Precedence Networks - Construction Project Scheduling and Control - page 111

Let us return to Figure 5.12: With activity A asacontiguous, it becomes entirely
critical, with both early and late dates as (0, 10). Activities B and C in Figure 5.13
become entirely critical, with both their early and late dates as (5, 10) for B and
(7, 10) for C. The same is true of activity C in Figure 5.14. Its early and late dates
become (9, 12). In Figures 5.15 and 5.16, all activities also become critical.
In the preceding cases, the finish date of each project does not change, despite
the change in assumption from interruptible to contiguous. The only change is that
activities with restricted float lose their float and become totally critical. However, this
may not always be the case. In Figure 5.24, if activities become contiguous, the dates
change per Figure 5.25, and the entire project is pushed back 2 days. CPM calculations
for such cases are similar to those described previously for interruptible activities, with
one exception: We must satisfy the following equation:

Dur = EF − ES = LF − LS

This can force the early dates to be pushed forward (to a later date) or late dates
to be pushed backward (to an earlier date). In the previous example (Figure 5.24), we
find that the early dates for activity B violate the preceding equation:

Dur = 6 < ( EF − ES )

We have to move one of the two dates. Moving the EF date to day 8 will satisfy
the equation but will violate the FF relationship between B and A. The only choice we
have is to move the ES date to day 4. Then, the early dates for activity B are (4, 10).
The same argument applies to late dates. All three activities in this case are critical and
have no float of any type.
To formalize the rules, we can say that activities with a combination (SS and FF)
relationship, say A and B, for example, will have two sets of early dates and two sets
of late dates. One set will prevail:

a. In the forward pass, the SS relationship (plus lag if any) determines the ES 1
date for activity B, which is ES (for A) + 2 (lag) = 2. The EF 1 for B is calculated
as EF 1 = ES 1 + Dur = 2 + 6 = 8. The first set of early dates for activity B is
(2, 8).

Figure 5.25 Network in Figure 5.24 with contiguous activities