(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?
(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.
Hello All,
I have had this issue on multiple projects and was wondering if this was a flaw in P6.
Whenever I apply the SF link, the link pulls the successor into non-working period. Here is how the 3 activties are linked.
Activity A <---- Milestone B ----> Activity C
Miilestone B is the main driving milestone. Activity A needs to finish the day before this milestone constraint date. However, even though the same calendar is applied to all 3 activities, Activity A seems to be pulled into non-working time period by Milestone B, which should not be allowed by P6 calendars at all costs. Does anyone know the reason behind this?
I know I will get quite a few here suggesting not to use SF links. I need to create a backward pass schedule based on Activity B milestones which are driven by a client. If you are going to suggest not using a SF link, please also suggest a working alternative to an SF link in this scenario.
Thanks all
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Thankyou Rafael and Tom for your reponses. Your suggestions from experience provide a good amount of insight in backward logic. As suggested, I have opted to use ALAP and it works. Now all activities are within working time periods. The only problem being Activity A risks being pulled back due to any scccessors, as Rafael suggested.
I have come across this situation various times in my line of work, particularly when I am building Interface Schedules where milestones are dependent on external constraints and the work needs to be scheduled accordingly.
Rafael, I looked into the Calendars and my Admin preferences are set to 8 hrs per day with the option 'Use assigned Calendar to specify' ticked. Some of my Schedules are based on 8 hr work days and some on 10 hr work days. I am assuming that as long as the option is ticked, the applied Calendar should specify the hrs/work day.
Beware that ALAP activities can be moved prior to milestone successor by another successor activity or resource leveling, many times without any warning.
No doubt both methods can yield inconsistent results under certain circumstances but if you take due care the use of ALAP approach it would be a better choice. Using maximal lags is even trickier so my choice would still be to use ALAP.
The only software I ever used with capability to model maximal lags was Projack Manager, software I believe is no longer available.
Going back to the original question the P6 calendar issue you never experienced with Asta I still wonder if the reference regarding P6 calendar issues I posted helped. SF relationship is rare but it happens. I also believe these calendar issues might also happen in P6 even if there are no SF relationships.
http://www.primaverascheduling.com/calendar-issues-in-primavera-p6/
I do not use P6 other than importing XER files into my software, still it is good to know in order to understand why some differences when importing XER files.
Wasi,
The dates you indicate are inevitable given the nature of the logical constraints. With B>SF>A, you are dictating to the software that A's finish MAY NOT PRECEDE B's start. Such time/date precedence relationships are at the core of the forward and backward passes, before calendars ever come into play. Since B starts at 0700 on 24Feb, the forward pass first set's A's finish at the earliest possible time (not before B's finish and within A's calendar) of 0700 on 24Feb, then it subtracts the 1day duration (according to A's calendar) to arrive at A's start at 0700 on 23Feb. There is no work during non-working time - A's work is done on the 23rd, but the night is spent idly waiting for the SF constraint to be met before the finish can be recorded. You may be tempted to modify the relationship to B>SF-0.01h>A, which gets A's finish back to the end of the previous day, but of course it's start is pushed back to the end of the day before as well (unless you shorten its duration by 0.01h). All of this is is pointless.
You have never explicitly stated exactly what the hard logical relationships are among A, B, and C, but I get the impression that A and B are preparatory works that must be done just in time for (finish) Milestone C. I guess you are using C>FF>B and B>SF>A to pull them along as C moves due to external constraints. Aside from your stated concern over the finish dates, such an approach also corrupts the flow of driving logic through the schedule network - float values and associated critical/near-critical definitions won't be reliable. If this is the case, you would be better served in my view to use the correct logic A>FS>B>FF>C, then apply just-in-time constraints (what we used to call Zero-Free-Float and what P6 calls "ALAP") to A and B.
Activity A needs to finish the day before this milestone constraint date.
Maximal-Type Relationships:
This is what I got if using Spider Project software, but it looks like P6 calendars are a bit tricky. Spider Project do not have such issues and seems like also Asta do not have such issues.
Try the following link to see if it helps.
http://www.primaverascheduling.com/calendar-issues-in-primavera-p6/
Good Luck
Hello Rafael and Zoltan,
Thanks for the input. Maybe i used the wrong terminology (backward pass schedule) which took the discussion in a different direction. Since I cannot attach images, I will try to explain the scenario in the most simplistic manner that i can. It is infact pretty simple and I am sure someone somewhere has come across the same issue in Primavera and knows a way around it. I have used similar logic in other software in the past (eg Asta power) for Interface schedules with success. Howeever, currently P6 is my only option.
Here is an example of my activity flow;
Activity A (1d) <---- Activity B (1d) <------ Milestone C* ------> Activity D (1d)
Milestone C is my main driving milestone for all activities surrounding it. For eg, if Milestone C must finish by 5pm on 24th Feb, the rest of the activities should ideally be;
Activity A 23Feb 7am-5pm <---- Activity B 24Feb 7am-5pm <---- Millestone C 24Feb 5pm* ----> Activity D 25Feb 7am-5pm
Milestone C is a variable (as always the case in Interface schedules) and moves frequently. I want all the remaining activities to constantly move accordingly as per Milestone C date.
When i implement this in P6, Activity A is being pulled past non-working time into the next day even though all activities have the same calendar.
Activity A 23Feb 7am-24Feb7am <--- Activity B 24Feb 7am-5pm<---Milestone C 24Feb 5pm*---> Activity D 25Feb 7am-5pm
Anyone knows why this would happen?
It looks like nothing is lost if using Spider Project as P6 Backward Schedule can be executed by making the network a closed one and applying a constraint to the end event, this can be done with any CPM software.
It looks like Spider is more functional as both Backward Schedule definitions can be performed if using Spider while P6 cannot.
It looks like Spider cannot calculate Backward Schedule as defined by P6. Sad, good to know.
It looks like P6 cannot calculate Backward Schedule as defined by Spider Project. Sad, good to know.
http://www.primaverascheduling.com/the-case-against-must-finish-by/
Anyway does the main issue regarding the SF link pulling the successor into non-working period was figured out and solved?
A Backward Schedule is where the backward calculation is performed first and then the forward calculation. It cannot be calculated merely by using some date constraints. A backward schedule is mostly used to determine what will happen to a schedule and its activities if it is required to finish on a specific dates. This is not the case in P6
Since this is a P6 fourm I will address it as such
The first pass is the forward pass this determines the early dates and begins with the project start date or the data date and goes forward in time and then the backward begins at the project must finish by date or if there is a constraint on the last activity.
If there is not a project must finish by date or a constraint on the last activity then the calculation for the late dates are assigned a Late Finish equal to the latest calculated Early Finish date. This is why if there is no finish contstraint or project must finish by date that the totla float will ALWAY be ZERO.
About Backward Schedule:
All CPM calculations include forward and backward pass that shall consider all constraints, including date constraints as well as resource constraints among others.
A Backward Schedule is where the backward calculation is performed first and then the forward calculation. It cannot be calculated merely by using some date constraints. A backward schedule is mostly used to determine what will happen to a schedule and its activities if it is required to finish on a specific dates.
This can be illustrated with the following example where for simplicity we are not resource loading and leveling. Resource leveling and constraining the backward pass is where many software fail.
In the following example the fist scenario do not fall within a job recess period. If the recess period impact in different ways different resources and activity calendars the calculation gets more complicated and so its analysis, so for purpose of this illustration I will keep it very simple. Note that in the scenario there is a scheduled recess of two weeks during November.
It is very simple just set the Target Finish date to the date you want the Backward Schedule to finish.
Among other purposes we use the Backward Schedule as the comparison scedule to calculate and manage buffers. Critical Chain approach to Feeding Buffer is flawed as feeding chains can change as soon as the critical path changes. Using activities to represent buffer is not a good practice, traditional CPM functionality is enough to handle buffers.
If your software do not provide for Backward Schedule calculation you can get it [or close to it] by trial and error moving the project start date until the finish gets equal [or close to it] to target finish date. Once again please be reminded that Backward Schedule as well as regular Schedule computations make use of forward and backward computations.
Could it be because of the Type of Milestone, Start Milestone or Finish Milestone? I do not have P6 to see what will happen if the milestone switches from Finish Milestone to Start Milestone and vice versa.
Take a look at the following image I got from the Web.
all you need is a fininsh on or before constraint on the last activity or a project must finish by date. This is where the backward pass begins to work backwards.
Zoltan,
How do you create backward pass schedules then?
Thats why I never ever ever use this type of realtionship I can't understand it and it makes no sense to me.
Thanks for pointing that out, Tom. Edited the post.
Wasi,
No one can help you without your images (which have been dropped by PP).
http://www.planningplanet.com/forums/improving-planning-planet-website/5...
Email: PPadmin@planningplanet.com
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