Class 104: Activity Relationships

Activity Relationships, also known as schedule logic, define the order in which schedule activities should happen. AACE states “Activity relationships determine how activities relate to one another and establish schedule logic.”¹

As stated in the definition above, activity relationships create the logic in a schedule.  AACE defines Logic as the “Relationship describing the interdependency of starts and finishes between activities or events. Every activity should have a predecessor (except for the initial activity or event), and every activity should have a successor (except for the ending activity or event). Activity logic is determined by need to meet competing constraints defined by contract requirements, physical capabilities of trades performing work, safety concerns, resource allocations, and preferential activity relationships.”¹

A predecessor activity is an activity that must start or finish, depending on logic, before its successor activity can start. Likewise, a successor activity is an activity that occurs after another activity.

There are four activity relationships used in construction scheduling. The four relationships are as follows:

Finish-to-Start:

The first and most common type of activity relationship is called Finish-to-Start (FS). Finish-to-Start is “a relationship in which the successor activity can start only after the predecessor activity finishes.”¹ These are the most commonly used activity relationships, and as an industry best practice, it is recommended to use them as often as possible.

Start-to-Start:

The next is Start-to-Start (SS).  Start-to-Start is “a relationship between activities in which the start of a successor activity depends on the start of its predecessor. The predecessor must start prior to the successor starting.”¹

Finish-to-Finish:

The next is Finish-to-Finish (FF). Finish-to-Finish is “a relationship in which the successor activity depends upon and can finish only after the predecessor activity finishes. The predecessor must finish first and then the successor can finish.”¹

Start-to-Finish:

Finally, we have Start-to-Finish (SF). Activity A must start before activity B can finish. Start-to-Finish relationships are the most seldom used activity relationships and using them is discouraged by industry best practices.

Activity Relationships Summary:

There are four types of activity relationships used in construction scheduling. As an industry best practice, it is recommended to use Finish-to-Start relationships as often as possible.

Key Terms:

     Logic – Relationship describing the interdependency of starts and finishes between                         activities or events

    Predecessor – An activity that must start or finish, depending on logic, before its                                           successor activity can start

   Successor – An activity that occurs after another activity

Resources:

1. AACE International Recommended Practice.  Cost Engineering Terminology. https://web.aacei.org/docs/default-source/rps/10s-90.pdf?sfvrsn=58