Precedence Networks - Construction Project Scheduling and Control - page 105

is when the start of Order & Deliver Equip. is not tied logically to another
activity, but its finish controls the Install Equip. activity near the end of the
project (let us imagine the activities in Figure 5.20 at the end of a large project
network). In this case, the ES of Order & Deliver Equip. is the start of the
entire project (day 0).
You should not leave any dangling activities. Instead, tie them to the most reason-
able predecessor or successor. For example, you can make Order & Deliver Equip. in
Figure 5.20 a successor to Submittals or another activity. Scheduling software such as
Oracle Primavera P6 lists all activities with no predecessors or successors in its schedul-
ing report. Ideally, you should have only one—not several—of each.
Alternative Approach
Let us consider the activities shown in Figure 5.21. After performing the calculations
for the three sets, we get the same results: the dates for A are (0, 10) and the dates
for B are (4, 12), for both early and late dates. However, are the three approaches
equivalent? The answer is no. In Figure 5.21a, the finish of A may be delayed without
delaying B (if A is interruptible). In Figure 5.21c, the start of both activities is indepen-
dent. In addition, if the duration of A changes, the three scenarios will yield different
results. The scheduler must be careful, when choosing the logical ties (relationships),
to reflect the actual relationship as much as possible.
In general, a positive lag means “after” and a negative lag (lead) means “before.”
The author has introduced a new relationship (DML) that performs more consistently
in all these scenarios without the pitfalls. It is explained in Chapter 12.
Figure 5.20 Another example of a dangling activity
Figure 5.21 Three alternative approaches (a) Activity B can start at least 4 days after the
start of Activity A (b) Activity B can start no more than (at most) 6 days before the finish of
Activity A (c) Activity B can finish at least 4 days after the finish of Activity A