Wingra Point Project Scheduling Network Diagram

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Using the Wingra Point project for the example, the following five figures will illustrate how to construct a Schedule Network Diagram using the critical path method. The critical path method is used to estimate the minimum project duration and determine the amount of schedule flexibility on the logical scheduled path to completion. The first step in developing a network diagram is to properly sequence activities. For this example, we will consider the project from start to construction of the foundation and footings and use key milestones as our activities.

In the above network diagram, Figure 7.13, you see that a box is set up for each activity. This is called Boxing the Network Diagram. Each square in the box represents the following values:

• Early Start: The earliest point in time the activity can start based on the schedule network logic and any schedule constraints.
• Early Finish: The earliest point in time the activity can finish based on the schedule network logic and any schedule constraints.
• Late Start: The latest point in time the activity can start based on the schedule network logic and any schedule constraints.
• Late Finish: The latest point in time the activity can finish based on the schedule network logic and any schedule constraints.
• Duration: How long the activity will take to complete from start to finish.
• Float: The difference between the late and early dates. Once the early and late start and finish dates have been determined, you can determine the float. For finish-to-start relationships, the difference between early and late start and early and late finish is the same. If you have a start-to-start or finish-to-finish relationship, you must take the respective differences and choose the lowest number as the float. There are two kinds of float:
  • Total Float: This float relates to the projects finish date. It is the amount of time that a scheduled activity can be delayed without delaying the finish date of the project or violating a schedule constraint.
  • Free Float: This float relates to the successor activity. It is the amount of time that a scheduled activity can be delayed without delaying the finish date of any successor or violating a schedule constraint.
• Path Convergence: This occurs when two tasks converge into the same successor.

The next step is to enter duration for each activity based on the information provided in the Excel file.

For this example, duration is represented as months. Activity A, Acquire Land, has a duration of 12 months for example.The next step is to conduct a forward pass.

Forward Pass

This is a technique for calculating the early start and early finish dates for each activity by working forward through the network diagram. It is common for scheduling software, by default, to assume a 5-day business week and a workday that starts at 8 am and finishes at 5 pm. For purposes of this example, it is easier to assume that the diagram starts on day zero (0). Doing this avoids subtracting 1 in the forward pass and adding 1 in the backward pass.

For example for Activity F, notice that the longest duration, 60 months, is the one that is used for the early start vs. Because the critical path for Activity F is the longest duration, that is the shortest path to completing the project, milestone, or phase.

The next step is to conduct a backward pass.

Backward Pass

This is a technique for calculating the late start and late finish dates for each activity by working backward through the network diagram.

As 60 months is the latest possible date that the last activity, F, can start, it is passed back as the late finish for all three preceding activities. Note, if the difference between the late finish and early finish and the late start and early start are not the same, there is an error in your backward pass.

The last step is to calculate float.

Calculate Float

Once the early and late start and finish dates have been determined, you can determine the float. Float is the difference between the late and early dates. For finish-to-start relationships, the difference between early and late start and early and late finish is the same. If you have a start-to-start or finish-to-finish relationship, you must take the respective differences and choose the lowest number as the float.

You can see for example that the total float for Activity B is 42 months. Plainly stated, Develop Site Plan (Activity B) can be delayed by 42 days and it will not delay the early start of Activity F.

Critical Path

The final step is to analyze the network diagram to identify the critical path. This is the path with the least amount of float. Add up the float values for each of the paths that go from start to finish. The path with the lowest number is your critical path. If you wish to shorten the overall duration of your project, you must reduce the length of the lags for the activities on the critical path. The critical path is the sequence of activities that represents the longest distance path through the project, which determines the shortest duration.

Wingra Point Project Scheduling Network Diagram