Do you use late-date constraints? Activity constraints define important dates in the life of the project. But too many constraints make a schedule rigid and not dynamic.
Most scheduling software provides the scheduler an assortment of constraints to further define a date’s importance. These dates are typically important milestones in the project including project reviews, decision points, delivery dates, and project completion. Overrunning these milestones is not an option.
To force these events to occur by preferred dates the project manager may direct the scheduler to insert ‘finish on or before’ (FOOB) activity constraints to hold these events in place. These constraints affect the ‘late dates’ in critical path method (CPM) scheduling. Any schedule that overruns these constraints will indicate negative float, which is unacceptable.
This article demonstrates proper usage of late-date constraints.
Constraints are agreeable to most scheduling guidelines. But too many constraints makes for a rigid schedule. Constraints should therefore be used sparingly. The Naval Facilities Engineering Command (NAVFAC) guideline says all constraints must be contractually designed, which significantly limits their usage.
Though wide adoption of constraints is discouraged, constraints, however, serve a useful purpose in schedule development. In the planning stages they tell whether need dates are feasible. Missing these key dates generates negative float in the schedule.
During the planning stages of the project it is helpful to hold need dates in place with activity constraints. The scheduler then examines the impact of need dates on negative float. To eliminate negative float the scheduler must change logic, add resources, or shorten activity durations. After the scheduler adjusts the schedule to address negative float the activity constraints can be removed. The correct or preferred dates then calculate, accordingly, without the constraints.
Below in Figure 1 is our constrained demonstration project.
Figure 1
This is a tile manufacturing and installation project. Note, in particular, the FOOB activity constraints on ‘receive tiles on site’ and ‘installation’. Both these constraints are generating negative float in the schedule, as evidenced by the yellow negative float bars on the Gantt chart.
Also observe the negative total float in the activity table total float column. So the constraints are indicating that our plan misses the preferred key dates. This is helpful. The first step in resolving negative float is to see its effect on the schedule.
As mentioned above, we have several options for resolving this negative float, including shortening activity durations. We decide the best way to meet the ‘receive tiles on site’ constraint is to speed up production of ‘manufacture tiles’. In Figure 2 we highlight ‘manufacture tiles’ and review its original duration, which is 10-days.
Figure 2
In Figure 3 we adjust the ‘manufacture tiles’ original duration to 7-days and recalculate the schedule.
Figure 3
Great! Our schedule now meets the ‘receive tiles on site’ need date, as displayed in Figure 3. Note the zero total float for ‘receive tiles on site’ in the activity table.
We still have negative float in our schedule due to the FOOB constraint on the installation activity. To correct this installation related negative float we adjust the original duration of ‘mobilize on site’. We highlight ‘mobilize on site’, Figure 4, and review its original duration, which is 7-days.
Figure 4
We shorten the original duration of ‘mobilize on site’ to 5-days, and recalculate the schedule, Figure 5.
Figure 5
Our resulting schedule is free of negative float. We then remove the activity constraints, Figure 6, and recalculate the schedule.
Figure 6
And the schedule now calculates the correct need dates but without constraints.
Summary
Late-date constraints help specify important dates in the schedule. But their rigidity effects the dynamic quality of the schedule, and, therefore, constraint usage should be limited. Broad adoption of constraints, however, is helpful in the planning stages of the project to support schedule adjustments to meet key dates.
If need dates are feasible, the negative float free schedule will support these preferred dates. At this point the scheduler can remove unneeded constraints and the desired need dates calculate without constraints. So broad adoption of constraints in planning a project is helpful, and limited usage appropriate in finalizing the project schedule.