Using the Start On Or Before constraint in Primavera P6. Projects generally come with finish date restraints, i.e. deadlines. There are scenarios though when a start date constraint better suites the schedule narrative. In general, activity durations and relationships should drive the start and finish of all schedule activities. But sometimes you want to emphasize the importance of a particular date in the life of the project.
In these instances, a constraint can add clarity to the true project situation. Primavera P6’s Start On Or Before (SOOB) constraint, in particular, confirms the start of an effort. It puts a binding restriction on the latest a project activity or phase can start. If the schedule shows a missed SOOB the project manager may decide to halt the respective effort before it begins.
This article demonstrates the utility of the SOOB constraint in Primavera P6 Professional.
The SOOB is the more conservative sibling of the Finish On Or Before (FOOB) constraint. The FOOB restricts the finish date of a project, while the SOOB says the project should not even begin beyond a certain date. If you miss a FOOB date, that’s good to know, but it may be too late; once commenced the effort may have to proceed to conclusion to the project’s detriment.
Missing the FOOB constraint date may adversely impact the quality of the end product. The quality of the product may also be negatively affected by missing the SOOB, but that’s OK. If a schedule indicates a missed SOOB the associated project effort can be aborted even before it begins. Thus, avoiding the risk of a poor quality end product. Let’s explain with a demonstration.
We have in Figure 1 our demonstration project.
Figure 1
This project specifies the installation of underground piping across a roadway. The project excavates a trench across the road, installs piping, then backfills the trench & repaves the roadway. It’s the roadway repaving that is the critical issue in this project. Asphalt paving is negatively impacted by cold winter conditions.
Our project takes place in Canada where asphalt work is not recommended beyond the last week in November. In light of this limitation on asphalt paving it would be good to insert a SOOB constraint on this paving effort. The SOOB is a better fit than the FOOB. If the schedule indicates a missed SOOB date the paving effort is stopped before it begins.
If the schedule has a missed FOOB date it may already be too late to stop the asphalt paving effort. The SOOB, again, provides a less risky and more conservative restriction than the FOOB.
Let’s proceed and insert a SOOB constraint on the asphalt paving roadway activity, Figure 2.
Figure 2
Select asphalt paving roadway activity and in the bottom details choose the status tab. In the constraints section set the primary constraint to SOOB. The constraint date is set to 24-Nov-2019, Figure 3, which is the last Sunday in November. Our constraint stipulates that he roadway paving must begin on or before 24-Nov-2019. If the schedule indicates a roadway paving later than 24-Nov-2019 the schedule requires adjustment.
Well, our SOOB constraint on asphalt paving roadway is generating six days negative total float on upstream activities. Observe the total float column in the activity table, Figure 3.
Figure 3
We also can activate the negative float bar, Figure 4.
Figure 4
Figure 5 displays the schedule and negative float bar on the Gantt chart.
Figure 5
So our schedule indicates negative total float, which is a dilemma. We, however, have a few options that include:
- Starting the project sooner
- Speeding up or fast track upstream activities
- Delaying the roadway paving till spring
- Delaying the entire project till spring
- Or any combination of the above
If fast tracking the upstream activities does not gain enough time, perhaps, it would be best to delay the entire project till spring. You would not want to leave a roadway unpaved for an extended length of time.
Summary
The ideal is for activity durations and network logic to drive activity start and finish dates. Sometimes though the situation calls for specifying an important project date. In our demonstration asphalt paving is seasonally restricted. And limiting the start date of asphalt paving instead of the finish date provides the project a more conservative restraint.
The SOOB constraint suites this restricted start date situation well. Thus, the SOOB constraint has its place in Primavera P6 Professional’s toolbox of schedule constraints.