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3. DEVELOPING

ACTIVITY-ON-THE-ARROW

NETWORKS

Objective:

To learn the principles of developing activity-on-the-arrow networks.

This is the first step in Critical Path Method (CPM) analysis.

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Summary:

3.1 Basic Components of the Activity-on-the-Arrow Network

3.2 Simple Logical Constructs3.3 Extended Logic Using the Dummy ____ _ Activity3.4 General Principles in Network Design3.5 The Systems Approach to Designing Large

Networks

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3.1 Basic Components of the Activity-On-The-Arrow Network A more sophisticated means of planning than bar

charts, eg:– they allow extensive analysis of the effects of delays

on the progress of a project.

Bar charts still play an important role in basic planning and in quick communication.

Activity-on-the-arrow networks can be converted into bar charts.

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Networks show the sequences, concurrency, and dependencies between activities.

Network development is concerned with the initial planning phase of managing operations, and can proceed without consideration of activity durations.

The inclusion of activity durations and resources can occur later.

Changing the durations of activities does not affect the structure of the network.

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There are two principle components: the arrow, and the event:

• The arrow represents an activity or process that typically takes time to perform:

– The tail of the arrow marks the start of the activity while the head marks its finish.

Fig. 3-1: Activity Arrows

designclear site

lay curbs cure concre

te

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– The activities represented can be very specific in nature (such as lay curbs) to very general (such as clear site).

– The activities in a network will be of approximately the same level.

– An activity may represent any process that takes time to complete, including design work and the curing of concrete.

– In some conventions, the length of the arrow is used to indicate the duration of the activity.

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• The second principle component, the event, represents an instant in time, in particular the start or finish of an activity.

– The ends of an activity arrow always connect to events.

Fig. 3-2: Activity Events

clear site3

events

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3.2 Simple Logical Constructs

Activity arrows and events are configured to represent the logical dependencies between the activities.

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• The simplest logical construct is a linear series of activities.– An event occurs when the preceding activity

has been completed.– An activity cannot start until the event that

precedes it has been reached.– Activities are executed in series, reading in the

direction of the arrows.

Fig. 3-3: Simple Activity Series

3 clear site

4 setting out

5 excavatefoundations

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• Several activities can precede an event (termed a merge event).

Fig. 3-4: Merge Event

set-up conc.batching plant

constructaccess road

excavate

foundations

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– The event does not occur until all merging activities have been completed.

– This construct is used to indicate a situation where an activity cannot start until several other activities have been completed.

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• Several activities can succeed an event (termed a burst event).

Fig. 3-5: Burst Event

7 sidewallsdown stream apron

up-stream

apron

– Once the event has occurred, all bursting activities may start.

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• Where activities can logically occur in parallel they should be represented as such.

Fig. 3-6: Parallelizing of Activities

11 1212 set out footing

(a) serial

12 excav. footing

13 construct frmwk.

14 position frmwk.

(b) parallel

11 set out footing

1212 excav. footing

construct frmwk.

14 position frmwk.

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3.3 Extended Logic Using the Dummy Activity

A limitation of activity-on-the-arrow networks is that they often require dummy activities:

– dummy activities do not represent any form of work and usually take zero time to complete.

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• Some software packages identify activities by their start and finish event numbers (their i-j identity).

– Can be ambiguous if two or more activities start and finish at the same events.

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• In Fig. 3-7a, for example, both activities have the i-j identity 21-23.

Fig. 3-7: Ensuring Unique Identity

(a) ambiguous identification

21 pour pier base A

pour pier base B

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Solve by introducing an

extra event and a dummy activity.

Note, dummy goes before the activity

(otherwise can change float - see

next lecture).

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dummy eventdummy activity

pour pier base B

(b) unique identification

21 23 pour pier base A

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• Sometimes burst activities have identical precedences.

– In Fig 3-8a, for example, C and D are both preceded by A and B.

(a) C and D have same precedences

Fig. 3-8: Use of Dummy Activities to Separate Precedences

D

C

A

B

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• In other cases, burst activities may only share some of their precedences.

– In this case, a dummy is required to isolate those preceding activities that are not common to all merging activities (Eg: see Fig 3-8b).

Fig. 3-8: Use of Dummy Activities to Separate Precedences

DA

(b) D has subset of C’s precedences

CB

dummy activity

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• Taking the previous example a step further:

– If another activity, P, was introduced that had the same precedences as C then it could emerge from the same event (see Fig. 3-9a).

Fig. 3-9: Use of Dummy Activities to Implement Complicated Precedences

DA

(a) Dummy to implement P preceded by A and B

CB

P

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– However, if P’s precedence was further limited (say to that of just B), a second dummy activity would have to be introduced (see Fig. 3-9b).

Fig. 3-9: Use of Dummy Activities to Implement Complicated Precedences

DA

(b) 2 dummies to implement P preceded by B only

CB

P

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• Activities that are lengthy may be divided into phases to enable overlap in their execution.

– The dependencies between the various phases of the activities are represented by an appropriate inclusion of dummy activities (see Fig. 3-10).

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Fig. 3-10 Dealing with Lengthy Activities

1212 excav. trench

(a) lengthy sequential activities

shoretrench

laypipe

excav. trn. 1

excav. trn. 2 excav. trn. 3

shore 1 shore 2

shore 3 lay pipe 1 lay

pipe 3

laypipe 2(b) phased lengthy sequential activities

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3.4 General Principles in Network Design

• In developing an activity network:

– First sketch it by hand.

Fig. 3-11 Drafting Network Diagrams(a) initial sketch

A

C

B

D

E

G

F

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– Once the basic scheme has been established, a tidier version may be produced, possibly by input to the computer.

Fig. 3-11: Drafting Network Diagrams(b) finalized diagram

clear site

excav. pad found.

constr. temp. haul road

constr. form

position form & fix steel

clean up

pour conc.

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• When determining how an operation should be connected within an activity network, ask three questions about it:

– Which activities must immediately precede this operation?

– Which activities can immediately follow this operation?

– Which activities can be executed concurrently with this operation?

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• When building complicated networks:

• Start by designing a high level activity network expressing the work involved in the project as general tasks.

• More detailed networks, breaking the project down into more specific activities, are then developed from this.

• Typically, two or three levels of network will be developed in the planning of a project.

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• Higher level networks are useful for– understanding the general nature of the work involved in a project; and

– establishing and monitoring progress on longer term goals.

• Low level networks, (those with more detail) can be used for:– establishing the precise amount of work involved in a project; and

– planning and monitoring work in the short term.

• Activities in a network should all be very approximately of the same level in terms of the time and resource commitment they involve. Eg:

– a network would not normally divide the project into some activities of a few hours duration and others of a few months.

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3.4 The Systems Approach to Designing Large Networks

• Large activity networks can be difficult to develop and maintain:– difficult to control number of activities;– difficult to read the network as too many activities;

• Solution is to adopt the systems approach to network design:– a top-down approach to breaking down a project into parts;– can control the extent of the breakdown and thus the number of

activities;– provides different levels of structure to the network that makes it more

manageable and readable;– for planning, break down should be component oriented (rather than say

trade oriented) since this reflects the way it will be built.

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OfficeFacility

Site work

Building

Preliminarysite work

U/G Utilities

Landscaping

Foundation

RC Stairwell

Steel Frame

Walls & Ceilings

Mechanical

PlumbingElectrical

Finishes

StructuralSystem

Interior

ExteriorEnvelope

Roof

CMU & Brick Veneer

Doors &Windows

Walls & Ceilings

Mechanical

PlumbingElectrical

Finishes

Walls & Ceilings

Mechanical

PlumbingElectrical

Finishes

3rd Floor

1st Floor

2nd Floor

Fig. 3-12: Component-Oriented Break Down of an Office Facility, in the Form of a Tree Classification

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Office Facility: LEVEL 1

Site work: LEVEL 1.1

Building: LEVEL 1.2

ETC…

Preliminary site work: LEVEL 1.1.1

U/G Utilities: LEVEL 1.1.2

Landscaping: LEVEL 1.1.3

Fig. 3-13: Component-Oriented Break Down of an Office Facility, in the Form of a Box Classification

Different levelsin the

break downof the project.

Different levelsin the

break downof the project.

Any box thatcontains activities

represents agroup or category

of activities

Any box thatcontains activities

represents agroup or category

of activities

Activities general to the entireproject would appearat the highest level:

eg: general mobilizationshould be at the outer level.

Activities general to the entireproject would appearat the highest level:

eg: general mobilizationshould be at the outer level.

Specific activities wouldappear in the category

most relevanteg: rough grade

should be at level 1.1.1.

Specific activities wouldappear in the category

most relevanteg: rough grade

should be at level 1.1.1.