Construction Scheduling. Critical Path and Total Float.

Can the Critical Path for a project have different Total Float values?

Does the Total Float value determine the Critical Path?

When the Baseline Schedule is developed, all the activities should have the appropriate calendars, durations and logic applied. This creates the schedule network of activities.

Within this network of activities, there are many paths which run through activities linked by mandatory or “hard” relationships. For example, the structural steel shop drawing development and submission should drive the review of this submittal which will then drive the procurement with lead time to drive the delivery and then the erection. Almost all the trades and materials have these “hard” relationships which form the work sequence for that network path.

There are also discretionary or “soft” relationships. For example, the ACT grid could be installed prior to the wall painting. I prefer to run the grid after the walls are painted, but it may be necessary to install the grid first to allow light fixture and register installation to start earlier. This may be sequenced at the discretion of the Project Manager or Superintendent.

There are external relationships, but these are usually handled within the schedule itself. But it is possible to link activities in one schedule to work in another schedule….

Once all the mandatory and discretionary relationships are assigned, and the calendars and durations are assigned we can “schedule” the project. This is what sets the schedule network up.

This is what also determines the Longest Path and Total Float values.

If there is not a finish constraint applied to the project, the end date will float dependent on the network. The Total Float value will be 0. If you sort the activities by Total Float value and look for the next continual sequence of work activities with a Total Float value close to 0, you will most likely find the Near Critical Path. Just be aware that this Near Critical Path is based on the Total Float values only and will not necessarily be the logical Near Critical Path.

If you have a constrained finish date for the project, the Baseline Schedule should have a Total Float value of 0. But as soon as you start updating actual progress, the work will force the various network paths to push or pull and this will change the Total Float values. The Longest Path will still be the Critical Path, but the Total Float values may be positive or negative values. Again, the next continual sequence of work activities with a Total Float value close to that of the activities on the Critical Path should be the Near Critical Path, based on Total Float values only.

Recognizing the Critical path is the Longest Path is the basis for properly managing the work. Recognizing the usefulness of the Total Float values for identifying Near Critical Path work is also valuable.

This is a fairly simple concept. But, there are many discussions about what determines the Critical Path.

Not using Longest Path as the definition to use Total Float values is a tool we use for analysis. Much as we use Progress Override instead of Retained Logic for analysis.

Interim Constraints play into the schedule network and affect the Longest Path and Total Float values. But this is another subject for the future….

I’m sure many of you have additional insight into this subject and can help explain the concept. Please share!

I’d love to hear what you think!

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Paul Epperson CCM, PMP, PSP, PMI-SP