If you set the calendar for relationship lag to the 24 Hour calendar you may find in certain instances that your critical path becomes discontinuous or disjointed. To avoid this situation you can adjust your definition of critical activities.
Lag modifies the relationship between two activities by inserting a waiting time between the predecessor and successor. In the finish to start (FS) relationship this means that not only does the predecessor have to be complete, but you must additionally wait a specified number of days before commencing the successor activity. This waiting time typically occurs during the normal work week.
However, when modeling curing processes, such as concrete, you want to include weekends in your lag day count. Primavera P6 Professional has the option to set the calendar for relationship lag to a 24 Hour calendar. This works great to model the 24 hour nature of curing processes, and, in particular, the continued curing of material during the weekends.
If, however, your cure process ends on the close of business Friday the upstream activities may generate 2 days total float. This causes the critical path to become disjointed; you lose your longest path. But there are ways you can address this broken critical path dilemma.
This article demonstrates ways to maintain a continuous critical path when modeling lag time using a 24 Hour calendar using Primavera P6.
We want to schedule the curing of concrete, so we set the calendar for scheduling relationship lag to the 24 Hour Calendar, Figure 1.
On our 8-hour per day 5-day work week this means every 8-hour period is 1-day, so a 24-hour period is equivalent to a 3-day lag. In our schedule it takes 4-days for the concrete to cure, so our equivalent lag is 12-days, Figure 2.
Note that it may be less confusing to enter the lag in hours and let Primavera P6 compute the number of days for you from your hours input. So our 4-day cure time lag is 4 x 24-hours or 96-hours. Enter ‘96h’ in the lag field and P6 will compute the equivalent lag value ‘12.0d’, which is what we want.
Our ’12.0d’ lag in the details successor frame correlates to a 4-day lag on the Gantt chart. And note that this Gantt chart 4-day lag, between pour concrete and strike forms, includes weekends. So the 24-hour lag works great, except, when the cure process completes at the close of business on Friday. A Friday cure time completion results in 2-days total float for activities upstream of activity ‘E – Strike Forms’. And our critical path becomes discontinuous and disjointed, Figure 3.
To address this broken critical path issue we adjust the definition of critical activities in schedule options. We change the definition of critical activities from total float less than or equal to 0-hours to 16-hours, Figure 4.
Thus, all activities with 2-days or less total float are captured in the critical activity definition. The resulting schedule is displayed in Figure 5; the schedule now has a continuous critical path.
Note that all activities upstream of activity ‘E – Strike Forms’ have two days total float, but these activities are now captured by our adjusted critical activity definition.
Alternatively, we could have toggled to set the definition of critical activities to those along the longest path, Figure 6.
The resulting schedule with longest path critical activity definition is displayed in Figure 7.
Again, we have an unbroken critical path although activities preceding activity ‘E – Strike Forms’ have 2-days total float. Defining critical activities as longest path activities is the preferred approach, except when the schedule has additional activity constraints that require monitoring.
Defining a 24 Hour Calendar for scheduling relationship lag works well to schedule the seven day weekly curing of material. The critical activity definition, however, may require adjustment. If you are not monitoring activity constraints then define critical activities as longest path.
If you want to watch the schedule in relation to activity constraints then set the critical activity definition to total float less than or equal to 2-days on a five day workweek calendar and 3-days on a four day workweek calendar.
Another non-lag approach is to model the cure time as an actual task assigned a 7-day workweek calendar. Refer to the following blog for details on this approach Primavera P6: A Simplified Procedure For Scheduling Cure Time.
This cure time task definition method also requires adjustment to the critical activity definition. But it also reserves the lag definition for workweek days; the 24 Hour calendar lag assignment affects all lag in the schedule.