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Primavera Scheduling

The Makings of a Complicated Schedule

Primavera SchedulingWhen does a Primavera P6 schedule become complicated? I realize that for new users it might seem like every schedule is hard; but their problem is a lack of training or experience, not the schedule itself. Schedules with thousands of activities take more time to assemble, but that alone does not make them more complicated. Besides, in nearly every schedule there is repetitive work, so we can often copy dozens (if not hundreds) of activities quickly and reuse them. If you do not already know how to do this, read Time-Saving Tricks in P6 You Must Use.

Having prepared hundreds of original plans and thousands of updates, I can say for certain that two characteristics in particular complicate a Primavera P6 schedule. There are other situations that make a scheduler’s life “interesting” but trust me, these two are the biggies:

  1. The schedule has more than one calendar
  2. The schedule is resource loaded

Having more than one calendar is, mind you, often a necessity. The project specifications may require that inclement weather days be added to the calendar. In other words, anticipated weather days must be blocked out similar to holidays and other non-work days. But not all activities will be affected by weather (shop drawings, permits, interior finishes, etc.) so we must also utilize a calendar that does not have weather days.

Another reason for more than one calendar would be that some activities can occur any day of the week without exception, such as concrete curing, or the project specifications identify certain tasks accordingly. So now we have a 7- day calendar in addition to a work calendar with weather, and a work calendar without weather. Just like that the schedule has three calendars!

Okay, but why is it such a big deal to have multiple calendars? To start with, strange things can happen as the schedule moves from one calendar to another. Here is one possible scenario:

The predecessor finishes on Saturday because it obeys the 7-day calendar. If the successor has a 5-day calendar, it cannot start until the following Monday. Even though both activities might be on the critical path, the predecessor will have one (1) day of Total Float while the successor has zero (0) Total Float. After all, the predecessor could have finished on Sunday and still not hold up the successor.

In Primavera P6, Total Float is relative to the activity calendar so two activities on the same path may not exhibit the same float value if they have different calendars. By the same token, activities with the same Total Float value may not be on the same path.

Here is another possible scenario: the predecessor has a calendar that is 7:00 am to 4:00 pm. The successor has an 8:00 am to 5:00 pm calendar. The relationship between them is Finish-to-Start. The successor will start the same day the predecessor finishes because there is still one hour left in the day (i.e. 4:00 to 5:00 pm). This can confuse a lot of people who never display the time of day on their schedules. They think the activities are overlapping somehow when in reality they are not.

Moving on the resources, there is a calendar issue here to consider as well. Resources cannot use project calendars. Only global and resource calendars can be assigned to resources. Our students know why we prefer to use project calendars on every project. But the moment resources are added we are stuck with global or resource calendars. There is some logic here on Oracle’s part. Resources are not project-specific and so only a shared type of calendar would be appropriate.

So think about it. We already had three calendars because of weather and the need for a 7-day calendar and now we have a fourth one! The schedule can get seriously weird at this point. What if the resource plays by different rules than any other calendar? Well, our strategy is to create a global calendar first, copy it as a project calendar, and also assign it to the resource. So the project calendar and the resource calendar are identical.

There are situations where the resource works different hours or different days and should take over in terms of calculating dates. Perhaps the resource is never available on Fridays. We have to consider whether the project or resource calendar is more important. This is why Primavera P6 offers Task Dependent and Resource Dependent activity types. There is no single right answer; it depends on the circumstances.

The question in my mind when it comes to resources is, “does the dog wag the tail or does the tail wag the dog?” If the dog is in charge then I want a Task Dependent activity. Then it won’t matter if the resource calendar does not align with the activity calendar.

Another consideration with resources is that we can allow Primavera P6 to change the activity duration – something that would never happen otherwise. Therefore, the Duration Type becomes a very big deal. Should we let this happen? It does introduce additional moving parts to a schedule that might be difficult to understand due to other reasons such as multiple calendars. Still, it does make sense that if I know the budgeted hours and the crew size, why not let Primvera P6 do the math?

And of course, if we resource-load a Primavera P6 schedule we can also resource level the schedule. I teach this in my intermediate and advanced classes, albeit with the caveat that it takes several iterations of resource leveling before we can be satisfied that the optimum answer has been found: finishing as quickly as possible using a reasonable number of resources. Not exactly something the Pharaohs worried about but for us mere mortals, time and resources are limited.

Sometimes, giving priority to the earliest activity that needs the resource yields the best result. Other times it is better to give priority to the activity with the least amount of float. If I want a very precise allocation of resources I will utilize the Activity Leveling Priority. Unless you know what all of this means then it would be unwise to resource level a schedule without guidance.

Owners may not like the idea of resource leveling because it reduces the Total Float that contractually is almost always shared between owners and contractors. But sequestering Total Float is not the idea. One of the best schedules I ever saw (that wasn’t mine) was a resource-leveled schedule on a project in Hong Kong. The owner thought it was madness but it definitely worked.

I have not touched upon everything to consider with multiple calendars and resources, but hopefully the point has been made. Complicated schedules are frequently unavoidable, often necessary, and must always be respected. Are you ready for the challenge?