Free Float Definition Calculation

The other type of float found in a network is called Free Float. Free Float is defined as the spare time available to an activity when all activities in the chain are started as early as possible. Thus, an activity may be delayed to the extent of its Free Float. Free Float for a given activity is calculated as:

Free Float (FF) = ES (following activity) — EF (activity being calculated)

Problem 3-7: Free Float

Apply this definition to Figure 3-15 and calculate Free Float for each activity of the network.


Examination of Figure 3-16 calculation reveals a general rule of Free Float, which is that Free Float occurs only when two or more activities have the same J Node.



It becomes obvious that in large networks with several thousand activities, it would be physically impossible to make all the calculations manually. As indicated earlier, computerization of the Critical Path Method of scheduling has solved a tremendous calculation problem. The result is a concise method of planning, scheduling and controlling major projects. However, the case of computer calculation and updating routines must not detract from the need for a cost effective scheduling program. Working at summary levels and only going into greater detail, when necessary, will usually produce the most effective program.


Figure 3-17 lists terms and definitions of precedence.

The following are examples of simple precedence networks.


On first inspection, this logic may appear to be satisfactory with the sequence of events being as follows:

1) Install pipework

2) Hydrotest pipework

3) Install electrics and instrumentation

4) Loop check instruments

The first detail that becomes apparent on examination is that all constraints are type FS (finish to start), which means that no

Figure 3-17. Precedence Diagrams - Terminology

General Activity

Duration Event


Preceding Event <PE)

Succeeding Event (SE)

Constraint Types

Finish to Start <FS)

Start to Start (SS)

Finish to Finish (FF)

Start to Finish (SF)

An item of work with a clearly defined beginning and end. e.g., INSTALL PUMP.

: Time required tocompletean activity, e.g., 5 days, 30 hours.

: Indicates a specific point in the course of a project, and is shown as an activity with no duration, e.g., FIRST OIL.

Defines the logical relationship between activities and events. This relationship is called a DUMMY in arrow networks and the subject shall be discussed fully in a subsequent section of the course.

Preceding activity identifier. A constraint showing the logical relationship between one activity and its previous activity.

: Succeeding activity identifier. A constraint showing the logical relationship between one activity and its following activity.

This is the most frequently used type of constraint and indicates that the SUCCEEDING activity cannot start until the PRECEDING activity is totally complete.

A constraint that indicates the SUCCEEDING activity can start immediately the PRECEDING activity has started.

A constraint that stipulates the SUCCEEDING activity can only be completed once the PRECEDING activity is complete.

A constraint that is rarely used and indicates that the SUCCEEDING activity cannot finish until the PRECEDING has started.

activity can start before the preceding activity is complete. Therefore, (2) cannot start until (1) is complete. Likewise, (3) cannot start until (2) is complete, and also (4) cannot start until (2) is complete. (Refer to Figure 3-18).

In practice, this logic is not correct as the piping system would be divided into more than one hydrotest. This would allow hydro-testing to begin as soon as sufficient piping had been installed. The same situation would apply to loop checking of instruments. This could start once the necessary instruments and electrics has been installed.

This overlapping of activities is shown on a network by the use of SS (start to start) and FF (finish to finish) constraints as shown on the revised Figure 3-18.


The revised sequences of activities as shown on Figure 3-19 are as follows:

1) Install pipework.

2) Start hydrotesting as soon as sufficient pipework is available.

installation must be complete before hydrotesting can be completed. This is shown by the FF constraint.

3) Electrical installation can start when pipework installation is complete.

4) Installation of instruments can start once the hydrotesting is complete.

5) Loop checking begins as soon as sufficient instruments and electrics have been installed, but cannot be finished until after all the instruments and electrics have been installed.

The ability to further define this logic is also shown on these diagrams. This would be achieved by adding delays to the constraints as shown by the number in the parentheses. This is known as lead or lag time. Without the lead/lag times, the network indicates that hydrotesting can start immediately after the piping installation has started. This in not strictly true, as the pipework associated with any hydrotesting would have to be installed prior to the start of testing.

When a duration is added to a constraint, the start of the activity is delayed by this amount. For example, by adding a duration of 4 to the SS constraint between the INSTALL PIPEWORK and HYDROTEST activity, hydrotesting would not start until four days after the start of the piping activity.

This method of overlapping network activities is a powerful planning tool and is used extensively in all precedence networks.

Figure 3-18. Network Logic.







Figure 3-18. Network Logic.



Figure 3-19. Amended Network Logic.


Figure 3-19. Amended Network Logic.


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Project Management Made Easy

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