Construction Planning Simplified
One of the crucial purpose of preparing any plan or schedule is to facilitate the construction team, not only in execution but to follow correct sequence, monitor productivity & take some control measures. To achieve this purpose the construction team including the front line supervisors should be familiar with the terms used in planning & scheduling. There are several terms which; though clear to an experienced planner; are somewhat baffling to construction professionals with no planning background. In this chapter I shall try to familiarize these terms to construction team through simple explanation.
Let's start with understanding difference between planning and scheduling.
PLANNING
The term Planning is associated with basic thinking about the methodology for delivering the project. Thus all the strategy about phasing, zoning, logistics, vendor deployment, sourcing Material etc. comes under purview of planning. In simple terms, you decide in how many parts the slab will be cast, how many number of columns sets to be fabricated, what quantity of shuttering is required. What should be the size of stores and yards, how to provide water and electricity for construction, where and what capacity batching plant is required, how many transit mixers required, what capacity of equipment viz. excavator, cranes, trucks, passenger-cum-material hoists etc., where and what size of approach roads to be made and how the logistics would be including when in stages it would all be removed. It also assists in decomposition of entire scope of work till activity level including decision as to how detailed the schedule to be.
SCHEDULING
Scheduling is all about applying
logical sequencing, timeline or durations and resources required to execute the
activities. It also includes the levelling of resources and timeline
adjustments. Thus all sort of dealing with numbers be it resources or time
comes under Scheduling.
Next, I am taking the dependencies of activities on each other, which decides the sequence. These terms are very logical and easy to understand.
HARD LOGIC / CAUSAL DEPENDENCY
The Hard Logic is causal dependency,
meaning that the activity can start only if certain other condition i.e. some
other activity is complete. It’s like concrete can be cast only after formwork
is complete or 2nd level column can be taken only after 1st
level slab is cast.
SOFT LOGIC / CONSTRAINED DEPENDENCY
Now, imagine casting of all the
columns of a floor say 36 numbers. Do we need to do the reinforcement and then
shuttering of all of them and cast together? Can we make 9 sets and cast these
in 4 stages or make 6 sets and cast in 6 stages? Sure we can, but then is there
a causal dependency or a hard logic? No! Except for dependency on number of
sets and number of days available in schedule, which are not the causal
dependencies. This is what is decided in ‘Planning’ and then when we link these
4 or 6 stages after one another is called as ‘Soft Logic’ or sometimes
constrained logic.
PREFERENTIAL LOGIC
Sometimes, there is a preference by
certain important stakeholders, when the activities are not dependent or
constrained in any other way; like South-West area to be taken up first to
satisfy ‘Vastu’ followers. Another example is the preference given to make
sample flat 100% ready as early as possible for marketing purpose etc.
In planning software the dependencies are represented by the links and there can be 4 types of links to show these relationships among activities. I am describing these next.
FS – FINISH TO START
This is the most common and useful link.
The succeeding activity cannot start without completion of predecessor activity
viz. you cannot do foundation without completing excavation. Thus the activity
of Foundation will be linked to Excavation as ‘FS’
SS – START TO START
Suppose you are doing Block work
masonry for a Floor having 2 housing units and you have separate activities for
each flat for better tracking, then these 2 Block work masonry activities can be
linked as ‘SS’. However it’s not considered as a very good option because the
activity is also having a dependency of Slab Formwork Removal. Thus at-least
one of these Block work masonry activity must be linked with Slab Formwork
Removal as FS.
FF – FINISH TO FINISH
Suppose there are 2-3 activities,
which can be done in parallel and all must finish to allow start of next
activity viz. you need to complete Formwork, Laying Conduits and Reinforcement
prior to casting slab concrete, you can link these to finish together to as FF
to allow next activity to start which would then be linked with any one of
these as FS.
SF – START TO FINISH
The logic is that the start of
succeeding activity only confirms the completion of predecessor activity.
Example to comprehend the logic is in 24x7 operations the start of 2nd
shift indicates the finish of 1st shift. Another example to
understand this link is the activity of Commissioning of HVAC System is
considered as finished only when HVAC System starts operating. This is a rare
link and is normally not used.
I believe knowing these links was helpful, however there are a lot of other technical points but only relevant to planners for giving appropriate links to make the schedule more efficient.
All these links can also be adjusted using ‘lags’ and ‘leads’. It would be easy to understand through practical examples.
LAG
In case of large footprint of a
building you can do part excavation and start casting PCC for footings, while
continuing excavation for the rest and subsequently cast PCC for remaining footings.
In such a case the activity of PCC for Footings will be linked to Excavation as
‘SS’ but excavation for few footings will take a few days, hence we need to
start casting PCC for footings after this period. In such conditions, we have
to give a lag of few days (say 4 days) and the modified link would be ‘SS+4’. Another
example can be that the activity of de-shuttering of column can start only
after a day of casting the column. Thus De-shuttering will have a link with
Casting of Column as ‘FS’ but with a lag of 1 day thus link will be modified as
‘FS+1’
LEAD
Think about the earlier example of
Slab. Formwork, Reinforcement and Conduit placement should be complete to allow
casting of slab. In general the reinforcement is last to complete though it
starts after Formwork and allows Conduit placement in between and gets
completed after Conduit placement. Sometimes even the stoppers of slab is done
after completion of reinforcement and thus Formwork may need to complete after
reinforcement. Thus these activities can be linked using ‘FF’ with some
modification. Conduit Placement finishes a bit ahead (say 2 days) of
Reinforcement Completion, though reinforcement has started earlier than
conduits, further the stoppers may get completed after reinforcement (say 1 day)
though it had started prior to reinforcement and conduit. In such a case
Conduit Placement will have a link ‘FF-2’ with Reinforcement and Reinforcement
may have a link of ‘FF-1’ with Formwork.
Although further detailed explanation and deliberation is required but that's essential only for planners and not necessary for construction team.
Next I take the widely used term ‘Float’.
There is free float and total float, but I am describing only the term float,
which is enough for the construction team to know.
FLOAT
If there are 2 or more activities of
either different duration or different start/completion times running in
parallel to each other and these need to be complete prior to start of a
succeeding activity. There are chances that one or more of them may complete
earlier than others. Such activities if delayed by a certain period will not
cause delay to succeeding activity. This period is called float in those
activities. For example, you need to complete Formwork, Laying Conduits and Reinforcement
prior to casting slab concrete. These have different duration and start/completion
time. Even if Formwork and Conduit is complete as per schedule but
reinforcement though on schedule but not complete, you cannot cast concrete. In
such a case conduit and formwork activity is said to have a float. The quantum
of float in number of days is the duration gap between the scheduled completion
of Formwork or Laying conduit and scheduled start of casting concrete, which is
the succeeding activity.
The last item in this article, but most widely used and important term is ‘Critical Path’
CRITICAL PATH
The previous example of ‘Float’
explains that there is a possibility that some activities although delayed a
bit may not impact the overall timeline of the project, however the remaining
activities if delayed even by a day, may impact the project timeline. All such
remaining activities, which if delayed, may delay the project are called
‘Critical Activities’. All such activities are normally connected to each other
and the whole network of these activities is called the ‘Critical Path’.
Naturally, these activities need to be monitored and controlled with great
caution and focus and thus are most important for the construction team to
know.
At the end of this article, I would also like to know from the readers if this article was useful, especially for the persons involved in core construction. It would also help, if you all can suggest as to what more I can take up for my next articles.
Abhay Gharpure
ADGP www.decodingcm.com/pm-blogs/ https://decodingcm.blogspot.com
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