Ban SS and FF Relationships?
Recently, I’ve seen a couple of CPM schedule specifications which do not allow any Start-to-Start (SS), Finish-to-Finish (FF) relationships or relationship lags. I do not understand this complete abolishment of these schedule tools.
I absolutely support limiting the use of SS and FF relationships, and relationship lags. The work being scheduled can almost always be modeled using FS relationships, if the work is broken down into small enough pieces. I also find that the use of a lead (negative lag) is rarely warranted.
However, there are legitimate reasons for applying SS and/or FF relationships to model work. And often times this includes the use of a lag to stagger the start of these work activities. Forcing the contractor to develop a schedule without the use of these tools, is not helping the project.
The contractor is forced to find “workarounds” to be able to model work that would best be modeled using these “forbidden” relationships.
One way to do this is, to assign the same predecessor for activities they intend to run concurrently. Simple enough. But this doesn’t accurately model the plan to execute the work for these activities.
A simple example is, starting electrical wall rough-in work soon after wall framing starts. These activities typically run concurrently with the electrical wall rough-in, waiting a day or two before having enough wall framing to work on to make it worthwhile to start. This is when using a SS relationship with a 1- or 2-day lag is valid. This models the actual plan for this work. We will also need a FF relationship, perhaps with a lag, to force the finish of wall framing to drive the ability of the electrical wall rough-in to complete. Both the wall framing and electrical wall rough-in can drive the typical in-wall inspection.
Removing the contractor’s ability to accurately model their plan to execute the project, by forcing the use of only FS relationships, could result in a common predecessor driving the start of both wall framing and the electrical wall rough-in. With that logic, the electrical wall rough-in could start without the start of wall framing. Does this logic really best serve the project?
Another way is to break the activities down into such small durations that we model the same work in a specific area with a series of 1- or 2-day duration activities to allow concurrent work with staggered start dates. However, it doesn’t make sense to break the work down into one day work activities to prevent the use of SS or FF relationships. This just makes the schedule confusing.
Typically, there is already a limit on activity duration in the CPM schedule specifications. If the duration for wall framing in one quadrant or area of the floor or project is expected to be 5 workdays, we would need to break the quadrant or work area into even smaller sections to allow the staggered start of electrical wall rough-in.
This will force us to do the same for almost all the other work in this quadrant or area. Pretty soon the schedule has far too many “micro-areas” to be a manageable schedule. There is “sweet spot” when it comes to activity decomposition. The activity is only small enough to “model and manage” the planned work. We should strive for that.
I propose owners limit the use of SS and FF relationships and relationship lags. Different types of projects will have a different percentage of instances when these will be valid. Basically, if the work isn’t broken down into enough detail, then the use of the SS and/or FF relationships shouldn’t be allowed or at least should be questioned. But there will be instances when these relationships, with appropriate lags, are valid and should be allowed.
Many owners handle this by having a percentage restriction on the use of these relationships and lags. This allows limited use which can be reviewed and questioned on a case by case basis. This is much more “mature” way of handling SS and FF relationships and lags.
Plan your schedule…
Paul Epperson CCM, PMP, PSP, PMI-SP
Paul has extensive experience as a Construction Manager. Over time, he became convinced that there is a critical shortage of skilled planning and scheduling professionals in our industry. In 2009, he backed away from his work as a Construction Manager and began focusing on planning and scheduling. He now serves our industry as a subject matter expert in this area.