The Negative Impact of P6 Activity Constraints
The Defense Contract Management Agency (DCMA) says that no more than 5% of tasks in the schedule should have hard constraints. This can seem lenient to some, because even soft constraints in the schedule can cause major critical path discontinuities, which is problematic.
The schedule should have one unbroken chain of critical tasks from project start to conclusion. The problem with P6 activity constraints is they cause breaks in the critical path. In this predicament the scheduler has no map of the shortest path through the schedule.
It is important to know the critical path for monitoring progress. In relation to schedule updates one of the attributes of a quality schedule is that it has a stable critical path that does not shift as progress is entered.
P6 activity constraints break up the critical path so that sections of it disappear. This is a worse situation than an unstable critical path, as your roadmap to achieve the schedule duration target is hidden. Hidden critical paths also prohibit schedule compression efforts.
Often, you shorten activities along the critical path to optimize the schedule. If these critical activities are unseen than your schedule optimization efforts become like blindly throwing darts at a dart board. The activities that can contribute to a shorter duration schedule are hidden from site.
This article demonstrates the negative impact of P6 activity constraints on the critical path through the schedule and proposes remedies when an activity constraint is a must.
If Figure 1 we have our demonstration Underground Pipe Branch Addition project schedule.
Let’s first examine the effect of a Mandatory Finish constraint on the Install Valve & Piping activity, Figure 2, which specifies that the installation of the valves and piping must complete (no exceptions) on Friday, June 24, 2024.
Note that testing the pipe at pressure is the final task to confirm the proper installation of the piping. After recalculating the schedule with the Mandatory Finish constraint, the most apparent issue is that now all activities in the schedule are critical, so our true longest path through the logic network is hidden, Figure 3.
Normally this issue is addressed by modifying the definition of critical activities in the schedule options. Critical activities were originally defined by Total Float. In Figure 4 schedule options we change this to define critical activities as Longest Path.
This, however, results in a fragmented Longest Path, Figure 5, which is not good.
Further, if we zoom into the relationship between Install Valve & Piping and Test Piping at Pressure, Figure 6, we find that the Finish-to-Start (FS) relationship between Install Valve & Piping and Test Piping at Pressure is violated.
To honor this FS relationship Test Piping at Pressure cannot begin until the conclusion of Install Valve & Piping, but, as Figure 6 displays, Test Piping at Pressure begins out of sequence. So, our Mandatory Finish constraint comes with a few issues: we lose the Longest Path, and we violate network logic.
Let’s ease up on our constraint restrictions and change the Mandatory Finish to a non-mandatory hard constraint, a Finish On or Before activity constraint, Figure 7.
When we make the Test Piping at Pressure activity constraint Finish On or Before and set Schedule Options ‘define critical activities as’ to Total Float and recalculate the schedule we lose sight of our sole Longest Path, Figure 8.
Too many activities have become critical, and we do not know the true Longest Path through the network logic. When we set the definition of critical activities to Longest Path for our Finish On or Before hard constraint, and recalculate the schedule, we find that we regain our true Longest Path through the network, Figure 9.
So, reducing the mandatory constraint to a less restrictive hard constraint helped to minimize the negative impact on our critical path and network logic. And the Finish On or Before activity constraint does not violate network logic, which is good.
Let’s now remove the Finish On or Before hard constraint. And next, we want to investigate the impact of a soft constraint on our schedule’s Longest Path. The scenario is our Pipe Insulator is not available to work on our pipe installation project until on or after July 8th, 2024. We therefore restrain the Install Pipe Insulation effort to begin on or after July 8th, Figure 10.
Again, the ‘define critical activities as’ Total Float and recalculate the schedule. The resulting schedule displays in Figure 11.
The soft constraint, surprisingly, manifested a major discontinuity in the schedule between Test Piping at Pressure and Install Pipe Insulation, Figure 11. Let’s examine the critical path, again, when we modify the definition of critical activities to Longest Path. When we recalculate the schedule, the results appear in Figure 12.
A Longest Path definition did not resolve our critical path discontinuity, Figure 12. And our simple Start On or After soft constraint is more problematic than our Finish On or Before hard constraint.
What we glean from this demonstration is that an activity constraint, even a hard activity constraint, may be agreeable. But we must be careful to examine each activity constraint closely to inspect the effect each has on the schedule.
Avoid mandatory hard constraints as their negatives are twofold:
1) They either hide or fragment the critical path depending on the definition of critical activities
2) They may violate network logic
A hard constraint may be acceptable but examine its attributes to learn its potential effect on the critical path. We tend to think soft constraints should cause no critical path issues, but that simply is not the true characteristics of soft constraints.
The DCMA does not forbid activity constraints but limits them to no more than 5% of tasks in the schedule to encourage their judicious application. Activity constraints are also agreeable to the Naval Facilities Engineering Command (NAVFAC), provided they are contractually defined.
So, insertion of P6 activity constraints is acceptable but restrained by NAVFAC to ensure a meticulous usage. The scheduler needs familiarity with the activity constraints probable effect on the critical path before insertion.
Construction Kick-Off Meeting
Let’s demonstrate the careful insertion of an activity constraint in a way that preserves the Longest Path and models the true schedule situation. We want to represent the scheduling of a Construction Kick-Off meeting for the Underground Pipe Branch Addition project.
The Kick-Off meeting is where team members introduce themselves. It fosters a positive collaborative relationship between sponsor and subcontractor. And it lays out the rules of the road for subcontractor success. It may include an important preconstruction safety briefing among other topics. The meeting is strategically located in the schedule where execution of project construction should begin in earnest.
The Kick-Off meeting is difficult to schedule because all participating team members must agree to the date it takes place. Good practice dictates the early planning of Kick-Off meetings. In some industries, often the date is agreed upon months in advance. A last-minute change to the date of the scheduled Kick-Off meeting is prohibitive because availability of participants is usually not flexible nor easily secured.
So, in this example, the Kick-Off meeting is a fixed date in the schedule that does not shift with the ebb and flow of other schedule activities, i.e., efforts.
In our Underground Pipe Branch Addition project, the Construction Kick-Off Meeting succeeds the Mobilize on Site effort, Figure 13.
The Kick-Off Meeting therefore follows the proper sequence of events. We also want to schedule it to take place on Monday, June 10th, 2024. Well, the natural flow of the network logic positions it directly after Mobilize on Site the week before on Wednesday, June 5th. Therefore, to hold the Construction Kick-Off Meeting at our desired place in the schedule we restrain it with a Monday, June 10th, Start On activity constraint, Figure 14, which is a hard constraint.
We recalculate the schedule, and the resulting network logic displays in Figure 15.
Well, the activity constraint generated a discontinuity between Mobilize on Site and Construction Kick-Off Meeting. We want to adhere to the requirement for the Longest Path to continue uninterrupted from project start to project complete.
To achieve this unbroken critical path, we have two options:
1) Modify the FS relationship between Mobilize on Site and Construction Kick-Off Meeting by inserting a 3-day lag
2) Extend the Mobilize on Site effort till Construction Kick-Off meeting naturally falls on the Monday, June 10th activity constraint date
Include Lag on Mobilize on Site
We proceed and insert a 3-day lag on the FS relationship joining Mobilize on Site and Construction Kick-Off Meeting.
We recalculate the schedule, and the resulting critical path displays in Figure 17.
And we have good news! Our Longest Path is restored. So, adding lag to soak up Total Float and remove a discontinuity is a viable option. Be forewarned though that any update to an upstream activity duration will require an update to this lag.
Let’s demonstrate this. If for some reason Site Survey’s duration estimate is updated to take 1-day longer, all activities on the critical path shift right 1-day, including Construction Kick-Off Meeting, Figure 18.
These affected activities now have 1-day negative total float. That is not good! This update requires us to modify the lag on Mobilize on Site; we must reduce the lag to 2-days. When we recalculate the schedule the Kick-Off Meeting lines up on its planned date, again.
Extend Mobilize on Site
Proceeding in our demonstration, we remove the 3-day lag on Mobilize on Site and reduce the Site Survey duration estimate back to 1-day. These changes set our schedule prior to the lag insertion. We proceed with option 2 and extend the Mobilize on Site effort to 4-days to eliminate its 3-day total float, Figure 19.
Again, we recalculate and find the schedule has a continuous critical path, Figure 20.
And, again, if an upstream activity duration is extended or delayed, we would have to modify the Mobilize on Site activity duration estimate, accordingly. This is, again, to realign the start of the Construction Kick-Off Meeting with the activity constraint.
Both the Mobilize on Site duration estimate extension and the Mobilize on Site FS lag insertion worked to restore the critical path. But I favor the Mobilize on Site duration estimate extension approach because the Mobilize effort is more fluid and possibly could take anywhere from one to four days. And the lag extension method is less transparent, as the lag appears as a simple line on the Gantt chart with no label.
This may leave stakeholders wondering what is really taking place between the Mobilize and Kick-Off meeting efforts. With the Mobilize on Site duration extension it is apparent that Mobilization is taking place during this four-day block of time leading up to the Kick-Off meeting.
P6 Activity Constraints Summary
P6 activity constraints come in three levels of increasing severity: soft, hard, and mandatory. Avoid mandatory activity constraints for two reasons:
1) They are prone to lose the Longest Path
2) They may violate network logic.
Both hard and soft activity constraints may break up the critical path. In some situations, a soft constraint may be more problematic to the Longest Path than a hard constraint.
A limited number of activity constraints (less than 5% activity relationships according to DCMA) are acceptable. So, a suitable activity constraint is possible but requires close examination of the schedule situation. Therefore, limit the number of activity constraints in the schedule to a few, which allows for proper examination and monitoring of each one.
If an activity constraint is essential, extending the duration estimate of an upstream activity that has total float is the preferred method to restore the longest path. This avoids the lag, so the true schedule situation is more transparent to other team members and stakeholders not intimately familiar with the schedule details.
For further guidance on the insertion of schedule constraints refer to the blog Guidelines for Insertion of Schedule Constraints.