What activities must precede the activity being examined? What activities can be concurrent with this activity? What activities must follow this activity?

WORKING ACTIVITIES. - With respect to a given activity, these representations indicate points in time for the associated activities. Although the boxes in the precedence diagram represent activities, they do not represent time, and therefore, they are not normally drawn to scale. They only reflect the logical sequence of events.

CRITICAL ACTIVITIES. - A critical activity is an activity within the network that has zero float time. The critical activities of a network makeup the longest path through the network (critical path) that controls the project finish date. Two slash marks drawn through an activity connector, as shown in figure 2-26, denote a critical path.

The rule governing the drawing of a network is that the start of an activity must be linked to the ends of all completed activities before that start may take place. Activities taking place at the same time are not linked in any way. In figure 2-24, both Activity 2 and Activity 3 start as soon as Activity 1 is complete. Activity 4 requires the completion of both Activities 2 and 3 before it may start.

Figure 2-26. - Designation of a critical path.

Use of Diagram Connectors

Within a precedence diagram, connectors are lines drawn between two or more activities to establish a logic sequence. In the next paragraphs, we will look at the diagram connectors commonly used in the critical path method (CPM).

FINISH to START connector, as shown in figure 2-27, is the most widely used connector in a precedence diagram. This connector represents the finish of one activity and the start of another. In the activity block there are boxes labeled EARLY FINISH (EF), LATE FINISH (LF) and EARLY START (ES), LATE START (LS), which is very critical in computing the forward pass and backward pass. These terms are covered later in this section.

START to START connector, as shown in figure 2-28, represents activities that can either parallel each other or that may start at the same time or be delayed a day or two. For example, pre-fab forms can start at the same time as building layout starts or setting forms can begin at the same time as excavation, but you might want to delay the setting of forms by 1 day for the EOs to excavate and to make sure they are clear of where your crew is working.

FINISH to FINISH connector, as shown in figure 2-29, is possible when the start of an activity is independent but is not completed before another activity is completed. Exercise care when you use this logic connection. If possible, the FINISH to START connector should be used for all planning.

Figure 2-27. - Finish to start connector.