Dependency relation query

Submitted by pmpnewster on Tue, 06/11/2013 - 08:12

 

Can someone please clarify the below doubt?

We have two sequential activities A and B (they could be part of a complex PDM network diagram) with some dependency (SS, FF, SF) specified.  NO LAGS or LEADS are given.

 

1. SS relnship:  Means that B cannot start until A has started. Fine.   BUT does SS relnship also require that B must start at the same time as A, i.e., they start simultaneously?

2. FF relnship: Means that B cannot finish until A has finished. Fine.   BUT does FF relnship also require that B must finish at the same time as A, i.e., they finish simultaneously?

3. Similar query for SF.

Pls remember: NO LAGS or LEADS are specified.

 

Any official or authoritative reference or link to the clarification will be appreciated...

 

Thanks.

Regards.

 

Forums: 
General Discussion
  • Log in to post comments
  • 511 reads
admin's picture

admin

Wed, 06/12/2013 - 01:48

Try to understand this with an example.

When you are calculating results of a poll, Activity A can be obtaining poll results and Activity B can be Entering the results in Database. For Activity B to start Activity A must have started. Meaning only if you get poll results from a station you can start entering in database. But you dont need all the results to start entering in database, in that sense once you have "Started" getting poll results, you can "Start" entering in database. 

So to answer your question, you dont need both activities to start simultaneiously, but its required that an activity has started for next one to start.

You can imagine another example for F-F relationship. To cook a dish with meat and veggies, veggies may take 15 mins, but meat may take 30 mins, but inorder to say that meal is ready both must have finished cooking, so you can say veggies and meat are cooked only when meat is cooked.

pmpnewster

Wed, 06/12/2013 - 02:56

 

Hi admin, thanks for the clarification.

That is what I had felt all along based on the pure definition of SS, FF etc. However, I recently came across two instances where the author mentioned about the requirement of simultaneous start in SS relnships. This can have big impact on PDM results. Hence, I was concerned.

If you don't mind, I have a follow-up question: When lead or lag are specified, then we should apply the proper + / - time unit values and treat it as a firm (set in stone) start/finish rule, correct? For example, SS + 3 means that B will Start 3 time units AFTER A Starts, no matter what is going on elsewhere in the PDM network, am I right?

Thanks.

Regards.