Precedence Networks - Construction Project Scheduling and Control - page 108

THE DETAILED APPROACH
Unlike the arrow and node diagrams in Chapter 4, precedence diagrams may show
the start or finish of an activity to be critical while the rest of the activity is not. This
is the situation in Figures 5.12 through 5.21. Let us add a twist to this scenario, as
shown in Figure 5.24.
In this case, both activities A and C are completely critical. Activity B has a critical
start and a critical finish but is not critical between.
Looking at Figures 5.12 through 5.24, we also notice that, contrary to the arrow
and simple node diagrams in Chapter 4, the duration of an activity may not always be
equal to its EF − ES or LF − LS . What does this mean? In Figure 5.17, for example,
activity A must start on day 0; otherwise, the start of activity B, which is critical, will
be delayed. The only restriction on the finish of activity A is that it should not delay
the LF of activity B, which is day 7. This leaves activity A with 7 days, but it has only
5 days' duration. Therefore, activity A has 2 days of restricted float , which we explain
later in this chapter. The crew for activity A has three options:
1. Start the activity on day 0, work nonstop, and finish early on day 5.
2. Start the activity on day 0, perform a certain portion, take a break of 1 or 2
days, and then resume and finish the activity by day 7.
3. Reduce production rate (by reducing crew size or work hours/day) so the
duration stretches from 5 to 7 days.
A forth float option, which activity A does not have, is to delay its start. If its start is
delayed, the start of B will be delayed, which will delay the whole project. This is why
we call it restricted float . This particular type of restricted float is called start-restricted
float. Looking at activity C in Figure 5.17, we also find that C has 4 days of float in its
start. However, no matter when it starts, it must finish on day 7; not before, not after.
We conclude that activity C has 4 days of restricted float, or finish-restricted float. The
work crew has the following three options:
1. Start the activity on day 0, finish a certain portion, take a break (or breaks) of
1 to 4 days, and return and finish the activity by day 7.
2. Delay the start of the activity till day 4, work nonstop, and finish on day 7.
3. Reduce production rate (by reducing crew size or work hours/day) so the
duration stretches from 3 to 7 days.
Figure 5.24 Network for the detailed approach