Good Use Of Relationship

Like they said, you must use the relationship that is the most logical...If something can’t start until the pred finishes, then by all means use FS...But if the succ can start before it finished, use a SS...Be sure to understand what exactly you are trying to do...And be aware of the activity and the driver(s) associated with it...

It is hard to answer your question, because we don’t know the circumstances of the particular activities....



Alot of times a SS & FF can be used with the Pred activity including lags for both SS and FF, and usually you have to adjust the duration to make both of the Pred. Relationships as drivers...The asterict that will be on the pred. Box...



For example, if the pred is ISO issue and the current activity is spool fabrication...You could say SS lag 5 FF lag 60...The fabrication can’t start till 5 days after the first ISO is issued, but can’t finish until 60 days after the last ISO is issued...Then you would adjust the spool fabrication duration to make both of the pred relationships as drivers...



Hope this helped...

Khawaja,



It will definitely help everybody if you explain your problem clearly. Go through Stephen’s answers - it is really good. Anoon has then explained FS as the only absolute relationship and others are speculative - which is very correct.

i don’t understand it to be honest! SS & FF then it’s parallel!

Further to what have been said below, be careful about about relationships when you got other affairs.



For me, the only absolute relationship is FS (Finish to Start) others are speculative.



Say for example: Simple Concreting Works (in m3), if you have to break it down into measurable activities you’ll probably get the following:



1. Rebars - Kgs.

2. Formworks - m2

3. Concrete Placement - m3

but you got other activities like concrete curing and stripping of formworks and not to mention the quality inspection works, which you cannot place concrete until you get approval (these are the other affairs).



So in order to get a good relationship, you need to know the activity inside-out!

In addition, you have to consult your Construction Manager to reconcile the construction methodology and sequences of activities that he wants to implement during the construction. This will defined your logic and correct relationship of your activities. From there you can define which activities run on parallel (SS) and which activities need finish before other activities start (FS) and so on (SF, FF). Check your design, drawings and procurement items if constraining your activity. Check your deliverable dates and milestones that need to attain. After getting all information and derived to your schedule then review it again together with construction manager for any changes. I believed in this method will direct you on the correct path and produced a meaningful schedule that gives you a correct total float.

khawaja,

stephen has answered all ur questions.

i just want to add.. To attain a realistic TF, ur relationship/logic (and lags) must be realistic and logical...

to me, FS is a "tight" relationship. The pred must fin for the succ to start. Indirectly, the whole chain is link from start to end of the activties.

SS "logically tie" the start of the pred only. if there is no FS or FF relationship to it, it means the fin of the pred can "float" to the end of the project.

FF "logically tie" the fin of the pred only. without any other SS or SF relationship, it means the start of the pred can be anytime. (if u use the "stretchable" option in p3, the pred will actually start on the start of the project).

Most of the time, the activties are not detailed enough to enbale FS, so we hv to use SS and/or FF... but the key point is be "realistic and logical" in the lag time. As in Stephen’s example, he explained why the 5 days lag...

so as long as ur logic and lags are realistic and logical, the rsulting TF should be realistic...

khawaja, I’m not quite sure I understand your question, but I’ll give it a shot...



1. In an FS relationship (unless there is negative lag!), the entire predecessor must be finished before the successor can start. If the link is instead SS (with or without an additional FF), then at least some of the successor can start before the predecessor is finished.



2. In general, FF+lag and FS+lag relationships are qualitatively different from SS+lag and SF+lag relationships.

a. In the two former cases, the time (and delay) is being added AFTER the completion of the predecessor. Therefore, for scheduling purposes, any lag where the relationship starts with F is like a separate activity, and can usually be modeled as such. If they are on the critical path, FF+lag and FS+lag can have their own separate DRAG and DRAG Cost.

b. In the two latter cases (SS+lag and SF+lag), the time (and delay) is because of WORK ALREADY MODELED in the predecessor activity! For example, if DIG TRENCH is an SS+5 predecessor of LAY PIPE, those 5 days are due to work scheduled for the first five days of DIG TRENCH. Therefore if that relationship (and lag) is on the CP, the DRAG and DRAG Cost belong to the predecessor (DIG TRENCH), and if we can get that first 200 metres of trench dug in three days, we will have reduced its DRAG (and the scheduled project duration) by two days.



3. I honestly have no idea what you mean by "a better way to attain a realistic TOTAL FLOAT" or why that goal is so important to you. DRAG, which might be important, is easier to compute with only FS relationships. But if you are going to limit yourself to FS relationships, you have to make sure that all activities are adequately decomposed ("DIG INIT. 100m. OF TRENCH" and "DIG REST OF TRENCH"), or you’ll end up with an unnecessarily elongated schedule.



In palindromic terms:



"No! I tag no legal SS lag ’elongation’."