Introduction :

Dans Primavera P6, les contraintes sont des dates fixes que vous assignez à une activité. Elles limitent la date de début ou de fin d'une activité, garantissant ainsi qu'elle ne commencera pas ou ne se terminera pas avant ou après une date spécifiée.

Constraints in Primavera P6 are Date-based restrictions, applied to activities for to guide or force their planning, in addition to (or sometimes to the detriment of) network logic.

Instead of relying solely on activity relationships (FS, SS, etc.), constraints allow you to control the start or end dates directly.

Typical use cases:

• Regulatory requirements (deadlines imposed by authorities)
• Contractual obligations (milestones, payments)
• Resource availability
• External dependencies (approvals, suppliers)

⚠️ Attention: improper use can generate:

• from Negative float
• an unrealistic plan
• a loss of flexibility

Types of Constraints in Primavera P6

The constraints are classified into two main categories:
👉 Hard constraints versus Soft Constraints

1. Hard Constraints

Strict constraints impose fixed dates and often ignoring network logic.

⚠️ Use with caution.

🔴 a. Start On

• Force boot to a specific date
• Ignore predecessors

Example:
Excavation permit from 15/06/2025

🔴 b. Finish On

• Force a completion date
• Can create a negative float

Example:
Structure finished on 30/08/2025 (contract)

🔴 c. Mandatory Start

• Forced start, with no regard for logic

Example:
Commissioning set for 05/01/2026

🔴 d. Mandatory Finish

• Fixed, even if logic indicates otherwise

Example:
Mandatory delivery by 31/12/2026

2. Contraintes souples

Soft constraints guides the planning without breaking the CPM logic.

👉 Recommended in practice

🟢 a. Start On or After (SOOA)

• Cannot start before a date

Example:
Steel assembly after delivery (10/05/2025)

🟢 b. Start on or before

• It must start before a deadline

Example:
Concrete pouring before 01/07/2025

🟢 c. Finish On or After (FOOA)

• Cannot finish before a date

Example:
Tests until 15/12/2025

🟢 d. Finish On or Before

• It must be finished before a date

Example:
Roof covered before 01/11/2025

Hard vs Soft Constraints: key differences

Example: Negative float caused by a constraint

Scenario:

• Activity: Final Testing
• Constraint Finalisation obligatoire – 30/10/2025
• Logical calculation: end on 05/11/2025

Result:

• Float -5 days

Indicate a scheduling conflict / constraint

Best Practices (Expert Level – Delay & Risk Strategy)

✅ 1. Use constraints sparingly

Too many constraints = artificial planning

✅ Prioritise flexible constraints

They maintain the integrity of the CPM model

✅ 3. Monitor the negative float

Critical drift indicator

✅ 4. Justify each constraint

• Contractual
• Regulatory
• Technique

Indispensable in the context of claims

✅ 5. Test the schedule without constraints

Check network logic robustness

Conclusion

Constraints in Primavera P6 are powerful management tools:

• Well used Realistic and controlled planning
• Misused Biased planning and legally fragile

Golden Rule:
Network logic must prevail — constraints must never replace it

In your projects, do you favour hard or soft constraints?
Have you ever managed a schedule with a significant negative float?