Construction Scheduling. Use of Relationship Lags.

When your contractor develops the project baseline schedule, do they use many relationship lags? Do you know?

A relationship lag is an imposed period of time between the predecessor and successor activity. A common use in a Start-to-Start (SS) relationship with a 2-day lag. Perhaps the contractor’s plan is to start finishing drywall 2 days after the drywall installation begins. This allows the work to run concurrently.

There absolutely are times when work can and should be modeled this way. However, care should be taken to ensure an open start isn’t inadvertently created. (This is a future topic)

The problem arises when the series of work activities is developed with SS relationships with an assigned lag. This may indicate the work is not developed to enough detail to allow Finish-to-Start (FS) relationships to drive the work. The issue is, once the start of the predecessor is actualized, the start of the successor is only dependent on the calendar set for use for lags. This may not match the number of days intended. If this work happens to fall on the Longest Path, this path is now driven by a calendar. I prefer to limit the use of SS relationships in general, much less with assigned an assigned lag.

Then there are the Finish-to-Finish (FF) relationships with lags. These allow the successor work to finish only after the predecessor’s finish date plus the lag duration. This is commonly used to close off SS relationships to model concurrent work with the finish dependency. However, often the only successor is the FF relationship with the lag. This creates an issue in itself. (This is a future topic)

The most serious issue is when the relationship is an FS with a lag. What does the lag model? Should there be an activity to establish this difference in the relationship finish to start? Most likely, the answer is yes. There is nothing wrong in using an FS with a lag to have the lag represent say, cure concrete. I personally prefer to add the cure concrete activity with the number of days required. This is more transparent. The lag is not visible and creates a non-work period seen in the Gantt Chart bars and can cause confusion.

Then there is the negative lag, or leads. Many schedulers like negative lags and use them as a viable way to model the plan. When I see an FS relationship with a negative lag, I wonder why the use of a SS relationship with a positive lag could not have been used. The use of the negative lag may better represent the thought that the successor cannot start until 3 days prior to the finish of the predecessor. And this 3 days is the critical period. I personally prefer to model this type of plan with a SS with a lag or to further break down the activities to allow the use of FS relationships to model the work.

Finally, there are the SS relationships with an assigned lag that is larger than the activity duration forcing the model to show the work as an FS relationship. Why not just use the FS relationship? Unless there is a reason to model the work this way, it just doesn’t make sense to do so.

The relationship lag, if used correctly, can allow the schedule to model the plan. It is my opinion that the use of lags should be kept to the absolute minimum and only used to model SS or FF relationships.

But remember, the lag follows the calendar set for use for lag calculation. If the predecessor calendar is set for use, and the predecessor is assigned to a 5-day work week calendar, the lag will follow that calendar and the duration will be in work days. So, a 3-day lag could become a 5-day delay between the relationships if the lag spans a weekend. This may not have been the case when project work was planned and the schedule was developed and the lag did not span a weekend. Remember, this is not transparent, only someone who understands the schedule software and has access to the native file format can verify this. Unless the schedule layout includes the columns which show relationships with lags and the column that shows calendars. Even then, you need to know what the actual non-work days for the specific calendar are.

I know many of you can offer additional comments and recommendations. I welcome your comments and input. My goal, as always, is to help our industry and help the projects we support….

I’d love to hear what you think!

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Paul Epperson CCM, PMP, PSP, PMI-SP

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