constraints

Constraints in | Primavera P6 | Project Management

What are the Constraints?

A constraint is a restriction or boundary on the start date or finish date of an activity. Constraining an activity’s start date or finish date is another technique used in Critical Path Method scheduling. Constraints are used to ensure that an activity follows a schedule or will occur before or after a certain date. The most used constraints are deadlines (no-later-than constraints) and potential delays (no-earlier-than constraints).

They can be attached to either the start or finish of an activity. For example, a deadline date can be put on the start/finish of activity (the activity must start or finish no later than the date specified) or a potential delay (the activity can start or finish no earlier than the date specified).

How do Constraints affect the CPM Dates?

Typically, the experienced scheduler or planner would first enter the project schedule on the software (Primavera P6, MS Projects, etc) without any constraints and examine the project purely on the CPM sequence.

Constraints are then added, one at a time — watching to see how each of them affects the entire schedule. Remember, constraints override the logic of the CPM.

If an activity includes a mandatory start constraint, that activity will start on that date regardless of the CPM logic and whether or not the predecessors are finished. Or, if the project is ahead of schedule, the constraint will not automatically adjust to a possible new early date. Again, constraints override logic, so they must be used with care.

Uses of Primavera P6 Constraints

Date constraints are most commonly used for deadlines and delivery dates. Not only do they affect the constrained activity, but also their predecessors and successors (and by extension the entire project). They should, therefore, be used sparingly. Too many constraints reduce the value of using CPM, as constrained activities can not be rescheduled as freely when other parts of the project are rescheduled.

Types of Project Management Constraints

1.Start Constraints

The Start Constraint specifies that an activity can start no earlier than, or no later than, a specified date. For example, if an activity can not begin until a piece of equipment has been delivered on a specific date, make that date an early start constraint. The activity may start on or after that date, as calculated according to the logic of the schedule, but not before. If an activity has to start by a certain date, attach a late start constraint.

2.Finish Constraints

Finish constraints are used to specify that an activity must be finished no earlier than or no later than a specified date. For example, if an activity has to be finished by a certain date, attach a late finish constraint. If an activity can start at any time but can not be finished until a specific date, possibly after a scheduled customer inspection, attach an early finish constraint.

3.Mandatory Constraints

Mandatory constraints force an activity to set a date for its start or Finish. No matter what happens, this activity will take place on a mandatory constrained date. Use the mandatory constraints with caution; check the dates of activity whether or not they are consistent with the logic of the schedule. An activity with a mandatory constraint could, therefore, be scheduled before its predecessor.

4.Start-on Constraints

Start-on restrictions are equivalent to applying both the “start-no-earlier-than” and the “start no-later-than” constraint to an activity. It sets the early and late start dates equal to the specified date but protects the logic of the schedule (unlike the mandatory start constraint) so that the activity can be either positive or negative.

5.Expected Finish Constraints

Expected Finish Constraints force the duration of an activity to depend on its scheduled completion date. Attach the expected finish constraint and the software will calculate the duration of the activity from its early start date to the expected finish date you specify. If the activity is underway, the software(Primavera P6, MS Projects, etc) will calculate its remaining duration as the difference between the data date and the expected finish date. Because it is calculated, the duration of the software is typically shown with an asterisk.

6.Float Constraints

Float constraints affect the scheduling of the activity, but, unlike date constraints, the logic of the schedule does not override the schedule. There are two types of float constraint: zero total float constraint and as late as possible float constraint.

7.Zero Total Float Constraint

The zero total float constraint causes the activity to be scheduled as soon as possible. It has zero total float. The activity will be scheduled to take place on the first dates it may occur and will appear to be critical. The float will disappear.

Types of Primavera P6 Constraints

  • Start On imposes the particular start date you choose. The Start On constraint can delay the early start of the activity or move forward the late start of the activity to satisfy the constraint date.
  • Start On or Before defines the latest date that an activity can start. This constraint only affects late dates & may decrease total float. When calculating a schedule, Primavera P6 imposes the start on or before constraint in the backward pass only if the calculated late start date will be later than the imposed date.
  • Start On or After defines the earliest date that the activity can begin. This constraint affects only the early dates. When calculating a schedule, Primavera P6 imposes the start on or after constraint in the forward pass only if the calculated early start date will be earlier than the imposed date.
  • Finish On imposes the specific finish date you choose. The Finish On constraint can delay the early start of the activity or move forward an activity’s late finish to satisfy the constraint date.
  • Finish On or Before defines the latest time an activity can finish. The finish on or before constraint affects only late dates.
  • Finish On or After defines the earliest date an activity can finish. The finish on or after constraint reduces float to coordinate parallel activities, ensuring that the finish of an activity is not scheduled before the specified date. It’s usually applied to the activities that have few predecessors that must finish before the next phase of a project.
  • As Late As Possible imposes a restriction on an activity with the positive float to allow it to start as late as possible without delaying its successors. When calculating a schedule, Primavera P6 sets the activity’s early dates as late as possible without affecting successor activities.
  • Mandatory Start imposes the early and late start dates you choose. Primavera P6 uses the mandatory early start date regardless of its effect on network logic. Mandatory early start date could affect the late dates for all activities that lead to constrained activity and all early dates for the activities that lead from the constrained activity.
  • Mandatory Finish imposes the early & late finish dates you choose. Primavera P6 uses the mandatory finish date regardless of its effect on network logic. This constraint affects the late dates for all activities that lead to constrained activity and all early dates for the activities that lead from the constrained activity.

Recommended: Primavera P6 Advanced Guide

See Also
DCMA 14 point schedule assessment
Schedule Percent Complete in P6
Retained Logic and Progress Override in Primavera P6
Out of Sequence in Primavera P6
Claim Digger in Primavera P6