Precedence Networks - Construction Project Scheduling and Control - page 99

to the project management topics listed in the Bibliography for more discussion on
this concept.
A PARALLEL PREDECESSOR?
You learned in Chapters 3 and 4 that a predecessor is an activity that must happen
before another one can happen. For example, if A is a predecessor of B, then A must
happen before B can start, but not necessarily immediately after A (Figure 5.10a).
If A is a predecessor of B with an SS relationship, both may occur in parallel
(Figure 5.10b). Would A still be called a predecessor ? The answer is yes because A must
start first (even with zero lag). If B starts several days after A starts, the logic is not
violated. However, if B starts first—by any amount of time—the logic will be violated.
Activity A, thus, controls activity B and is still called a predecessor . Note the direction
of the arrow in Figure 5.10b: it points from the activity that controls to the activity
that is controlled.
Realistically, a portion (maybe small) of A must occur before B can start. Popescu
and Charoenngam (1995) defined a predecessor as “an activity that has some mea-
surable portion of its duration logically restraining a subsequent activity or activities”
(p. 438). In reality, we may show two activities with an SS relationship and zero lag, but
a certain—possibly small—portion of the predecessor must occur before the successor
starts.
One observation that may occur in computer software: Oracle Primavera P6 pro-
vides a report after the CPM calculations mentioning dangling activities; i.e., those
with no predecessor and/or successor. To the program, activity B in Figure 5.10b
(assuming it has a successor, activity C) is not dangling because it has a predecessor
and a successor. In reality, it is dangling because its end is not connected to any activ-
ity. Thus, its late finish date will be same as the project completion date and it may
have large total float. The same situation can happen in cases such as when we use
a negative lag (lead). In the case shown in Figure 5.21(b), even though the end of
activity A show as connected to activity B, but in reality, activity A is dangling unless it
is connected to another activity. This is because the first 4 days of A are a predecessor
Figure 5.10 (a) Typical predecessor relationship; (b) parallel predecessor relationship