June_1971 — Page 35

Project network analysis

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The value of project network analysis (PNA) – also known as PERT or the critical path method for the planning, scheduling and con- trol of building work has in recent years been well illustrated by its application not only to complex large-scale projects but also to me- dium-sized ones. This feature sets out the basic principles of PNA and shows that its chief merit is a simple logic which imposes a clear and systematic approach to programming, and which is equally well suited to small projects not requiring computers for analysis.

PROJECT network analysis is a meth- od of planning, scheduling and con- trolling projects by recording their analysis in a diagrammatic form which enables each fundamental problem in- volved to be tackled separately.

The junction of arrows are called events. Sometimes an event corres- ponds to a well-defined stage, for ex- ample the completion of foundations, but usually an event serves no more useful a purpose than the separation of one activity from another.

ed on speeding up the relatively few 'critical' jobs; and (b) money is not wasted on speeding up non-critical jobs.

It employs a network diagram or flow chart to show the jobs to be done and their inter-relations. From this dia- grammatic 'method statement' and es- timates of times taken to do individual jobs, it calculates the project duration, the critical path (that sequence of jobs which defines project duration) and the float (or permissible delay) for non-critical jobs. A schedule or pro- gramme for the whole project is thus devised.

7. Schedules may be based on con- siderations of costs so as to complete projects in a given time at minimum expense.

What are the advantages of PNA? 1. It separates (a) planning the se- quence of jobs from (b) scheduling times for the jobs.

Procedure

Only one rule is needed to inter- pret an arrow diagram: at every event all activities represented by incoming arrows must be complete before any activity represented by an outgoing ar- row can start.

All projects consist of separate operations or jobs (called here activi- ties). The activities are inter-related technical or logical considerations will define a certain sequence in which the activities must be performed.

2. It shows the inter-relationships between jobs (not possible with a bar chart) and enables people to see not merely the overall plan, but the ways in which their own activities depend upon, or influence, those of others.

Fig. 1 is thus a more concise repre- sentation of the following method statement:

When the partitions are completed, the plumber's fittings and carpen- ter's fixing can be started, and also the window surrounds. The floor finish can be started when the car- penter's fixing is complete and the internal painting can commence when carpenter's fixing, plumber's fittings and window surrounds are finished.

The first step is to decide which activities are involved, and their se- quence. This requires care, thought and a good knowledge of the project in hand. In project network analysis the sequence is presented as a network of which the commonest form is the arrow diagram, each activity being re- presented by an arrow. Fig. 1 shows a typical sequence of activities forming part of a building project.

3. By setting out the complete plan for examination by everyone involved in the project it is easier to assess its soundness and so prevent unrealistic or superficial planning.

4. The effect on the project of al- ternative methods or individual job times can be examined at the outset.

5. The total requirements of man- power and plant can be readily cal- culated. By delaying or slowing down non-critical jobs, that is, those not im- mediately affecting the duration of the project, a schedule may be devised which makes the best allowance for any limitations in available resources.

6. The identification of the critical path has two immediate advantages: (a) If the completion date has to be advanced, attention can be concentrat-

Far East BUILDER, June 1971 Page 35

partitions

Fig. 1.

plumbers' fittings

window surrounds

finding

internal painting.

floor finish

The flow chart in Fig. 2 is an alter- native presentation of the method statement which does not require the use of dummy activities and may be preferred.

The duration of activities is not considered at this stage, so the arrow diagram is not drawn to a time scale. The arrows indicate the direction of time flow only.

plumbers' fittings

partitions

window surrounds

internal painting

complete

carpenters

fixing

floor finish

Fig. 2.

In practice, the necessary opera- tions or activities are listed and the ar- row diagram or flow chart drawn by

asking of each activity in turn:

The two unnamed broken arrows (called 'dummy' arrows) do not repre- sent real activities; they merely help clarify the sequence by showing, for example, that internal painting follows the completion of carpenter's fixing as well as of window surrounds.

What activities must precede it? What activities must follow it? What activities can be done at the

same time?

(0

partitions

(35)

Fig. 3.

plumbers fittings

(47)

window surrounds

(28)

Carpenter

(70)

internal paintin

floor finish

(17)

(50)

Once the sequence is known, es- timated times (in any convenient units) for each activity are added. Fig.

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Table 1