CONSTRUCTION MANAGEMENT AND
ADMINISTRATION
UNIT-II
Unit II: list of topics
1. Large scale production
2. Economies of large scale production
3. Stages of Construction planning and scheduling
4. Work Breakdown Structure
5. Bar charts, limitations of bar charts
6. Network techniques in construction management
7. CPM and PERT
8. Network Problems
Network techniques in construction
management
• Network techniques are effective tools for
– Planning
– Scheduling and
– controlling construction jobs
• Network techniques provide a rational approach
• Application of these techniques essential to meet higher
productivities with constraint on resources such as time,
capital, skilled man power, equipment e.t.c.,
• The two commonly used network techniques are
– Critical Path Method (CPM)
– Programme Evaluation and Review Technique (PERT)
CPM
1.
2.
3.
4.
5.
6.
7.
Developed by US Du Pont
Corporation & Remington Rand in
1956
Activity oriented
Single time estimate
Deterministic approach
activities are shown as a network
of precedence relationships using
activity-on-node network
construction
CPM is used for repetitive types of
projects where the time estimates
for various activities are either
known or can be determined
accurately
CPM places emphasis upon
optimising allocation of resources
and minimizing overall project
cost
PERT
1.
2.
3.
4.
5.
6.
7.
Developed by US Navy during
1957-58 while working on Polaris
Missile program
Event oriented
Three time estimates-To , Tl, Tp ,
Probabilistic approach
activities are shown as a network
of precedence relationships using
activity-on-arrow network
construction
PERT is used for pioneering type
(R&D) projects and where prior
data about activity times is not
available
PERT lays emphasis on reducing
project completion time without
cost constraint
Program Evaluation & Review Technique
Three time estimates used in PERT are
1. Optimistic Time Estimate (To): shortest possible time under ideal conditions
2. Most likely Time Estimate (Tm): time for completing activity under normal
conditions
3. Pessimistic Time Estimate (Tp): Maximum time required to complete activity
under extremely adverse conditions in which everything goes wrong
• Expected Time Estimate (Te) = (To + 4Tm + Tp )/6
• Estimation Trend assumed to follow Beta Distribution curve
• To < Tm < Tp
• Std.Deviation of activity =(Tp-To)/6
• Higher the SD greater the uncertainty
• Variance of activity =(SD)2
– Variance reflects the spread of a value over a normal distribution
Three time estimates and Expected time
S.No Activity
To
Tl
Tp
Te
SD
V
1
Driving precast piles for a bridge abutment
22
30
50
32
4.67
21.78
2
Erecting roof trusses for factory shed
11
14
17
14
1.00
1.00
3
Concreting foundation for turbo-generator
3
5 1/4
6
0.50
0.25
4
Fabricating sheet metal AC ducts for an auditorium
12
16
17
5
15.5
0.83
0.69
5
6
Immediate
predecessor
Opt.Time
-
10
22
22
20
2
4
b
-
20
20
20
20
0
0
c
-
4
10
16
10
2
4
d
a
2
14
32
15
5
25
e
b,c
8
8
20
10
2
4
f
b,c
8
14
20
14
2
4
g
b,c
4
4
4
4
0
0
h
c
2
12
16
11
2
5
I
g,h
6
16
38
18
5
28
j
d,e
2
8
14
8
2
4
Activity
a
Most likely Pessimistic
Time
Time
Expected
Time
Std.Dev Variance
Terminology used in Network Techniques
• Activity (ij): a specific task, operation, job or function which consumes time &
resources and has a definite beginning and end
• Event: An instantaneous point in time marking the beginning or end of one or
more activities. It consumes no time or resources
• Network / Flow diagram: the diagrammatic representation of a work plan
showing the activities, step by step, leading to the established goal. It depicts
the interdependence among the various activities
• Duration (tij): Estimated time required to complete an activity
• Dummy activity: activity with zero duration and no resource consumption
SUCCESSOR
PRECEEDING
ACTIVITY
EVENT
Basic Rules for developing Network
• No activity can commence until all preceding activities have
been completed
1
Build Wall
3
Plaster wall
4
• A dummy activity is introduced in the network either to show
dependency or to avoid duplicate numbering of activities
2
1
CC Pier
3
Erect Beams
4
Basic Rules for developing Network
• Event numbers must not be duplicated
• 3logics considered to place an activity in the network
– Preceding activity
– Succeeding activity
– Simultaneously occurring activity
• No activity should lead back to a previous event (no looping)
• Only one start and only one finish
• No dangling activity
Network elements
• Sequential Activities
1
Build Wall
3
Plaster wall
4
2
• Diverging Activities
1
3
2
• Converging Activities
CC Pier
1
• Parallel Activities
A
C
3
B
D
Erect Beams
Network Representation
• Activity On Node (AON)
– Activities are represented on nodes and arrows are used to show the
dependency relationships
A
– Duration is also indicated in the node
2
start
B
3
• Activity On Arrow (AOA):
–
–
–
–
Activities are represented by Arrows drawn left to right
Activity description written above the arrow and duration below it
An event is graphically represented by a number enclosed in a circle
The beginning of an activity is marked by a “tail event” or preceding
event and the end by a “head event” or succeeding event
Pour concrete
Tail Event
1
2
2 Days
Head Event
Network Analysis
• Earliest Event Time(TE): It is the earliest possible occurrence of an
event i.e. the earliest possible time when all activities leading to
an event will be completed.
• Early Start Time (EST)
• Early Finish Time (EFT)
• Latest allowable Event Time (TL): ): It is the latest possible
occurrence of an event without delaying the project completion
time i.e. the latest time when all activities leading to an event
may be completed without delaying project completion time
• Late Start Time (LST)
• Late Finish Time (LFT)
Float in Activities
• Total Float (FT): The maximum time by which an activity can be
delayed without delaying project completion time.
– FT =LST-EST=LFT-EFT
– Free Float= TEj-EFT
– Independent Float=(TEj – TLj)- tij
• Critical Activities: Activities which have zero total float
• Critical Path: The path joining the critical activities
Network Analysis
Forward Pass:
Determine EST and EFT for each Task
– For all Initial Tasks, EST = 0, EFT equals EST plus Duration
– The EST for all other tasks with tail [i] is equal to the largest value of EFT for
all tasks with head [i]
– Project Completion Time is the largest value of EFT for all Final Tasks
Backward Pass:
Determine LFT and LST for each Task
– For all final Tasks, LFT =EFT, LST equals LFT minus Duration
– The LFT for all other tasks with head [j], is equal to the smallest value of LST
for all tasks with tail [j]
– At least one Initial Task must have LST = 0; none may be negative
Activity Duration
(ij)
(Tij)
1-2
2
1-3
3
2-4
4
3-4
5
4-5
6
Earliest
EST
Latest
EFT
LST
2
3
6
8
14
2
0
4
3
8
0
0
2
3
8
Total
Float
LFT
4
3
8
2
0
2
8
14
0
0
1. Draw Network
2. Calculate float
3. Identify Critical Path
Critical Path: 1-3-4-5
1
2
2
4
4
5
3
3
6
5
Activity Duration
(ij)
(Tij)
a-b
6
a-c
8
b-c
4
c-d
0
b-d
3
c-e
6
d-e
10
e-f
3
Earliest
EST
0
0
6
10
6
10
10
20
Latest
EFT
LST
6
8
10
10
9
16
20
23
0
2
6
10
7
14
10
20
Total
Float
LFT
6
10
10
0
2
0
10
10
20
0
1
4
20
23
0
0
1. Draw Network
2. Calculate float
3. Identify Critical Path
Critical Path: a-b-c-d-e-f
a
6
b
d
3
4
10
0
8
e
c
6
3
f
Activity Duration
(ij)
(Tij)
1-2
5
1-3
4
2-3
0
2-4
4
2-6
7.5
3-4
5.5
3-5
6.2
4-5
6.3
4-6
5
5-6
4.3
Earliest
EST
Latest
EFT
LST
5
0
4
5
5
5
9
5 12.5
5 10.5
5 11.2
10.5 16.8
0
1
5
0
10.5 15.5
16.8 21.1
LFT
5
5
5
0
1
0
6.5
13.6
10.5
21.1
5
10.6
10.5
16.8
1.5
8.6
0
10.5
16.1
16.8
16.8
21.1
21.1
2
1
4
5.6
0
5.6
0
7.5
5
4
5.5
3
1. Draw Network
2. Calculate float
3. Identify Critical Path
6
4
5
Total
Float
4.3
6.3
6.2
5
Critical Path: 1-2-3-4-5-6
Network Development
Events
1
2
3
4
5
6
7
8
9
10
Immediate
Predecessor
-
1
2
2
2
3,5
3,4
3,7
7
3,6,8,9
4
7
9
8
1
2
3
5
10
6
Network Development
Activities: A,B,E,Q,K,X,J,Z,G,F,C
Logic
1.
A&B can be carried out at the same time. They represent the beginning of the job
2.
K follows E
3.
X depends on Q&K
4.
Neither F nor G can be started before B is completed, but they can be concurrently performed
5.
E&Q follow A
6.
Q must be carried out before J
7.
C depends on the completion of F&G
8.
E&Q can be executed at the same time
9.
Z can only be started when C,X and J are finished
10. Z is the last activity
4
E
A
2
5
9
J
6
F
3
7
X
Q
1
B
K
G
C
8
Z
10
Network Development
Activities: M,N,O,P,Q,R,S and T
Logic
a. Activities M,N and Q can start concurrently and represent start of the project
b. Activities O&P are concurrent and depend on completion of both M&N
c. Activities R&S are concurrent and depend on the completion of O
d. Activity T depends upon the completion of P,Q&R
e. The project is complete when S&T are completed
3
N
1
M
4
O
2
Q
P
S
6
R
T
5
Network Development
• Previous class problem
Activity
Immediate
predecessor
a
b
-
c
-
d
a
e
b,c
f
b,c
g
b,c
h
c
I
g,h
j
d,e
d
j
a
e
f
b
g
c
h
i
Advantages of CPM Network in execution of
projects
• Calendar-wise construction schedule of activities useful to draw schedule of
men, machinery and material
• Inter-relationship and sequence of various activities are clear from network
• Procurement of various resources can be done according to the schedule
• Realistic time to complete the project can be ascertained by going into the
details of time and activity sequence
• During execution of the work the productivity constants can be revised
• In the event of the program getting upset due to some unforeseen reasons, a
revised CPM chart can be prepared and prompt action can be taken to avoid
further loss of resources
• The network scheduling ensures the optimum use of the men, machines and
material
• The executive gets a reliable and valuable aid to assess progress of the work
Clarifications and discussion…..