Basic Networks - Construction Project Scheduling and Control - page 25

Brief Explanation
In example 3.1, the project has only five activities, each represented by an arrow, as
shown in Figure 3.1. Each arrow connects two nodes (depicted by circles with num-
bers in them): the from node and the to node . These nodes represent events: an event
is a point in time when an activity starts or ends. In computer software, events with
significance can be created as milestones , and they are either start milestones, such as
Notice to Proceed, or finish milestones, such as Substantial Completion.
The Logic
In example 3.1, activity A starts the project. Activities B and C follow, but inde-
pendently. In reality, depending on the availability of resources and other factors,
activities B and C may occur concurrently, overlap, or occur consecutively. However,
both B and C cannot start until A is complete. Activity D must wait until activity B is
complete. Once both C and D are complete, activity E can start. The end of activity E
means the project is completed. Activity A is considered a predecessor activity to
activities B and C. 1 Similarly, activity B is a predecessor to activity D. Conversely, we
can say that activities B and C are successor activities to activity A, activity D is a suc-
cessor to activity B, and so on. Activity A has successors but no predecessors. Activity E
has predecessors but no successors. All other activities have both successors and
predecessors.
As mentioned previously, a node in an arrow network represents an event or a
point in time. This event is the starting or ending point of an activity (or activities).
Node 10 represents the start of activity A (and hence the start of the project). Node 20
represents the end of activity A and the start of activities B and C. Node 30 represents
the end of activity B and the start of activity D. Node 40 represents the end of activities
C and D, and the start of activity E. Node 50 represents the end of activity E and the
project.
Notation
The arrow diagram is also called the I-J method because each activity is identified by
the two nodes that define its start and end. For example, activity A is also known as
10-20, B as 20-30, C as 20-40, and so on.
Dummy Activities
Let us consider a simple project.
1 If you think of the activity as a college course, the predecessor(s) is similar to the prerequisite course(s).