Class 107: Activity Constraints

August 16, 2021 0 Comments

What are Activity Constraints?

An activity constraint is a restriction that is placed on an activity to control the activity’s start date, finish date, and/or duration. Common examples of activity constraints include start-no-earlier-than, finish-no-later-than, mandatory start, and as-late-as-possible.1

Activity constraints are often utilized in construction schedules to identify the contractual completion date. Often called “Substantial Completion,” this activity is rarely the last activity in the schedule. However, placing a constraint on the “Substantial Completion” activity prioritizes the work sequence for the project and defines the schedule’s critical path. Activity constraints can also be applied for financial restrictions, material delivery dates, or crew availability.

Other constraints that can be applied to the schedule include: “(1) minimum time constraints to create a time buffer between two activities; and (2) minimum distance/location constraints to denote the minimum physical distance between two activities to allow for the proper or safe execution of linear construction activities.”2 These constraints apply more to project management rather than to project scheduling, but they are often incorporated into a schedule by using activity constraints.

Example of an Activity Constraint:

In the example above of a schedule for a high-end mixed-use skyscraper, the schedule utilizes a finish constraint on the activity called “Initial TCO.” The asterisk on the finish date of 02-Mar-18 indicates the activity’s finish date is constrained. While this activity is not the last in the schedule, nor is it a Substantial Completion activity, the constraint indicates “Initial TCO” is likely a contractual completion milestone. Therefore, the critical path of the project is calculated by analyzing the schedule logic to the “Initial TCO” milestone.

Why Constrain an Activity?

An activity constraint can be applied to define a certain activity sequence to achieve a specific milestone date. Large schedules can often have multiple concurrent critical paths, so applying a constraint to one or more milestone can help prioritize a particular activity sequence. Constraints are also helpful for documenting impacts to the schedule, such as weather impacts and major changes.

Sometimes, the presence of an activity constraint, particularly a finish constraint on Substantial Completion, can cause a critical path to have negative float, meaning the critical path has been delayed to the extent that achieving the constrained activity on time is infeasible. In this case, the negative float value indicates the number of days that the critical path is behind schedule as defined by the activity constraint.

The most common activity constraints are as follows:

• As Late as Possible

• Mandatory Start & Mandatory Finish

• Start On, Start On or Before, & Start On or After

• Finish On, Finish On or Before, & Finish On or After

Activity Constraint Summary:

1. An activity constraint is a restriction placed on an activity to control the activity’s start and/or finish date. 

2. An activity constraint is most often used to define a contractual completion date that must be achieved and must be on the critical path.

Key Terms:

Activity Constraint a restriction that is placed on an activity to control the activity’s start and/or finish date to indicate a date that will not change, regardless of other changes to the schedule

Critical Path the longest continuous chain of activities which establishes the minimum overall duration

Floatamount of time an activity can be delayed without impacting the overall project



1. 10S-90: Cost Engineering Terminology.

2. Bramble, Barry B. , and Callahan, Michael T. Construction Delay Claims. Wolters Kluwer Legal & Regulatory U.S., 2017.