Precedence Networks - Construction Project Scheduling and Control - page 101

Figure 5.13 Three-activity project, with 10-day duration, using a finish-to-finish relationship
not be a problem). Consequently, let us try to tie the ends of the activities with FF
relationships rather than the SS relationships (Figure 5.13).
In Figure 5.13, the finish of each activity is restricted to the finish of the predeces-
sor. Activity B cannot finish till A is finished. Activity C cannot finish till B is finished.
No restriction is put on their start other than the start of the project. Thus, not only is
activity A (the activity with the longest duration) critical but also the ends of activities
B and C are critical. This criticality can change if, for example, activity B has 12 days'
duration, as shown in Figure 5.14.
In Figure 5.14, B has become the driving activity. It has no control over the
completion of its predecessor (activity A), but it has control over the finish of activity
C. Activity B automatically becomes critical (since it has the longest duration). The
end of activity C is still critical, since it is tied to (follows) the end of B. Activity A has
2 days' “unrestricted” float.
If we tie both the start and the end of the activities shown in Figure 5.13, we
get Figure 5.15. The relationships shown in Figure 5.15 combine those shown in
Figures 5.11 and 5.13. As mentioned previously, in the case of two activities tied with
two relationships, in most cases, one of these two relationships governs (i.e., becomes
driving). In an unlikely case, both relationships govern (become driving). The main
difference between Figures 5.15 and 5.11 is that we did not allow activities B and C
in Figure 5.15 to finish before day 10, the finish date for activity A. There are no
differences—as far as dates are concerned—between Figures 5.13 and 5.15. However,
a logic difference exists between the two cases, but as a result of the durations of
Figure 5.14 Three-activity project, with 12-day duration, using a finish-to-finish relationship
Figure 5.15 Three-activity project, with a 10-day duration, using a combination relationship