1 3. D EVELOPING A CTIVITY- O N- T HE- A RROW N ETWORKS Objective: To learn the principles of developing activity-on-the-arrow networks. This is the first step in Critical Path Method (CPM) analysis.

2 Summary: 3.1 Basic Components of the Activity-on-the-Arrow Network 3.2 Simple Logical Constructs 3.3 Extended Logic Using the Dummy ____ _ Activity 3.4 General Principles in Network Design 3.5 The Systems Approach to Designing Large Networks

3 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.

4 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.

5 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 design clear site lay curbs cure concrete

6 –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.

7 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 site 3 events 4

8 3.2 Simple Logical Constructs Activity arrows and events are configured to represent the logical dependencies between the activities.

9 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 excavate foundations 6

10 Several activities can precede an event (termed a merge event). Fig. 3-4: Merge Event set-up conc. batching plant construct access road excavate foundations 7 –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.

11 Several activities can succeed an event (termed a burst event). Fig. 3-5: Burst Event 7 side walls down stream apron up-stream apron –Once the event has occurred, all bursting activities may start.

12 Where activities can logically occur in parallel they should be represented as such. Fig. 3-6: Parallelizing of Activities set out footing (a) serial 12 excav. footing 13 construct frmwk. 14 position frmwk. (b) parallel 11 set out footing 12 excav. footing construct frmwk. 14 position frmwk.

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.

14 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.

15 In Fig. 3-7a, for example, both activities have the i-j identity Fig. 3-7: Ensuring Unique Identity (a) ambiguous identification 21 pour pier base A pour pier base B 23 Solve by introducing an extra event and a dummy activity. Note, dummy goes before the activity (otherwise can change float - see next lecture). 22 dummy event dummy activity pour pier base B (b) unique identification pour pier base A

16 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

17 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 C B dummy activity

18 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 C B P

19 –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 C B P

20 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).

21 Fig Dealing with Lengthy Activities 12 excav. trench (a) lengthy sequential activities shore trench lay pipe excav. trn. 1 excav. trn. 2 excav. trn. 3 shore 1 shore 2 shore 3 lay pipe 1 lay pipe 3 lay pipe 2 (b) phased lengthy sequential activities

General Principles in Network Design In developing an activity network: –First sketch it by hand. Fig Drafting Network Diagrams (a) initial sketch A C B D E G F

23 –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.

24 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?

25 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.

26 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.

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.

28 Office Facility Site work Building Preliminary site work U/G Utilities Landscaping Foundation RC Stairwell Steel Frame Walls & Ceilings Mechanical Plumbing Electrical Finishes Structural System Interior Exterior Envelope Roof CMU & Brick Veneer Doors & Windows Walls & Ceilings Mechanical Plumbing Electrical Finishes Walls & Ceilings Mechanical Plumbing Electrical Finishes 3 rd Floor 1 st Floor 2 nd Floor Fig. 3-12: Component-Oriented Break Down of an Office Facility, in the Form of a Tree Classification

29 Office Facility:LEVEL 1 Site work: LEVEL 1.1 Building: LEVEL 1.2 ETC… Preliminary site work:LEVEL U/G Utilities:LEVEL Landscaping: LEVEL Fig. 3-13: Component-Oriented Break Down of an Office Facility, in the Form of a Box Classification Different levels in the break down of the project. Different levels in the break down of the project. Any box that contains activities represents a group or category of activities Any box that contains activities represents a group or category of activities Activities general to the entire project would appear at the highest level: eg: general mobilization should be at the outer level. Activities general to the entire project would appear at the highest level: eg: general mobilization should be at the outer level. Specific activities would appear in the category most relevant eg: rough grade should be at level Specific activities would appear in the category most relevant eg: rough grade should be at level