S-S Relationship with different dates

nigel,



perhaps your contractor used the Line of Balance approach. This kind of approach is good for repetitive jobs (which have little element of uncertainty) eg different typical floors of a building.



In such a case the critical path would probably be passing through the SS lags. However when the project finish date is affected this critical path may go through some SS lags and through the body of the network it may pass through the FF lags.



This is the reason free float must be monitored at each update

Clive,

in pipeleine construction project Ditching, Welding, Coating, Lowering to Ditch and Backfilling have both SS AND FF dependencies with Lags. We prefer to use volume (or quantity) lags (like after 500 meters have been done).

This is how people work keeping the distance between different work types and performing activities in parallel.

Why reflecting the real situation is called Lazyness?

Clive,

I totally agree with you when you say SS & FF relationships are for lazy planners.

We have had an experience whereby our contractor has submitted his schedule with their "Ladder Logic", and they have used SS and FF links throughout their whole schedule.

All this has done is constrained the "front" end and the "back" end of the schedule and shows a pretty picture and not true logic. That can be clearly shown when the monthly updates are calculated, and the schedule is depicting "on schedule", but in reality, progress on site is down by 9%, and all they are doing is manuipulating the schedule and eating up whatever float that had in their logics.

That’s my 10 cents worth.

"Suppose activity A will start today, then activity B will start after one week with SS relationship.

How can it be? Did you experience submittal like this? needs help.."





Suppose A is erection of piping

B is installation of valves on these pipes

B can start only once A started hence SS relationship



However, valves need to be delivered in order to be installed, and they are due next week



subsequently a SS link and starting a week later!



hth

even if we used SS and FF relationship.



still i can not understand the bad effect to use SS and FF relationship .

hi all,



I would reiterate that for initial planning purpose the SS relationship may seem useful (cosmetically shows the correct logic of the schedule).



But the moment such activities are in progress ONE MUST THINK WHETHER SS RELATIONSHIP HOLDS GOOD FOR THE REMAINING DURATIONS ALSO ON EACH UPDATE.



Most of the time SS relationship will not mimic the actual situation at site once the updates of the schedule are in progress.

To all,



The basic approach of all scheduling techniques is to create or form a network of activity & event relationships that graphically portrays the sequential relations between the tasks in the project. Tasks that must PRECEDE or FOLLOW other tasks are CLEARLY identified.

I believe SS relationship with time lag can only be use in graphical representation of Schedule in Level 1~3 and not in detailed schedule.



benito

Thanks,Clive



however i would like to clarify that most of the activites in roads and infrastructure projects have to be overlaped we can not use only FS relation otherwise the project duration will enlagred unnecessary.

To make these overlap between the activities we have to use SS and FF relationships.



can you explaine to me what do you mean by (generally SS and FF are lazy realtionships ).

Hi,

Perviously i gave a practicl example in which we have to use relation Start to Start but in the same time we have to use Finish to Finish relation in order to avoid open ends.



I will repeat that practical example for your infomation and benefit discussion.



In roads works , we can not start spray betuminous prim coat until we start laying of aggregate road base layer and we ca not finish spray betuminous prim coat until we finish laying of aggregate road base layer in the same time we can not wait until finishing of all woks for aggregate road base layer in order to start spray betuminous prim coat.



another example, We can start laying of asphalt base course only after two days from start spraying of spray betuminous prim coat and we can finish laying of asphalt base course only after two days from finishspraying of spray betuminous prim coat .

Is it theoritically correct to have start to start (SS) relationship with different early start dates in each activities.



Suppose activity A will start today, then activity B will start after one week with SS relationship.

How can it be? Did you experience submittal like this?

If multiple calendars are used, then yes, it is both theoretically and practically possible if A & B have different working calendars.



Resource leveling systems could also affect the "pure" CPM logic.



Bernard

InterPlan Systems

Hi,



Be very cautious while giving relationships. Most of the planners do not think how these relationships will behave when the activities are in progress.

Surely what we are trying to do as planners is to decide the optimum method of construction given the particular constraints of the project.



If activity B cannot start until after activity A has started, then a S-S link is the correct one.



If there is a logical delay between the two starts - concrete curing or attaining strength, paint films drying or similar, then there will be a S-S link with a lag.



If activity B cannot finish before activity A as well, then a F-F link should be used too. If there is no logical link then don’t use one!



There is more danger in putting illogical links into programmes than there is in having loose ends and associated float in a programme. This can be edited out on the final pass through the programme before issuing.

selection of the proper relatio between the activities and proper links is on of the most important task during the preparation of the work program.



some times we have the chance to selest between alternative links if the logic and sequance of the works are allowing for this and many times we have to use a certain relation in order to reflect the proer logic and sequances of the activities.



for example, in the road works, we have to spray betuminous prim coat above the aggregate road base layer and it has to get 48 hours as curring time befor staring the aspalt works. in this particular case we have to link the activity "spraying of betuminous prim coat " with it’s predessor "laying,mixing, watering, copaction and testing of aggregate road base layer " with relation SS with lag +2days and FF with lag +2 days so we can not avoid using cetain relation and we have to reflect the actual logic and sequances of the activities.

I would think that there would only be a difference if either activity or the logic were using different calendars.



Not to forget that if the scheduled start for an activity is close to shift end on one day there may not be sufficient time for the engine to place a portion, (= to the minimum scheduling time period), of the activity on that day, this will cause the Successor to spill into the next time period which may in fact be the next day.



So I would say it is entirely theoretically possible, dependant on the software being used and the way it has been set up to schedule, and also the underlying activity settings.



So that’s my 2 bobs worth.



Andy

Hi,



If we are using only the SS relation in the links, as per my knowledge the link is not closed. Then, it may be required to introduce FF relation in the same link, with certain lag, to close the relation. Hope I am not wrong.



GG

Hi，



Maybe we can use FF+ (-lag),but some people don’t like -lag.



if the software can use %lag as MSP or pertmaster,it will be better

I don’t think we have enough information to really advise to use a FF relationship. What if Task A has a long duration (ex: 35 days) and Task B has a short duration (ex: 3 days).



I’d use a SS with +7 days lag to cover this.

Hello Ulysses,



Your point is about the logic not about the technic of making SS link right ?



In my opinion the SS relationhsip is problem when the predecessor activities was being delay, then u will get a mistake in forecasting the start of the SS successors. To overcome this problem pls cover it with FF relationship.



Other way u can use as single FS with negative lag but..some person doesn’t like this trick.



it answers u ?

Hi Ulysses,



This is possible if the relationship is Start to Start (SS)

with a lag. So, if Activity A has Activity B as a successor

with SS + 7 days lag, then you will have a case as you

mentioned.



Regards,

Ramesh