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Contiguous or Interruptable

9 replies [Last post]
Wed, 2003-09-03 06:25
Joe Mansour
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Hello everybody,

I use in my schedules the "interruptable" option in scheduling.
I find it more representative of the logic, especially when I use the SS and FF relationships. (This will show a bar longer than the duration of the activity).

Consultants do not like.

What do you think about that, and do you think Consultants can impose the "contiguous" option on the Contractor?

Joe
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Thu, 2012-05-03 14:30
#1
Ronald Winter
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David,

  I am afraid that you do not understand the P3 implementation of Continuous Activities.  Retained Logic and Continuous Activities mode have practically nothing in common.   You must be assuming their functions instead of researching them.

  If an activity has not started, then Continuous Activities is in effect only if there are non-balancing logical constraints to both the start and the end of an activity.  In this case, Retained Logic has no effect, as the activity has not started.

  If the activity is statused with an Actual Start, then Retained Logic may come into play but Continuous Activities mode does not.  The start of the activity has already been determined.

  This stuff has been reviewed, researched, and implemented since 1983.  Its logic is quite defenseable.

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Thu, 2012-05-03 10:41
#2
David Barnett
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I don't understand the use of retained logic (RL) and cintiguous activities (CA) in relation to construction. Almost all forms of contract have an explicit duty placed on the contractor to mitigate delay. Generally in construction that is expressed by starting an operation in a different location if there is a problem in the planned location. If you can't get at a certain chainage section on a road then you go elsewhere. If you get into trouble on a building you can jump a floor or start another team ten floor up. 

By using RL and CA this goes against the whole concept of the duty of flexibility and mitigation the contractor has to attain. He can still change his planned method but he has no way of showing the benefit. Why would anyone want that?

Yet here I sit looking at a claim on a project with one of the largest PMs in the world involved and they have changed the Conditions of Particular Application to specifically state that RL & CA must be used. 

I find this topic so bewildering I can't understand why we even have RT & CA. Was it a legacy method left over from the early days of computerised CPM which was subsequently upgraded to allow for progress override and interruptable activities and someone forgot to delete Plan A.

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Fri, 2003-09-12 23:08
#3
Tomas Rivera
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Joe:

Your latest post shows a more complete explanation of what is going on about the subject.

But, going back to your original post, I have a couple of comments.

First, I agree with you that using interuptible mode might be a better option for many cases. It will allow the contractor to start some activities earlier and by doing so we might get several things: lower the risk of delay, more latitude for resource management and sometimes shorten the project schedule. But also, Ronald has a good point about some activities not being desirable to start and stop before finished.

Second, regarding your original question, it usually is the contractors prerogative to decide how he plans to do the job and meet the contractual finish date of the project. Unless of course, it is specified otherwise in the contract, which usually is not the case. Therefore, I do not think anybody can impose on the contractor anything that is not spelled out in the contract. We are playing by the rules of the contract, the law and government regulations. Anything else is free range for the contractor to manage his resources, get a profit and be able to meet contractual requirements.

Tomas Rivera
Altek System
Scheduling for high performance construction projects
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Fri, 2003-09-12 11:43
#4
Bernard Ertl
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Joe, thanks. That was very interesting. I assume this came from:

Construction Delay Claims

Bernard Ertl
InterPlan Systems Inc. - Project Management Software, Project Planning Software
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Fri, 2003-09-12 10:12
#5
Joe Mansour
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Hello Ronald and Tomas,

Let me start by saying that I this issue is not a minor one, if you want to be in full control of your schedule.

I would like to share with you an extract from a very interesting book by "Bramble and Callahan" on this subject:

"Primavera software may calculate early start dates in a questionable manner under the contiguous activity calculation mode. Early start dates for activities without predecessors are calculated by subtracting the activity’s duration from any finish date provided through a finish-to-finish relationship with another activity. The forward-pass calculation performed by the computer under the contiguous calculation mode is actually going backward.

An activity may have simple predecessors, start-to start predecessors, and/or finish-to-finish predecessors. Primavera’s calculation method, although identified as an early start date, is in fact a late start because the calculation is based on the finish of a finish-to-finish relationship, ignoring any other relationships. The problem is not obvious when there are multiple relationships affecting an activity.

Subsequent activities with start-to-start relationships may also be affected by the method of calculation of that early start date. Thus, the contiguous calculation mode can affect the early start date of activities far ahead in the schedule. The calculation method can also reposition the critical path.

When finish-to-finish relationships have the potential of being the controlling relationship, delaying the finish of one of the activities may cause the project schedule to appear to finish earlier. Adjusting the logic to add appropriate predecessors may permit the calculation of proper early start dates. Often, adding appropriate predecessors requires dividing the activity. In cases in which activities may be divided into four or five parts, the true critical value of the various parts of the activity becomes apparent.

Classic CPM theory would calculate the early start date from the project’s notice to proceed or data date. Many schedulers consider the classic theory the proper calculation method for the determination of early start and early finish, The issue revolves around the selection of which float values to use in precedence schedules and proper relationship rules. Start float, activity float, and finish float can vary dramatically, with complex relationships. However, under classic scheduling theory, there can be one early start float, multiple activity floats for different portions of the activity, and one finish float."

How about that?

Joe
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Mon, 2003-09-08 11:10
#6
Ronald Winter
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What is more realistic to you might not be so for another. My experiences with tasks being performed by subcontractors is that they do not want to mobilize and send a crew out to the field until there is enough open area for them to work continuously until finished. Then they leave.

Subcontractors cant make a profit if their men are sitting around waiting for some other company to get out of their way to continue. Wise Subcontractors who bid the lowest and make the most in profits demand that the Continuous Mode be used to schedule their work.
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Mon, 2003-09-08 06:02
#7
Joe Mansour
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When using the interruptable option linking two activities with SS and FF, the early start of the successor activity is more realistic than when using the contiguous option.
In that case, how could you explain using the contiguous? Wouldnt the interruptable option be a must?
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Wed, 2003-09-03 16:49
#8
Tomas Rivera
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Hi guys:

Let me add one more thing.

If you are trying to improve your schedule, some times using interruptible mode will give a better project finish date.

Tomas Rivera
Altek System
Scheduling for high performance construction projects
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Wed, 2003-09-03 10:32
#9
Ronald Winter
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Your situation is a little ‘backward.’ Usually, it is the Contractor who likes the continuous mode.

If you intend to work whenever work is available, even if it means that you ‘break-up’ the work into different time periods, then the Interruptible setting most closely reflects the described work. If you intend to wait and not work on a task until all resources are available and you are able to work without pause, then the Continuous mode best describes the work.

The Continuous mode is less aggressive in pursuing the work. It is also the most efficient method from the Contractor’s view point. Now you can see why I say, ‘backward.’ Usually, the Owner wants the schedule to reflect Interruptible work. Continuous mode reduces your float and gives you more chances for Owner-caused delay.

Finally, the issue of being told which mode to use. If your contract stipulates which mode to use (or gives the Owner the choice of determining this,) then you must use it as specified. If it does not, then the Owner is imposing additional requirements on you that are not part of your bid price. All in all, this is a small issue to worry over.
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