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
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