Basic Networks - Construction Project Scheduling and Control - page 34

B E

F

J

A C G

PF

K

H

M

D I L

Figure 3.15 Solution for example 3.7 as a node network

through 3.7 as node diagrams. Figures 3.11 through 3.15 show the results. As shown
in Figure 3.15, by using a node diagram, we can solve Example 3.7 without using the
annoying eight dummy activities.

Lags and Leads

In some situations, an activity cannot start until a certain time after the end of its
predecessor. A typical example is concrete operations. Let us imagine this sequence:

1. Form the concrete column.
2. Install the steel reinforcement (commonly known as rebar ).
3. Place the concrete.
4. Wait for the concrete to set (attain sufficient strength).
5. Strip the forms.

Note that the fourth step is not a “real” activity to which we must allocate
resources and a budget. It is merely a waiting period, commonly known as a lag .
A node network can accommodate such a lag if we simply put the lag on the
relationship line between Place Concrete and Strip Forms, as shown in Figure 3.16a.
This 3-day lag means a minimum waiting period of 3 days. Waiting less than 3 days
violates the preceding logic, whereas waiting more than 3 days does not violate the
logic. In some networks, the lag number is put inside a little box for better visibility.
Thus, a lag is defined as a minimum waiting period between the finish (or start)
of an activity and the start (or finish) of its successor. Arrow networks cannot accom-
modate lags. The only solution in such networks is to treat it as a real activity with a
real duration, no resources, and a $0 budget (Figure 3.16b).
With arrow networks, an activity is defined as “a unique unit of the project
which can be described within prescribed limits of time” (Harris, 1978, p. 18) or
“a time-consuming task” (Callahan, Quackenbush, and Rowings 1992, p. 29). Note