Part 1
This is the first of a series of three articles by Mr. P.A. Smith, AIOB, lecturer at Hong Kong Technical Col- lege. It describes the com- ponent parts and method of building up a network dia- gram and presents logically the development and me- thod of applying network analysis techniques. Part II of the series, to be publish- ed next month, will illus- trate the whole process by means of a worked example. Part III will describe exten- sions of the basic technique. The series is based on a pa- per presented to the En- gineering Society of Hong Kong and is published by kind permission of the So- ciety.
NETWORK ANALYSIS IN CONSTRUCTION
by P.A. Smith, AIOB
NETWORK Analysis is a general
term which is used to describe the relatively new planning techni- ques of
CPM Critical Path Method. PERT-Programme Evaluation and Review Technique.
Many people are aware of the po- tential contribution of Network An- alysis Techniques in planning work. Fewer people know the
the detailed mechanics of the techniques and fewer still appreciate that the techni- ques have application in all branches of engineering and administration.
Historical
Network techniques developed along two parallel streams-one mili- tary, the other industrial.
The military version, known as PERT, was originally devised for the Polaris Missile Programme in 1958 by the Programme Evaluation Branch of the Special Projects Office of the U.S. Navy in collaboration with Lockheed Missile Systems Division and the con- sultant firm of Booz-Allen & Hamil- ton.
―
The network was built from "mile- stones"
events to be achieved by specified dates in order to comply with the then Navy system of control reporting. The industrial version known as CPM began in 1956 when the Integrated Engineering Control Group of the American chemical firm E.I. du Pont de Nemours & Company began to look for new ways of scheduling projects. J.E. Kelley of that company and M.R. Walker of Remington Rand produced the Cri- tical Path Method.
The first application of the method was made in 1958 in the construction of a new chemical plant.
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CPM/PERT.
CPM differs from PERT in the following three ways:-
(1) CPM is built up from jobs. (2) No allowance is made for un- certainty in the estimates of job durations.
(3) Times are to some extent re-
lated to costs.
The division between the two tech- niques has become blurred with the passing of time. It is now common to use the PERT method of three time estimates for CPM where there is uncertainty in the estimates of job durations.
Most of the examples in the ap- plication of network analysis techni- ques given in this series will be drawn from the construction industry.
Network analysis is a powerful management technique for the plan- ning and control of projects; it is ex- tremely simple in principle but its ef- fects are far reaching.
In applying the technique, the pro- ject is broken down into separate ac- tivities each of which may take only a very small fraction of the total pro- ject time. The inter-related sequences of these activities are also studied.
This attention to detail by itself. usually, effects substantial improve- ment in the management of the pro- ject as it reveals at an early date most of the potential difficulties. The tech- nique can also be made to predict the demand on resources, period by period, and thus promotes improved utilisation of available resources.
It has been said that the most im- portant characteristic of the techni- que is its ability to direct manage- ment attention to jobs which control the duration of the project, the so- called critical or nearly critical ac- tivities.
The first step in the preparation of
the arrow diagram is to draw up a list of all the individual separate tasks (or activities as they are usually known) which have to be accomplish- ed to move the project from start to completion.
Arrow Notation
Each individual activity is repre- sented by an arrow. The length or slope of the arrow has no significance. although it will be found more con- venient if the horizontal projections of the arrows progress from left to right across the diagram.
Events
The beginning and end of jobs are events; these are represented by num- bered circles called nodes.
Beginning Event
10
Node
Fig. 1.
End Event
Job
20
Arrow
Nod>
An 'event' indicates the completion of a certain stage in the project.
Establishing Sequence
Establishing the sequence is the most important part of network an- alysis and calls for a detailed tech- nical knowledge of the process and the ability to apply this knowledge logically.
In order that the logic of the net- work is correct and that jobs are not missed, the following questions should be asked of oneself for each activity.
Far East Architect & Builder June, 1967
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