Hi Dytham,
One case would be contractual. The principal requires that a commissioning
plan is submitted no later than 9 weeks before the start of commissioning.
The start of commissioning may change as construction changes. It may arrive
earlier than originally planned or iot may come later. Either way the
contractor needs to ensure that the plan is submitted before that deadline.
So the logic is, the finish of the commissioning plan must occur 9 weeks
before the start of commissioning. Thios translates as "Commissioning plan
completed" has a predecessor of "Commissioning" with a SF -9wks link. Now
as the date of commissioning changes with construction so the date by which
the plan needs to be completed changes. The trick then is to make sure that
this becomes a critical activity, (you don't want to risk delaying the
contract by not submitting in time) so a deadline needs to be set on the
"commissioning plan completed" activity. Without a deadline and since this
method does not have a successor, it would have a large total float value.
To set a dynamic deadline, copy the finish date and then paste it using the
paste special Link, option into the deadline field.
Normally we think that every activity 'drives' a subsequent activity, but
documentation is often not the prime driver but a precondition to starting
the next activity but is not the driver for the activity. That means that if
the documentation is not furnished in time then there is a risk that the
activity (commissioning for example) may not be allowed to commence. If you
set a normal sequence and have the documentation as one of the drivers for
commissioning, you will see that the documentation does not need to be
started for a period of time out and delay starting. Then if construction
progress goes better and commissioning could start earlier, it is not always
that obvious that the plan needs to start or should have started.
I feel it comes down to the difference between one activity 'driving'
another and one activity being a contractual precondtion. I suggest that the
precondition date is driven by the activity that it is a precondition for.
For the example you have raised, I believe this process works well. The
driver for the meeting date is the expected comepletion date. Since the
completion date may change then so will the meeting date because of the
completion date, and not the other way around.
You do need though, negative lag since you need to tell the activity it
needs to finish a fixed period of time before the start of the driver
activity.
What I do is set the date for the activity as a milestone using it's own
finish date, and link the actual activity of work having a duration as a
standard FS link to the milestone. That way if the milestone overruns the
deadline, you have time to review the start or duration of the work activity.
Hope this helps and hasn't confused you further.
Regards
DavidC