Precedence Networks - Construction Project Scheduling and Control - page 115

Remedy for Interruptible Activities

As mentioned earlier, the interruption in the activity duration meant in this context is
a planned one; not stoppage due to unpredicted and/or uncontrolled circumstances.
Some CPM software programs allow the user to choose between contiguous and inter-
ruptible activities. However, unfortunately this choice is on “all or none” basis, thus,
it is safer to stay with the contiguous option then deal with interruptible activities on
a case by case basis.
As you saw earlier in the interruptible activities case, an activity may have a critical
start but a flexible finish, or vice versa. In other words, it may have a time slot to fill
that is larger than its duration (see many examples in Figures 5.12 through 5.18). The
difference in this time slot (LF−ES) contains the duration plus the restricted float.
If this activity becomes contiguous, the restricted float disappears and the activity is
pushed forward or backward (depending whether its finish or start was critical). We
proved in Example 5.2 that going from interruptible to contiguous type of activity may
cause a later completion date for the schedule. For the case shown in Figure 5-25, two
solutions are suggested:

2 3

0, 10

5, 12

2, 10

A
10

B
6

C
7

0, 12

2, 12 5, 12

Figure 5.29(a) Network with interruptible activities

2 3

0, 10

7, 14

4, 10

A
10

B
6

C
7

0, 10

4, 10 7, 14

Figure 5.29(b) Network with contiguous activities

1. Stretch the activities that have restricted float till they fill the “restricted dura-
tion”, i.e., eliminate restricted float. Unrestricted float (where the entire activ-
ity can move as one unit) can stay without being absorbed. In this case, the
contractor has to reallocate (reduce) resources so the total resource allocation
stays the same after stretching.
2. Split the activities that have restricted float. The duration of each portion
depends on the dependency between that portion and the predecessor or
successor, which usually equals the lag between them. In our case, activity