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Float for Start to Finish Activities

8 replies [Last post]
Tue, 2004-11-23 12:58
Asif Anwar
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Dear Planners

I have a programme with some of the activities are scheduled as Start to finish activities.

e.g. the predecessor for activity no. 1 is start to finish activity from activity no. 2. Now activity no. 1 does not have any successor, as its successor is activity no. 2 which cant be put as successor here because it will then become a loop. Hence the float for activity no. 1 will be until the end of the project.

Can anyone advise how to remove this float and what successor should be assigned to activity no. 1?

Asif
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Sun, 2005-01-16 10:42
#1
Jonathan Kirby
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PowerProjects has a link function that caters for this type of requirement where you need a preceeding activity [like deliveries] to follow the dominant work activity.
Using a FF link you can then set a maximum slope [in days or weeks or whatever] in the link properties.

The preceeding activity will then stay at a set time before the succeeding activity.

This also works with FS links .

Very useful for Procurement and design milestones, once set up you can quickly and reliably produce revised schedules of the milestones. [code them and set up a filtered standard report with the report riter]

Hope this helps.
JK
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Thu, 2004-12-02 09:21
#2
Asif Anwar
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Thanks all planners for the inputs.

The best solution which i used is to assign the successor activity of activity no.2 to activity no. 1 with a lag of same duration as activity no. 2. This solution is a bit similar to one of the suggestion given to split activity no. 2 into 2 activities. This way I managed to remove the lag.

1. Activity no. 1 gets the start to finish link from activity 2.

2. Activity 3 which is the successor of activity 2 gets the FS link from activity no. 1 with the same duration as activity 2 (if the activity 2 and activity 3 have FS relationship) or same lag as between Activity 2 and Acitivity 3.

Any better option, please advise?

Asif
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Wed, 2004-11-24 05:36
#3
Erik Jonker
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Asif,

The logic being used is not pure logic, but since it is an inherited program and you do not want to change it, I suggest you do the following: Add the construction activity’s successor logic to that of the delivery activity, just add the duration of the delivery activity to the lag. Your float should be correct.

There is a major problem with this logic, if the delivery is late, your construction activity will not move out, you might be sitting without material. The delivery will only drive the construction successors.

The correct way would have been to use a normal FS or SS+FF relationship. Asign an as late as possible (MSP) or a Zero Free Float (P3) constraint to the delivery activity. This will have the delivery show when it is required.

Regards,

Erik Jonker
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Wed, 2004-11-24 05:06
#4
Jaco Stadler
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I would suggest a Finish to Finish Link with a Lag. Example one brick delivered will not help you finsh the building. You need the last brick deliverd to enable you to finish the building on time. Even though your start is depended on enough bricks. (Not first Delivery sufficient that is the reason for S-Curves)

You have a choice of two things.

1) FF with a lag
2) add an Activity #300. (Sufficient Delivery to Start)

Your Logic Will Look Something Like This
#100) Delivery of Material Succesor = FF + Lag to #200 Construcution / Sucessor SS + Lag to #300 Sufficient Delivery
#200) Construction Successor = FF Complete Project

# 300) Sufficient Delivery to Start Succesor = SS #200 Construction.

(Without Drawing this I hope I got this right)

The above will stop your Network Logic Problem without having to manipilate the schedule with constraints.

Cheers I hope this help.



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Wed, 2004-11-24 04:32
#5
Asif Anwar
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Dear Planners

I should explain it a bit more.

Lets say, the activity 1 is delivery of material to site(offsite work activity). And for explanation, take activity 2 which i defined earlier as activity 100 in this case. Now this activity 100 is onsite activity, and it is governed from the predecessors from the previous onsite activities.

What is done in the programme is that, activity 1 is defined as a predecessor from activity 100 (start to finish) as the delivery to site will be governed with the actual start of activity 100. If activity 100 is delayed due to delay in onsite works, the activity 1 should also be delayed accordingly.

I know this is not a preferred solution and i have never done this before, but what programme i have been inherited from the tender team is like this and there are 1000s of such activities which at this stage will not be practical to change from start to finish logic to finish to start.

The one way, i do know is to put finish constraint to activity 1 to remove this float but this will cause a problem whenever i will reschedule the programme as there are 100s of such activities. But i reckon this may be the only solution.

I hope the above clarifies my problem.

Asif
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Wed, 2004-11-24 03:46
#6
Jaco Stadler
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Can activity 1 Finish After Activity 2 If not why not use a Finish to Finish Activity.

If yes why dont you link it with the project completion date and take up the float on the lag.

Use a Finish Constraint.

What is the logic Behind the linking.

Can Activity 2 Start Before Activity 1
Can Activity 2 Finish Before Activity 1
Can Activity 1 Start Before Activity 2
Can Activity 1 Finish Before Activity 2.

What Happen After the Activity is Finish.

Can you please give the activity descriptions this will help in the assesment of the logic that can be applied as you know their is a lot of link / Lags / Constraint that can be applied to make something "work". If you have a problem that you need a start and finish logic "loop" this say your are at lvl 2 (No Critical Path identifeid by logic (As per my definition of a Lvl 2) it is first when you get to lvl 3 when you will be able to link logically the activity’s if you encounter this problem in a lvl 3 schedule it means you need to break it down to a further lvl because the activity is at Lvl 2.

Cheers

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Wed, 2004-11-24 02:19
#7
Raj Maurya
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Asif,
The easiest way to set constraint as float zero on activity 2 and connect with the end as its successor. I wonder why you need to define start to finish relationship in your schedule? Generally we use it to connect start milestone to finish milestone with this relationship.
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Tue, 2004-11-23 16:46
#8
Tomas Rivera
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Asif:

Probably the best way to do it would be to split activity 2 in 2 activities. Activity 2A would be predecessor to activity 1, Activity 2B would be successor from activity 1 and from activity 2A also.

If you do not want to split activity 2 in two parts, I would choose to have activity 2 as successor from activity 1 and not use activity 2 as predecessor to activity 1. Also I would make the duration for activity 2 as long as necessary to overlap the start of activity 2 with the finish of activity 1. For this to work a third activity needs to come into the picture. We need activity 3 to be predecessor to activity 1 and activity 2. Also, the only predecessor to activity 2 should be activity 3.

The second alternative does not reflect the true situation you described as well as the first alternative. Both alternatives take care of the float.

Tomas Rivera
Altek System
Detailed scheduling of high performance
construction projects

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