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. Developed by US Du Pont Corporation & Remington Rand in 1956

2. Activity oriented3. Single time estimate4. Deterministic approach5. activities are shown as a network

of precedence relationships using activity-on-node network construction

6. CPM is used for repetitive types of projects where the time estimates for various activities are either known or can be determined accurately

7. CPM places emphasis upon optimising allocation of resources and minimizing overall project cost

PERT

1. Developed by US Navy during 1957-58 while working on Polaris Missile program

2. Event oriented3. Three time estimates-To , Tl, Tp ,

4. Probabilistic approach 5. activities are shown as a network

of precedence relationships using activity-on-arrow network construction

6. PERT is used for pioneering type (R&D) projects and where prior data about activity times is not available

7. PERT lays emphasis on reducing project completion time without cost constraint

Program Evaluation & Review TechniqueThree time estimates used in PERT are1. Optimistic Time Estimate (To): shortest possible time under ideal conditions2. Most likely Time Estimate (Tm): time for completing activity under normal

conditions3. 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

1 Driving precast piles for a bridge abutment 22 30 50

2 Erecting roof trusses for factory shed 11 14 17

3 Concreting foundation for turbo-generator 3 5 1/4 6

4 Fabricating sheet metal AC ducts for an auditorium 12 16 17

5

6

32

14

515.5

SD

4.67

1.00

0.50

0.83

V

21.78

1.00

0.25

0.69

Activity Immediate predecessor Opt.Time Most likely

TimePessimistic

Time

a - 10 22 22

b - 20 20 20

c - 4 10 16

d a 2 14 32

e b,c 8 8 20

f b,c 8 14 20

g b,c 4 4 4

h c 2 12 16

I g,h 6 16 38

j d,e 2 8 14

Expected Time

20

20

10

15

10

14

4

11

18

8

Std.Dev

2

0

2

5

2

2

0

2

5

2

Variance

4

0

4

25

4

4

0

5

28

4

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

ACTIVITYPRECEEDING SUCCESSOR

EVENT

Basic Rules for developing Network

• No activity can commence until all preceding activities have been completed

• A dummy activity is introduced in the network either to show dependency or to avoid duplicate numbering of activities

CC Pier1 3 4

Erect Beams

2

PreCast Beams

Build Wall1 3 4

Plaster wall

• 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

Basic Rules for developing Network

Network elements

• Sequential Activities

• Diverging Activities

• Converging Activities

• Parallel Activities

Build Wall1 3 4

Plaster wall

CC Pier13

2PreCast Beams

CC Pier1

3Erect Beams

2PreCast Beams

C D

A B

Network Representation

• Activity On Node (AON)– Activities are represented on nodes and arrows are used to show the

dependency relationships– Duration is also indicated in the node

• 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

1 22 Days

Tail Event Head Event

start

A2

B3

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

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

Network Analysis

Activity (ij)

Duration (Tij)

Earliest Latest Total FloatEST EFT LST LFT

1-2 2 0

1-3 3

2-4 4

3-4 5

4-5 6

20 3

3 82 6

8 14

2 40 34 83 88 14

20200

1. Draw Network2. Calculate float3. Identify Critical Path

1 2 4

3

52

3

4

5

6

Critical Path: 1-3-4-5

Activity (ij)

Duration (Tij)

Earliest Latest Total FloatEST EFT LST LFT

a-b 6 0

a-c 8

b-c 4

c-d 0

b-d 3

c-e 6

d-e 10

e-f 3

60 8

10 106 10

6 9

0 62 106 10

10 107 10

02001

a b d

c f

6

8

3

0

3

10 2010 16

20 23

14 2010 2020 23

400

4

e6

10

Critical Path: a-b-c-d-e-f

1. Draw Network2. Calculate float3. Identify Critical Path

Activity (ij)

Duration (Tij)

Earliest Latest Total FloatEST EFT LST LFT

1-2 5 0

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

50 4

5 95 5

5 12.5 13.6 21.1

5 10.510.6 16.810.5 16.816.1 21.1

010

1.58.6

5 11.25 10.5

10.5 16.8

16.8 21.121.116.815.510.5

05.60

1. Draw Network2. Calculate float3. Identify Critical Path

6.5 10.55 51 50 5

05.6

15

2

3

4

6

54

47.5

5.5

6.26.3

54.3

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

1 2

4

3

5 6

7

8

9

10

Network DevelopmentActivities: A,B,E,Q,K,X,J,Z,G,F,CLogic1. A&B can be carried out at the same time. They represent the beginning of the job2. K follows E3. X depends on Q&K4. Neither F nor G can be started before B is completed, but they can be concurrently performed5. E&Q follow A6. Q must be carried out before J7. C depends on the completion of F&G8. E&Q can be executed at the same time9. Z can only be started when C,X and J are finished10. Z is the last activity

1A

B

2E

Q

4K

9 Z 10

3F

G

J5

X7

6 C

8

Network DevelopmentActivities: M,N,O,P,Q,R,S and TLogica. Activities M,N and Q can start concurrently and represent start of the projectb. Activities O&P are concurrent and depend on completion of both M&Nc. Activities R&S are concurrent and depend on the completion of Od. Activity T depends upon the completion of P,Q&Re. The project is complete when S&T are completed

5

T

1 M

Q

N O

P2

3

6

4

R

S

• 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

g

b

c

a

d

h

j

fe

i

Network Development

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…..