June 11, 2015
The first step in developing a schedule is the planning stage, which involves listing all activities – defined as tasks that take time to perform – and arranging them in a logical sequence. A list of construction-related activities is typically generated by reviewing the project's contract documents and procedures to be used in administering the construction. Then, after discussing the project with project personnel, the work should be broken down into individual work areas, or subproject categories. For example, in a typical office building these major "breakdown" categories could include, among others, the following:
- major procurement items
- site work
- underground piping and utilities
- foundations
- structural steel
- floor systems
- roofing
- electrical
- plumbing
- mechanical
- architectural interior finishes
Each of those major categories can then be further subdivided for a greater level of detail. Then the listing of activities can begin. In CPM, determining significant activities and the degree of detail to use in planning is a challenge. A basic rule of thumb is that the number of activities should not be so small as to prevent a suitable tracking mechanism nor so large that the sheer number of activities overwhelms the user and defeats the purpose of the schedule. In general, when determining the number of activities to include, one should consider the following factors:
- Number of bid packages
- Type and complexity of the project
- Structural breakdown of the job
- Equipment deliveries (employer-furnished and other)
- Labor trades performing the work
- Shop drawings (submittals and approvals)
- Interfaces with third parties as well as with other contractors and the employer
- Subcontracting work
- Testing and close-out
- After drawing up the list of activities, one creates the network logic. To create the proper sequence for the CPM diagram, one must constantly consider:
- What other activity(ies) must be completed before this activity can start?
- What other activity(ies) can be going on while this activity is under way (i.e., current)?
- What activity(ies) cannot start until after this activity is completed or, at least, under way?
The use of milestones (usually shown as imposed start or completion dates on portions or all of the work) may dictate to the contractor, within limits, the sequence of performance. Otherwise, the contractor should be allowed to sequence its work in the most desirable and advantageous manner to the contractor. The resulting series of logic networks for each category of the project, often referred to as "fragnets" (fragmentary networks), are then linked together to form an overall schedule, which show the relationships among all the tasks required to complete a project. Each activity event on the network is given a narrative description. Activities are also identified by an event numbering system.
After the elements of the overall plan discussed above are accomplished, it is reviewed and revised as necessary, and the planning stage ends.