Lester diagram

With the development of the network grid, the drafting of an arrow diagram enables the activities to be easily organized into disciplines or work areas and eliminates the need to enter reference numbers into the nodes. Instead the grid reference numbers (or letters) can be fed into the computer. The grid system also makes it possible to produce acceptable arrow diagrams on a computer which can be used 'in the field' without converting them into the conventional bar chart. An example of such a computerized arrow diagram, which has been developed by Claremont Controls as part of their latest Hornet Windmill program, is given in Figure 23.1. It will be noticed that the link lines never cross a node!

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Completed or partially completed o-o o-o 0*0

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activities activities

Figure 23.1 AoA network drawn on grid

Critical path o-o

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activities activities

Figure 23.1 AoA network drawn on grid

Critical path

A grid system can, however, pose a problem when it becomes necessary to insert an activity between two existing ones. In practice, resourceful planners can overcome the problem by combining the new activity with one of the existing activities.

If, for example, two adjoining activities were 'Cast Column, 4 days' and 'Cast Beam, 2 days' and it were necessary to insert 'Strike Formwork, 2 days' between the two activities, the planner would simply restate the first activity as 'Cast Column and Strike Formwork, 6 days' (Figure 23.2).

While this overcomes the drafting problem it may not be acceptable from a cost control point of view, especially if the network is geared to an EVA system (see Chapter 32). Furthermore the fact that the grid numbers were on the nodes meant that when it was necessary to move a string along one or more grid spaces, the relationship between the grid number and the activity changed. This could complicate the EVA analysis. To overcome this, the grid number was placed bet-ween the nodes (Figure 23.3).

It can be argued that a precedence network lends itself admirably to a grid system as the grid number is always and permanently related to the activity and is therefore ideal for EVA. However, the problem of the congested link lines (especially the vertical ones) remains.

Now, however, the perfect solution has been found. It is in effect a combination of the arrow diagram and the precedence diagram and like the marriage of Henry VII which ended the Wars of the Roses, this marriage should end the war of the networks!

The new diagram, which could be called the 'Lester' diagram, is simply an arrow diagram where each activity is separated by a short link in the same way as in a precedence network (Figure 23.4).

In this way it is possible to eliminate or at least reduce logic errors, show total float and free float as easily as on a precedence network, but has the advantages of an arrow diagram in speed of drafting, clarity of link presentation and the ability to insert new activities in a grid system without altering the grid number/activity relationship. Figure 23.5 shows all these features.

If a line is drawn around any activity, the similarity between the Lester diagram and the precedence diagram becomes immediately apparent. See Figure 23.6.

Cast column

Cast beam

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