1 Lecture by Junaid Arshad Department of Engineering Management Abridged and adapted by A. M. Al-Araki, sept. 2013 WBS: Lowest level OBS: Lowest level.

1

Lecture by Junaid Arshad Department of Engineering Management

Abridged and adapted by A. M. Al-Araki, sept. 2013

WBS: Lowest level

OB

S:

Low

est

level

Topics Covered

Network RepresentationNetwork DevelopmentDummy ActivityNetwork Scheduling Practice ProblemsCPM Network Calculations

Even though a linked Gantt Chart can include precedence relationships between activities, it is an unsuitable tool if these relationships grow beyond a certain complexity.

In such situations, the scheduling should be carried out by a network technique.

However, the presentation of the schedule to the project participants may still be done in a Gantt Chart format.

Network RepresentationNetwork Representation

Arrows and nodes are used for construction

of Network.

There are two types of network representations:

1. Activity on arrow (AOA)

2. Activity on node (AON)

Arrow Node Node

1. Activity on arrow (AOA)

Activity Event Event

In AOA Arrow shows activity and precedence relationships

2. Activity on node (AON)

Node

In AON Arrow shows precedence relationships only

Activity

Following information is required in order to develop a network:

A list of all activities

A list of all precedence relationships

An estimated duration of each activity (required for network scheduling)

Network DevelopmentNetwork Development

For structur

e of network

Problem 1

Activity Immediate Predecessor

(s)

A -

B -

C A

D B

Table for network development

Problem 1

A

B

C

D

Activity on arrow (AOA)

Act-ivity

Immediate Predecess

or

A -

B -

C A

D B

Problem 1

1

3

A C

B D

2

4

Activity on arrow (AOA)

Problem 1

Activity on node (AON)

D

CA

B

START END

An activity inserted into the network to maintain the logic of the network.

A dummy takes zero time and do not consume any resources.

Dummy ActivityDummy Activity

2 31A

C

BD

A precedes B and C, B and C precede D

4

Dummy ActivityDummy ActivityIn

corr

ect

Dis

pla

y

2 41A C

B

D

3

5

Add a Dummy as shown by dash line below.

Corr

ect

Dis

pla

y

The purpose of network scheduling is o To determine when the project will finisho To determine which activities directly

influence the project duration

Two techniques of network scheduling are:1. Critical path method (CPM)2. Program evaluation and review

technique (PERT)

Network Scheduling

PERT and CPM Developed in 1950’s

CPM by DuPont Cooperation for chemical plants (1957)

PERT by Booz Allen Hamilton with the U.S. Navy, for Polaris missile (1958)

Consider precedence relationships and interdependencies

Each uses a different estimate of activity times

Is the project on schedule, ahead of schedule, or behind schedule?

Is the project over or under cost budget? Are there enough resources available to finish

the project on time? If the project must be finished in less than the

scheduled amount of time, what is the way to accomplish this at least cost?

Questions which may be addressed by PERT & CPM

Six Common Steps to PERT & CPM

1. Define the project and prepare the work breakdown structure.

2. Develop relationships among the activities. (Decide which activities must precede and which must follow others.)

3. Draw the network connecting all of the activities

4. Assign time (and/or cost) estimates to each activity

5. Compute the longest time path through the network. This is called the critical path

6. Use the network to help plan, schedule, monitor, and control the project

CPM calculates a network in several steps:

For each event:

o Earliest possible time: The earliest possible time the event can occur

o Latest possible time: The latest possible time the event can occur

CPM Network Calculations

For each activity:

o Early start (ES): The earliest possible start time for the activity

o Early finish (EF): The earliest possible finish time for the activity

For each activity:

o Late start (LS): The latest possible start time for the activity so as not to delay the completion time of the entire project.

o Late finish (LF): The latest possible finish time for the activity so as not to delay the completion time of the entire project.

Continue to next lecture

20

Topics Covered

CPM Calculations for AOA

and AON Networks Slack Time / Float Critical Path, Critical Activity Practice Problems

CPM Calculations for AON Network

Provides activity information

Early start (ES) & late start (LS) Early finish (EF) & late finish (LF) Slack (S) / Float (FL)

Identifies critical path

Forward and Backward Pass

Forward pass is a technique to move forward through network diagram. Backward pass is its opposite.

Early Start (ES) and Early Finish (EF) use the forward pass technique.

Late Start (LS) and Late Finish (LF) use the backward pass technique.

Note: If the float of the activity is zero, the two starts (ES and LS) and the two finish (EF and LF) are the same. Hence, If float of activity is zero, ES = LS and EF = LF.

Early Start and Early Finish Steps

Begin at starting event and work forward ES = 0 for starting activities

ES is earliest start EF = ES + Activity time

EF is earliest finish ES = Maximum EF of all predecessors for

non- starting activities

Late Start and Late Finish Steps

Begin at ending event and work backward LF = Maximum EF for ending activities

LF is latest finish; EF is earliest finish LS = LF - Activity time

LS is latest start LF = Minimum LS of all successors for

non-ending activities

Latest Finish

ES

LS

EF

LF

Earliest Finish

Latest Start

Earliest Start

Activity Nam

eActivity Duration

AON Network Calculations

Slack Time is tis the amount of time an activity may be delayed without affecting the project deadline. This is also referred as float.

An activity having zero slack time is called critical activity.

The concept of critical activities is that it draws the attention of the project manager to the activities that needs the closest monitoring.

Any delay of a critical activity leads to an equivalent delay of the total project.

A path having longest duration (completion time) in the network is called critical path.

Slack Time/FloatSlack Time/Float

Gantt Chart for General Hospital Early Start and Finish

ES EFES EF

A Build internal componentsB Modify roof and floorC Construct collection stackD Pour concrete and install frameE Build high-temperature burnerF Install pollution control systemG Install air pollution deviceH Inspect and test

1 2 3 4 5 6 7 8 9 10 1112 13 1415 16

LS LF