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In construction planning in primavera do we use negative float? If yes why ?
please any one can explain and anser this question for my understanding
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Mike and Rocky
I agree 100% with you both, unfortunately some specifications require them; for this my approach is to temporarily disable the constraints in order to get an unobstructed view for float values and schedule logic, especially when using resource leveling as traditional computation of these under finish constraint yields wrong results.
The Owners want to see the activities that fall within a chain with a broken finish no latter than constraint marked as critical in the Gantt report.
Within my software I can toggle on and off these constraints in several ways:
a) I can use formulas and user defined fields,
b) I can use what we call Reference Books to manipulate the database tables and
c) I can apply the constraints to open end milestones that can be triggered of and on using a functionality that allows me to toggle on and off any link, either manually or in a near future using formulas to set up the conditions.
Still I would prefer a simple programmed command to do so, easy life is good.
Option C I would leave it for temporarily applying my own non contractual constraints when prohibited by the specs. Yes even for the contractor temporary use of such constraints can be useful, but do not misunderstand me, I mean temporary.
Mohamed,
As you can see Owners want to be warned about activities that are critical not only to overall project finish date but also critical to intermediate contractual milestone. Just temporarily trigger them off for an unobstructed view of float and logic, and then restore them. Usually they are very few, in my last job we had only one intermediate finish constraint date for the delivery of a sanitary sewer lift station to be powered by a new electrical substation at a pre-determined date.
Regards,
Rafael
Hello everyone,
new to PP..just want to add something on this thread...
we don't use Negative float.... "float by definition is the amount of time an activity can be delayed"...
if it is negative then you can't use it..
regards,
the rock
Hi All
Just a small entry into the discussion.
Use of end constraints is a bad thing.
They restrict the dynamic use of logic and give a false result.
Never use them.
Best regards
Mike Testro
ps to Rafael - good to see you back on the forum.
There should be atleast one constraint say for project finish milestone. now other milestones can be connected with this milestone with suitable lag which shall automatically impose constraints to individual milestones.
regards
Tanveer,
The constraints only freeze the late finish date of the activity in question, the computations of early dates is not affected, only backward pass computations. When the float displayed is either 0 or negative it is a warning although perhaps with some unrealistic negative values, but still a valid warning about some milestone will not meet a target finish date.
The Owner has always the option to require the contractor to keep the contractual target date and make the adjustments to his schedule.
Then the contractor will look at his schedule and most probably will be in need to release the artificial constraints to follow the logic to solve the issue. After he solves the issue in any of the options feasible (if still feasible) then he will set back the constraints in his schedule, and if the issue was solved there will be no negative float and the late dates will be latter than the early dates releasing the anomaly negative float creates.
Yes as owners you have a lot to say about the schedule, even when the means and methods are the responsibility of the contractor, within his means he is still responsible of complying with the contractual dates.
Keep in mind that if the Owner caused delays to the contractual dates he might still make the call to keep the original contractual dates. In this case in the absence of an agreement for acceleration it becomes Constructive Acceleration and the contractor can claim for it. The specific procedure to require the contractor to keep schedule might vary depending on the wording of the contract. As an Owner you might even ask for acceleration without any agreement whatsoever, usually there is a mechanism for this, here we call it a Change Directive. A directive will not prevent the contractor to make a claim if latter on an agreement on the implied change is not reached, a well written contract will even provide for such not uncommon situations.
Contractual Constraints are not a bad idea, usually a real need, it is more a matter of how to deal with them within the CPM schedule. Contract finish date is the mother of all constraints and as soon as the job start lagging the owner has the right to require the contractor to keep schedule as to meet the mother of all constraints, in a way the approved schedule creates many contractual constraints that might even vary as the job progress.
Regards,
Rafael
Dear
MOHAMED RIAZ For ur information i would like explain about float. 1st of all you should know what it is the meaning of float. (Float = Amount of Time). and we have three type of floats which is following : 1. Positive Float 2. Negative Float 3. Total Float. & ur question is regarding negative float if u hv negative float in ur schedule it means ur schedule is going to be overlap (delay). hope this will help u .............if any correction than let me know...... thanks.......... IRFAN KHAN
hi Mukesh
can you tell me how i can restrict my milestone dates with out using constraint
Tanveer
Hi Rafael
How you are doing. Replying to Mohammad you just said a pitfall encountered in using milestones occurs when we try to restrict our milestone to a particular date. May be I am wrong but what I understood in that making constraint is not a good idea. If so what we do the client needs to finish some activity on a particular date
Thanx - Tanveer
You are correct, float value may be negative only when there is any constraint at project level or at activity/milestone level which is not being met either with early dates or resource vailability.
Regards
Even substandard software that create negative float under resource leveling do not create negative float automatically, first you have to create the constraints either at activity or milestone level or at the project level using an overall project finish constraint, otherwise no negative float will be shown. Is a mathematical computation that fools the traditional CPM backward pass computation that the user toggles on with the use of certain date constraints (availability of such constraints might vary among the software). Many experienced schedulers and particularly the forensic analyst avoid the use of negative float and do not use the constraints that create it.
Perhaps there is a single exception and it might be Spider Project, developed by a mathematician that understands how resource constrained float is determined and how the traditional computation of negative float under resource leveling is wrong.
The concept of negative float has the practical value of identifying the chain(s) that will not meet their milestone dates but the computed value is at times wrong. The correct approach is to identify these chains by displaying 0 float but no other value.
In the following example the displayed float value is set to 0 as the negative values can be wrong under resource leveling, the next column was determined not by the software but by defining a formula to compute the float under the traditional computation for illustration purposes.
The issue of negative lag is another issue, is a negative lag value the user enter at the link level.
Regards,
Rafael
negative float are calculated by all scheduling software automatically, if applicable. Negative float are not entered by planning engineer. I think you are actually asking about negative lag; not negative float.pl. confirm?
Mohamed,
The use of negative float is misleading and under resource leveling is erroneous. The following article identifies how it is produced and ways to tame it.
Proper Use of Milestones and Constraints
“One of the biggest pitfalls encountered in using milestones occurs when trying to restrict a milestone to a particular date in the schedule, as opposed to being logic driven. ”
Not only individual constraints can create negative float but also a project must finish date constraint can create negative float.
Each software has its ways, in Spider Project there is no negative float but it is either positive or 0 float as negative values under resource leveling are wrong. Still to find the logic under such constraints we must recur to our own ways, fortunately deleting and restoring back the constraints is easy by using what we call "Reference Books" our way to manipulate the database tables. In this way we get a clear warning on activities that fall under a chain that will not met such constraints, we can easily release them to understand the logic along the longest path(s) and then restore the constraints at the click of the mouse.
For non contractual milestones, especially when the contract limits the use of finish constraint only to contractual milestones I would consider a fixed milestone next to my non contractual milestones.
Best regards,
Rafael
Email: PPadmin@planningplanet.com
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