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Zero Free Float constraints

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Kumar I
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In what situations do we use Zero Free Float Constraints in a schedule,what are the advantages and draw backs

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Roberto Jr Bustos
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im not sure whether your post is a followup to this old thread (2005).  but to answer your question.

constraints can be based on the requirements of your project.  ZFF normally is used to make your early dates equal your late. basically scheduling the activities on the late dates.  examples are like what was mentioned, JIT deliveries.

most projects requires contractual milestones.  in transportation industry, there are access restraints, road closing and opening etc.  we can put constraints not just ZFF to specify the importance of a specific date for a specific milestone.

putting constraint to some contractual milestone makes the series of activities related to that M/S critical even if it doesnt belong to the longest path.  You are probably right, dont put constraints when it is really not needed.  but in most cases, especially for projects with stage completions, it is always needed.

 

Evaristus Ujam
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The employment of floats in a schedule is rather one way of mitigating risk rather than the other way round. This is given a situation whereby the optimist believes that everything as programmed will go smooth. The pessimist on the other hand sees monkeys everywhere and plans for it. The question is in which school are you? Both views should be accommodated and there lies the need for brainstorming before every time frame is concluded for a given schedule. Contingencies should be provided and imbedded in any schedule, especially when objective doubts exist as per a given budget element of time and cost needed to accomplish a given project. Failing to plan for such unforseens when there is a probability of its occurence means, we already have planed to fail. This way, floats be it free or total is a friend rather than an enemy.

Rafael Davila
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Sorry,

Seems like my interpretation on how Zero Total Float is implemented is wrong, it do not delays the activity as Zero Free Float does. The application of Zero TF constraint will not yield late schedule but an Early Schedule with all activities critical.

Zero TT and Zero FF

Best Regards,

Rafael

Rafael Davila
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In CPM scheduling terms logic is determined by the relationships between the activities, that is by the links, typical constraints do not change these, they remain the same.

Very few software provide functionality to change logic as conditional events do happen, one example is Spider Project.

www.youtube.com/watch?v=A314H5ds7Xw

Your sample table is in error, activity B TF is 7. Zero free float constraint can make an activity critical if the free float and total float are equal, therefore by applying zero free float constraint to all activities you end up consuming all activities free float as well as all activities total float, making them all critical, this is one way of forcing your schedule to become equal to the late schedule.

Constraints shall be used whenever they are required to represent true need. If you have a real need to model just in time deliveries because there is no room on site to store the material if you want to have a correct schedule then it is not optional but mandatory that you model just in time deliveries, if you can model it with the use of no free float constraint then let it be. In this case if you do not model just in time delieveries your model will be wrong as it will be displaying impossible early dates.

Good Practice Protocols that blindly put a limit on constraints based on quantities instead on a logical reason do more harm than good. Constraints were created because they are needed, that some make wrong use of these is another thing.

Mohamad Abdelsala
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Through this example its clear is by applying the zero free float constriant Activity D which is non critical will be critical this the 1st thing ,2nd thing the logic will completly change . am i right? so my opinion using any constraint should be in only special and rarely cases pls ur advise guys

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Regards,

Mohamad

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Mohamad Abdelsala
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Thanks Mike and Rafael

Rafael Davila
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Mohamad,

When your software do not provides the functionality to get Late schedule S Curve then you might need to resort to use zero free float constraint. By applying zero free float constraint to all activities you will get late schedule, same as if using zero total float constraint in all activities.

Beware that zero free float constraint and zero total float constraints are different, they have their use as real life is full of constraints.

The issue with constraints is that they fool the traditional calculations of CPM, they can easily be abused and usually in order to follow logic you have to temporarily toggle them off and on. For years I have been waiting for some functionality that will allow you to toggle on and of all constraints by type.

That constraints can be misused is no reason to ban them, otherwise if misused water can drown you then water shall be banned.

Kumar,

With the appropiate use of zero free float constraint you can model deliveries to be just-in -time, a need if you have no on-site storage capacity as pointed out by Brandon. This will provide the reader with the necesary dates for the delieveries be just-in-time.

Using Baseline functionality and custom formulas you can compute and display the slippage caused by the ALAP constraint, kind of a "Start Float" metrics.

ALAP Slippage

Regards,

Rafael

Mike Testro
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Hi Mohamad

I am assuming that when you say Zero Free Float constraints you are talking about tasks starting As Late As Possible thus placing all float at the start of the task not the end.

You are quite correct to be suspicious of a programme that uses this type of artificial restraint because it disguises the true critical path.

I would sugest you save the file in a different name and then remove all the ALAP logic to make it ASAP - you will then get a better idea of what the programme is saying.

Best regards

Mike Testro

Mohamad Abdelsala
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Hi Mike ,

yes its very old as i notice now :)

but i was searching about zero free float constraints , the problem am checking a time schedule right now and i found that the scheduler did all the activities with zero free float constraints that make me confused and angry in the same time lool

for me what he did is destroying the whole time schedule am i right in that?

best regards

Mike Testro
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Hi Mohamad

You have joined an old thread but the answer is always the same.

Never use artifical contraints in a programme becuase they distort the true critical path.

If there is a reason why a task must start on a particular point in the programme then put in a series of tasks and logic to fix the date.

If you are waiting for a decision from the designer then put in a task that describes that decision and monitor its progress.

Best regards

Mike Testro

Mohamad Abdelsala
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Actually i need to understand why i should use Constraints in my Program if its not Required i think its too risky but i found alot of schedulers they r using constraints and i wonder who is rite??

 

thanks

Raja Izat Raja Ib...
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like Mr Shaw say... its good if u are ready with the material and your area readiness, the curve will show drastic jump but at the end it depends either it will show u, slow moving progress or ???....the important thing make sure the work is in sequence as at site, afterwards at site its self will wasting time by doing this double job
Brandon Shaw
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I use them when wanting to delay an activity as late as possible without delaying a succeeding activity. ie, in a baseline schedule. It’s risky business I know, but if you have a heads up on material, such as it’s already on site or you control all resources for that job it isn’t that bad. Also Job status doesn’t affect anything here. Just make sure you’re justified in this change.