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Start-to-Start relationship, predecessor stops enforcing successor start date

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Nelson Bond
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Very newb question here...I have Activity (Struct) Start-to-Start relationship to Activity (Pipe).  

NOTE: All of the date adjustments described are by dragging the middle of the gantt bars such that the duration remains the same.

I move the predecessor (Struct) into the future; successor (Pipe) Start date adjusts to match.

As I continue to move Struct into the future, the Pipe always adjusts such that Start dates are the same.

Now, here is the real question...I start moving Struct backwards in time..the successor Pipe Start date adjusts to match, UP TO A CERTAIN POINT.  At a certain point, after moving Struct, Pipe doesn't move. In this case, the date is Jan 13.

I cannot figure out what rule is being applied, or what condition exists, that marks the start of this behavior where the successor Start date for the SS relationship stops being enforced?

Any help is much appreciated!  thanks

 

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Replies

Nelson Bond
User offline. Last seen 6 years 23 weeks ago. Offline
Joined: 14 May 2013
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Gary, thanks again for the insight and help.  I understand exactly what you are saying.

I am responsible for moving some data from one application into P6, so that is why I am new to the program.  Using the software is not a part of my job, per se, but I do need a better understanding.

I am looking at some training guides and so forth to gain some insight...

Gary Whitehead
User offline. Last seen 26 weeks 5 days ago. Offline

Nelson,

It’s not really good practise to move activities around by dragging them on the gantt chart, for the reasons you are encountering.

 

Primavera is the same as other software in that predecesors drive, and successor do not. I suspect Struct is driving pipe (until you move them about enough so that the constraint takes over), and Pipe is driving the next activity.

 

If you remove all the constraints, you will see what you expect to see –the relationships driving the dates.

 

By dragging the bar around, you are effectively telling primavera “I don’t want this activity to start when the relationships say it should, but on a date I choose instead” –hence it adds a constraint for you.

 

Start on or After constraint means just that –Primavera won’t allow it to start any early than the constraint date, even if the predecessor relationships say it could (hence the relationship is no longer driving).

The constraint will however allow the activity to start any time after that constraint date, so if you have a start no earlier than 16th May constraint, and a predecessor relationship that says start on 17th May, then the relationship will be driving. –Makes sense?

 

General rule of thumb: Don’t use constraints if you can avoid it. If you need to change the dates of an activity, this is 95% of the time best done by applying progress (actual dates & remaining duration), adjusting durations, or amending logic. If you change dates using constraints, you start to change your programme from a dynamic schedule into a glorified to-do list.

If you have to use constraints, make sure you know how they work first, so you can select the most appropriate one & understand the implications on your schedule

 

The below link will take you to another thread in which I describe how all the constraint types available in P6 work. Might be worth a look.

 

http://www.planningplanet.com/forums/planning-scheduling-programming-discussion/532479/differnce-between-mandatory-constraints-and

 

Cheers,

 

G

Nelson Bond
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Joined: 14 May 2013
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Gary, thanks for the help...Pipe Driving and there is a constraint, Start on or After date.

When I first enter P6 and attempt to move activities via the Gantt bars, it always throws a dialog prompting me to add a constraint.

If I answer Yes, I can adjust activity Start dates by moving the Gantt bars.

If I answer No, I cannot adjust activity Start dates by moving the Gantt bars.

So, I always answer Yes and I guess that where the constraint is getting set for the first activity I move.

I don't understand why both activities are marked as Driving, until I move Struct Start date to be earlier than the constrained start date for Pipe...then Struct is no longer Driving and my original situation occurs.

I am used to software wherein the Predecessor is always Driving, not Successor.

Confused about the way Start dates seem to dictate the Driving property.  I obviously need some training!

Gary Whitehead
User offline. Last seen 26 weeks 5 days ago. Offline

Nelson,

 

If after scheduling (F9) you are still encountering this issue, click on your pipe activity, and got to the "relationships" tab of the activity details (view menu > show on bottom > activity details)

One of the columns available in the predecessor section is called "driving" -if it is not shown, you can add it by right-clicking on the predecessor section, and choosing "customise predecessor column"

The relationship that is forcing the start date of the activity will be ticked as driving.

 

If there is no relationhsip ticked as driving, then something else is telling the software that pipe cannot start before January 13. This is most likely either:

-A constraint (from activity details, go to "status" tab and check the constraints section)

-Or the data date (when you hit F9, if the current data date is Jan 13, the software will not allow an activity that has not been marked as actually started to start before this date)

 

Cheers,

 

G

Abilash Palakkada...
User offline. Last seen 3 years 45 weeks ago. Offline
Joined: 25 Aug 2012
Posts: 47

pls do scheduling after moving the struct activity towards left, in order to reschedule the sucessors