Line Of Balance

Network analysis is essentially a technique for planning one-off projects, whether this is a construction site, a manufacturing operation, a computer software development, or a move to a new premises. When the overall project consists of a number of identical or batch operations, each of which may be a subproject in its own right, it may be of advantage to use a technique called line of balance.

The quickest way to explain how this planning method works is to follow a simple example involving the construction of four identical, small, single-storey houses of the type shown in Figure 45.1. For the sake of clarity, only the first five activities will be considered and it will be seen from Figure 45.2, that the last of the five activities, E-'floor joists', will be complete in week 9.

Assuming one has sufficient resources and space between the actual building plots, it is possible to start work on every house at the same time and therefore finish laying all the floor joists by week 9. However, in real life this is not possible, so the gang laying the foundations to house No. 1 will move to house No. 2 when foundation No. 1 is finished. When foundation No. 2 is finished, the gang will start No. 3 and so on. The same procedure will be carried out by all the following trades, until all the houses are finished.

Another practical device is to allow a time buffer between the trades so as to give a measure of flexibility and introduce a margin of error. Frequently such a buffer will occur naturally for such reasons as hardening time of concrete, setting time of adhesive, drying time of plaster or paint.

Table 45.1; can now be partially redrawn showing in addition the buffer time, which was originally included in the activity duration. The new table is now shown in Table 25.1.

Figure 25.3 shows the relationship between the trades involved. Each trade (or activity) is represented by two lines. The distance between these lines is the duration of the activity. The distance between the activities is the buffer period. As can be seen, all the work of the activities A to E is carried out at the same rate, which means that for every house, enough resources are available for every trade to start as soon as its preceding trade is finished. This is shown to be the case in Figure 25.3.

However, if only one gang is available on the site for each trade, e.g. if only one gang of concreters laying the foundations (activity B) is available, concreting on house 2 cannot start until ground clearance (activity A) has been completed. The figure would then be as shown in

Table 25.1

Activity Activity Adjusted Dependency Total float Buffer letter description duration (weeks) (weeks)

(weeks)

A Clear ground 2.0 Start 0 0.0

B Lay foundations 2.8 A 0 0.2

C Build dwarf walls 1.9 B 0 0.1

D Oversite concrete 0.9 B 1 0.1

Completion of floor joists (E) = (2 x 4) + 3 + 2 + 2 = 8 + 3 + 2 + 2 = 15 weeks

Figure 25.3 Line of balance

Completion of floor joists (E) = (2 x 4) + 3 + 2 + 2 = 8 + 3 + 2 + 2 = 15 weeks

Figure 25.3 Line of balance

Figure 25.4. If the number of concreters could be increased, so that two gangs were available on site, the foundations for house 2 could be started as soon as the ground had been cleared.

Building the dwarf wall (activity C) requires only 1.9 weeks per house, which is a faster rate of work than laying foundations. To keep the bricklaying gang going smoothly from one house to the next, work can only start on house 1 in week 7.2, i.e. after the buffer of about 2.5 weeks following the completion of the foundations of house 1. In this way, by the time the dwarf walls are started on house 4, the foundations (activity B) of house 4 will just have been finished. (In practice of course there would be a further buffer to allow the concrete to harden sufficiently for the bricklaying to start.)

As the oversite concreting (activity D) only takes 0.9 weeks, the one gang of labourers doing this work will have every oversite completed well before the next house is ready for them. Their start date could be delayed if necessary by as much as 3.5 weeks, since apart from the buffer, this activity (D) has also 1 week float.

It can be seen therefore from Figure 25.4 that by plotting these operations with the time as the horizontal axis and the number of houses as the vertical axis, the following becomes apparent.

If the slope of an operation is less (i.e. flatter) than the slope of the preceding operation, the chosen buffer is shown at the start of the operation. If, on the other hand, the slope of a

Completion of floor joists (E) = (2 x 4) + 2(buffer) + 2.8 + 2 + 2 = 8 + 2 + 2.8 + 2 + 2 = 16.8 weeks

Figure 25.4 Line of balance

Completion of floor joists (E) = (2 x 4) + 2(buffer) + 2.8 + 2 + 2 = 8 + 2 + 2.8 + 2 + 2 = 16.8 weeks

Figure 25.4 Line of balance succeeding operation is steeper, the buffer must be inserted at the end of the previous operation, since otherwise there is a possibility of the trades clashing when they get to the last house.

What becomes very clear from these diagrams is the ability to delay the start of an operation (and use the resources somewhere else) and still meet the overall project programme.

When the work is carried out by trade gangs, the movement of the gangs can be shown on the LoB chart by vertical arrows as indicated in Figure 25.4.

Readers who wish to obtain more information on LoB techniques are advised to obtain the booklet issued by the National Building Agency.

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