Tips on Linking Activities – Chapter 01
Further to my Tips on ‘Activity Description’ or ‘Naming the Activity’, I am now giving some tips on linking the activities. Every planner knows that there are 4 type of links available for use; which are:
Finish to Start – FS – Activity ‘B’ can start
after finishing Activity ‘A’
Start to Start – SS – Activity ‘B’ can start
after starting Activity ‘A’
Finish to Finish – FF – Activity ‘B’ can
finish after finishing Activity ‘A’
Start to Finish – SF – Activity ‘B’ must
start for finishing Activity ‘A’
Most commonly used Link is ‘FS’ and least or almost never used, is ‘SF’. The remaining 2 Links of ‘SS’ and ‘FF’ is used by some planners however not all of them are aware of the limitations which are must while using these links. Off course, there are some adjustments in terms of ‘Lead’ and ‘Lag’ which are also used along with the Links.
While ‘Primavera’ allows the use of SS & FF links to same activity to give a good amount of control; Microsoft Project” does not allows the same. This poses some issues, particularly while using ‘SS’ link as some of the control is lost on the activity, which may distort the outcome of schedule update as it may not correctly predict the impact on overall schedule.
I am giving an example to explain the same:
Assume a Floor in Building has several Flats
and you are making a schedule for finishing the flats. Consider the activities
of Block Masonry, Plastering, and Painting. Please note there would be several
more activities but for this example I am discarding those activities. Please
also note that every flat has several rooms and thus several walls and one can
make a micro for every room separately for a very detailed schedule, however it
might not serve the purpose and become too cumbersome to handle. As you are
also aware that not all the walls of the flat will start together and end
together and even any particular wall also can be taken to only a certain
height in a day for stability purpose (Technical Requirement). Thus it is also
not necessary that only after finishing a wall the second wall will be taken up
for masonry. Instead of complicating the schedule, I am considering the
activity at ‘per Flat’ level.
The Typical List of Activities would be:
Block-work Masonry - Flat 101
Plastering – Flat 101
Painting – Flat 101
Block-work Masonry - Flat 102
Plastering – Flat 102,
Painting – Flat 102
Block-work Masonry - Flat 103
Plastering – Flat 103
Painting – Flat 103
Considering the Gang Movement and if it fits into overall time line, consider initiating the works of Block Masonry flat after flat i.e. the activities of Masonry may have the Link ‘Finish to Start’ The Duration for Block Masonry is say 20 days per flat. The next activity is that of Plastering and assume plastering of a flat also takes around 15 days per flat, but remember that plastering can start only after 7 days of curing of the wall. The next Activity is of Painting and again the Painting can only start after 7 days of curing of plaster and assume that it takes 10 days for the entire flat. All the duration assumptions are based on the productivity of the works.
As you are aware that work of masonry is happening wall by wall in a flat and there could be some of the walls that can be ready in say 5 days’ time after initiating it and thus Plastering can be initiated after 7 days of completion of first few walls although the masonry for entire flat is not complete. Similarly in 3 days these walls can be plastered and one may initiate the painting work after the curing period of 7 days, although not all walls are plastered.
In such conditions one may initiate the Plastering activity and then the Painting activity in a flat and in such a case the Link considered would be ‘Start to Start’ with some adjustment in the form of Lag. The Typical Schedule Links and Bars would be as given below:
There are some issues, which need attention. Although for starting the activity the time for few walls (5 days) and technical condition of 7 days of curing is met with by giving a Link SS with a lag of 12 days; however look at the end date for walls which is 20th day after start and thereafter a gap of 7 days is required for curing only then the plastering can be initiated for the last of the walls & then, it would take minimum 3 days to complete the plastering of these walls. Thus the Plastering would complete only after 10 days of finishing the Block Masonry. This condition is not getting met with.
Similarly consider the Painting activity. Although the initiation of the activity is being taken care of by giving a lag of 10 days in start but the end date for plaster has to be beyond 7 days of ending the plaster and then some 4-5 days for the Painting works itself including Primer and Paint in several coats. Again this condition is also not met with.
Also notice that between the 2 flats there is a gap of 5 days for plastering and 10 days for painting though the block masonry activity has no gap. This will cause idling of manpower of plastering and painting.
Now consider giving a link of ‘Finish to Finish’ in such a case. The last of the Walls would need a curing period of 7 days and then the plastering would take 3 days. Similarly last of the plastering activity would need 7 days of curing and then painting would take 5 days more.
Now the Schedule would look like this:
In this case the technical conditions are met with giving the necessary time for ending the last of the activities of Plastering and Painting. It may also be noted that although the activities of plaster and painting could be started earlier, these are initiated based on the duration and technical conditions based on the possible end date.
Even in this case there is idling of manpower of plastering and painting activities.
Imagine what can happen in case the duration for these activities were the other way, i.e. the Masonry takes 10 days, Plastering 15 days and Painting takes 20 days.
The Schedule with Links of ‘Start to Start’
will look like this:
In this case the technical condition to start the works is met with due to link and end technical condition is being met with due to duration of activities of Plastering and Painting; however the Plastering activity in 2 flats is overlapped by 5 days necessitating additional Plastering manpower and similarly the Painting activity in 2 flats has an overlap of 10 days again necessitating additional manpower for Painting works.
The Schedule with Links of ‘Finish to Finish’
will look like this:
Although the technical conditions for end are met, there is an issue in the start of activity. Plastering cannot be started as per schedule above as the technical conditions are not meeting. There are also issues of overlapping but that’s anyway need not be considered as technical conditions itself are not being met with.
Clearly the links of ‘Start to Start’ or ‘Finish to Finish’ on their own are not sufficient for preparing the Schedule. Either you are forced to play with the Durations/Resources or put a constraint on either, the end date (in ‘Start to Start’ Link) or the finish date (in ‘Finish to Finish’ Link). Also, planners should not get carried away with the end dates alone, which are varying in each scenario as none of them are correct either because of technical criteria or resource criteria.
The planners have to consider all these
aspects and check each & every activity primarily to ensure that the
technical conditions are met with for both start and end dates for activities
and secondarily to check either the idling or the over-allocation of resources
while using the links.
Happy Planning!
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