Precedence Networks - Construction Project Scheduling and Control - page 109

A forth float option, which activity C does not have, is to start early and finish early
on day 3. It cannot finish before activity B is finished.
Looking at Figures 5.14 through 5.16, we find activities A and B in Figure 5.16
with start-restricted floats . Activity C in Figure 5.14 and activities B and C in
Figure 5.15 have finish-restricted floats .
Activity B in Figure 5.24 must start on day 2 and must finish on day 10. We
note that it has only 6 days' duration but must fill an 8-day time interval. We call the
difference (8 − 6 = 2) start-finish-restricted float , or, for short, double-restricted float.
Activity B in Figure 5.11 has unrestricted float. The work crew can use this float
in one of three ways:

1. Start the activity on day 0, work nonstop, and finish early on day 5.
2. Start the activity on day 0, finish a certain portion, take a break (or breaks) of
1to5days,andreturnandfinishtheactivitybyday10.
3. Delay the start by 1 to 5 days, work nonstop, and finish on or by day 10.

This type of float is similar to the total float discussed in Chapter 4. We call it
unrestricted float. Total float is the combination of all types of float, whether restricted
or unrestricted.

Definitions

Start-restricted float : The amount of time we can delay the rest of the work in the
activity, after it has started on its critical start date, without delaying the entire
project.
Finish-restricted float : The amount of time we can delay the start of work on the
activity, or the rest of it if it has started, then finish on its critical finish date without
delaying the entire project.
Double-restricted float : The amount of time we have to delay the rest of the work
in the activity after it has started on its critical start, then finish on its critical finish
date without delaying the entire project. It happens with combination (SS + FF)
relationships; and both of them are driving.
Unrestricted float : The amount of time we can delay all or part of the activity
without delaying the entire project.

The Equations

For calculation purposes, let us put both start-restricted float and finish-restricted float
under one category, single-restricted float ( SRF) , as compared with double-restricted
float (DRF) or unrestricted float (URF) . As mentioned previously, in this chapter,
total float (TF) includes all types of float, restricted or unrestricted.
For calculations, follow these steps:

1. Calculate the DRF:

DRF = max[( EF − LS − Dur ) , 0] (5.1)