether the activities are con-
tiguous or interruptible makes a substantial difference in the calculation method used
and possibly in the calculated finish date of the entire project.
Interruptible Activities
Let us start with the simple three-activity project depicted in Figure 5.11. The three
activities are tied with only SS relationships, with no lags. Theoretically, all three activ-
ities can start at the same time. The project is complete when the activity with the
longest duration, A, finishes. Thus, activity A is critical. Activities B and C have floats
equal to the difference between their duration and the duration of A (the float for B
is 10 − 5 = 5 and the float for C is 10 − 3 = 7). Now, let us add some lags, as shown
in Figure 5.12.
In Figure 5.12, we are forcing a delay in the start of activities B and C, maybe
because we want to see a certain percentage of activity A finished before B can start,
and a certain percentage of activity B finished before C can start. We find that, in this
example, activity B is the critical activity, since it takes 11 days to finish (6-day lag +
5 days' duration). The problem is that activity C may finish early, on day 10, which
is 1 day before activity B is scheduled to finish (in reality, this situation may or may
Figure 5.11 Three-activity project using a start-to-start relationship
Figure 5.12 Three-activity project using a start-to-start relationship with lags