When do you apply hard or soft constraints in Primavera P6? This is a question I had recently while working with a client and while there are no hard and fast rules as project specific circumstances can differ, here are some guidelines that may help. Many times the project schedule is influenced by certain constraints on the project. These constraints can be contractual, external, or internal. An explanation and example of each follows:
- Contractual constraints are typically from legal agreements written in the contract. An example is a delivery date in the contract – must be met and if missed will result in some financial or other penalty.
- External constraints are constraints that the project manager has little or no control over, such as weather or environmental constraints. Examples include:
- Winter shutdown periods – A dredging company must be out of the water before November 15th, when the lake or river freezes.
- Fish spawning season – legally construction work in a river or stream cannot happen during fish spawning season in some states.
- Internal constraints are constraints that the project manager may have some control over. An example is resource limits – only finite availability of particular skills to perform the work.
This article examines the difference between applying a Finish On constraint and a Mandatory Finish constraint in Primavera P6.
Mandatory contractual or external constraints may warrant the use of harder constraints in Primavera P6. However, it may be safer to utilize a softer Finish On constraint instead of the harder Mandatory Finish constraint.
Primavera P6 – Soft Constraint
Figure 1 displays a project with four tasks labeled alphabetically. This schedule has a February 13th, 2015 Finish On constraint to describe the importance of completing the project by this date. Note the negative total float warning you that the project is not going to meet the completion date unless the schedule is adjusted. However, the Gantt chart still describes the correct sequence of tasks according to the predecessor/successor logic. Note that task D cannot commence until task C and B are complete.
Primavera P6 – Hard Constraint
Figure 2 displays the same project with a Mandatory constraint. This schedule has a February 13th, 2015 Mandatory Finish constraint to describe the absolute necessity of completing the project by that date. Note though that activity D is shown commencing before the completion of activity C. This is a violation of the logic that specifies that activities B and C must complete before the start of activity D. The danger is the project team may get the go ahead to commence activity D when the reality is that it cannot be successfully performed without the prerequisite completion of activities B and C.
Sometimes constraints on the project schedule are both out of the project manager’s sphere of influence and critically important. The problem with these constraints is that yielding to them may cause your scheduling logic to be violated. This calls into question the entire accuracy of the project schedule.
Where possible a Finish On constraint in Primavera P6 is preferable to the Mandatory Finish hard constraint. If a delivery or completion date is absolutely essential you may want to consider scheduling your project from that date backwards.