Basic Networks - Construction Project Scheduling and Control - page 26

Example 3.2

Draw the arrow network for the project given next.

Activity IPA

A -

B A

C A

D B, C

Solution A

The main problem in this case, as shown in Figure 3.2, is that both activities
B and C start from node 20 and finish at node 30. Hence, both are identified
as 20-30.
This situation will create an identity problem. To solve this problem, we
introduce a fictitious activity and an additional node. This fictitious activity is
called a dummy activity , d. It is treated in the critical path method (CPM)
calculations (discussed in Chapter 4) and in computer programs as a real
activity even though it is not.

Figure 3.2a Improper solution for example 3.2

Solution B

In this case, activity B is identified as 20-30, whereas activity C is 20-40,
as shown in Figure 3.3. Note that the dummy can be inserted in different
positions, as shown next. They all serve the same purpose.

Figure 3.2b Proper solution for example 3.2