Using a Constraint in Primavera to Use Up Free Float

Written on . By Tracy Mah

There are times in your schedule when you need to use a constraint to delay an activity to start as late as possible without affecting any activities after it. For example, if you wanted to delay spending/costs, you will want to order the parts as late as possible. In Primavera P6, constraints are used to manipulate the start or finish of individual activities.

First off we’ll go through some definitions:

Constraint = An externally imposed factor affecting when an activity can start and/or finish. In Primavera P6, each activity can have up to 2 constraints on it.

Total Float (TF) = The maximum amount of time an activity can be delayed from its early start without delaying the entire project. Late Date – Early Date = Total Float

Free Float (FF) = The maximum amount of time an activity can be delayed without delaying the early start of the succeeding activities

When a project is scheduled, Primavera undergoes some calculations that determine the earliest and latest dates each activity can start and finish. From this calculation, it determines the Total Float for each activity. The Total Float is then used to determine the ultimate date the project can finish at. Projects along the critical path are usually the sequence of activities in the project that will take the longest time to complete (ie, longest path).

Here is a sample situation:

Lets concentrate on Activities A, B and C for now.
– In this situation, Activity C can’t start until both Activity A & B are completed
– We see here that Activity A has a duration of 15 days, while Activity B will only take 5 days to complete
– As we can see, Activity A will determine when Activity C can start, which won’t be until the start of Day 16.
– Activity A has a Total Float=0 and Free Float=0
– Activity B has a Total Float=10 and Free Float=10. What this means that Activity B can start anytime between Day 1 to Day 10 without it delaying Activity C from starting on Day 16, ie, Activity B’s start date can freely float for 10 days

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Let’s add a constraint on Activity B and Activity F to delay the start of each.

In Primavera P6 Windows Client: simply go to the <Activities> screen and highlight the activity. Then under the <Status> tab,<Constraints> section, open the drop down menu and select <As Late As Possible>.

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In Primavera P6 Web Access: Simply go to the <Projects> section, <Activities> tab and highlight the activity. Then under the<General> tab, <Constraints> section, select <As Late As Possible>.

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Now remember, your schedule won’t change until you <Schedule> your project. Let’s see what happens to Activity B and Activity F:

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Notice how both Activity B and Activity F, are delayed to use up all the Free Float available.

Primavera P6 has several other types of constraints that you can use to change the start and finish dates of your activities.

Learn more about Primavera P6 Project Management software.

Posted in Primavera, Scheduling, Web Access