Lecture_27_Critical Path in Network Analysis

27.1 Critical Path in Network Analysis
27.1.1 Basic Scheduling Computation s
The notations used are
(i, j) = Activity with tail event i and head event j
Ei = Earliest occurrence time of event i
Lj = Latest allowable occurrence time of event j
Dij = Estimated completion time of activity (i, j)
(Es)ij = Earliest starting time of activity (i, j)
(Ef)ij = Earliest finishing time of activity (i, j)
(Ls)ij = Latest starting time of activity (i, j)
(Lf)ij = Latest finishing time of activity (i, j)
The procedure is as follows
1. Determination of Earliest time (Ej): Forward Pass computation
? Step 1
The computation begins from the start node and move towards the end node. For
easiness, the forward pass computation starts by assuming the earliest occurrence time of
zero for the initial project event.
? Step 2
i. Earliest starting time of activity (i, j) is the earliest event time of the tail end event
i.e. (Es)ij = Ei
ii. Earliest finish time of activity (i, j) is the earliest starting time + the activity time
i.e. (Ef)ij = (Es)ij + Dij or (Ef)ij = Ei + Dij
iii. Earliest event time for event j is the maximum of the earliest finish times of all
activities ending in to that event i.e. Ej = max [(Ef)ij for all immediate predecessor
of (i, j)] or Ej =max [Ei + Dij]
2. Backward Pass computation (for latest allowable time)
? Step 1
For ending event assume E = L. Remember that all E’s have been computed by forward
pass computations.
Lecture 27
Critical Path in Network Analysis
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? Step 2
Latest finish time for activity (i, j) is equal to the latest event time of event j i.e. (Lf)ij = Lj
? Step 3
Latest starting time of activity (i, j) = the latest completion time of (i, j) – the activity time
or (Ls)ij =(Lf)ij - Dij or (Ls)ij = Lj - Dij
? Step 4
Latest event time for event ‘i’ is the minimum of the latest start time of all activities
originating from that event i.e. Li = min [(Ls)ij for all immediate successor of (i, j)] = min
[(Lf)ij - Dij] = min [Lj - Dij]
3. Determination of floats and slack times
There are three kinds of floats
? Total float – The amount of time by which the completion of an activity could be
delayed beyond the earliest expected completion time without affecting the overall
project duration time.
Mathematically
(Tf)ij = (Latest start – Earliest start) for activity ( i – j)
(Tf)ij = (Ls)ij - (Es)ij or (Tf)ij = (Lj - Dij) - Ei
? Free float – The time by which the completion of an activity can be delayed beyond the
earliest finish time without affecting the earliest start of a subsequent activity.
Mathematically
(Ff)ij = (Earliest time for event j – Earliest time for event i) – Activity time for ( i, j)
(Ff)ij = (Ej - Ei) - Dij
? Independent float – The amount of time by which the start of an activity can be delayed
without effecting the earliest start time of any immediately following activities, assuming
that the preceding activity has finished at its latest finish time.
Mathematically
(If)ij = (Ej - Li) - Dij
The negative independent float is always taken as zero.
? Event slack - It is defined as the difference between the latest event and earliest event
times.
Mathematically
Head event slack = Lj – Ej, Tail event slack = Li - Ei
4. Determination of critical path
? Critical event – The events with zero slack times are called critical events. In other
words the event i is said to be critical if Ei = Li
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? Critical activity – The activities with zero total float are known as critical activities. In
other words an activity is said to be critical if a delay in its start will cause a further delay
in the completion date of the entire project.
? Critical path – The sequence of critical activities in a network is called critical path. The
critical path is the longest path in the network from the starting event to ending event and
defines the minimum time required to complete the project.