Chapter 30
Resource Loading
Abstract
This chapter describes a manual method for allocating and smoothing resources, if a computer is not available, or for examination purposes, by adding these to the activities of a bar chart (Gantt chart), which was itself created by abstracting the activities and durations from a network. The resources on the bar chart can then be summated for each time period to produce a histogram and the cumulative totals can then be plotted to produce an S-curve, which shows the planned rate of resource usage. The chapter explains that by utilizing the floats, the resources can be smoothed to reduce peaks of resource usage. The relationship between a network, bar chart, histogram and S-curve is shown in one diagram.
Keywords
Bar chart; Histogram; Resource smoothing; Resources; S curve
Most of the modern computer programs incorporate facilities for resource loading, resource allocation and resource smoothing. Indeed, the Primavera P6 program shown in Chapter 51 features such a capability.
In principle, the computer aggregates a particular resource in any time period and compares this with a previously entered availability level for that resource. If the availability is less than the required level, the program will either:
1. Show the excess requirement in tabular form, often in a different colour to highlight the problem; or
2. Increase the duration of the activity requiring the resource to spread the available resources over a longer period, thus eliminating the unattainable peak loading.
The more preferable action by the computer is option (1), i.e., the simple report showing the overrun of resources. It is then up to management to make the necessary adjustments by either extending the time period – if the contractual commitments permit – or mobilizing additional resources. In practice, of course, the problem is complicated by such issues as available access or working space as well as financial, contractual or even political restraints. Often it may be possible to make technical changes that alter the resource mix. For example, a shortage of carpenters used for formwork erection may make it necessary to increase the use of precast concrete components with a possible increase in cost but a decrease in time. Project management is more than just writing and monitoring programs. The so-called project management systems marketed by software companies are really only there to present to the project manager, on a regular basis, the position of the project to date and the possible consequences unless some form of remedial action is taken. The type of action and the timing of it rest fairly and squarely on the shoulders of management.
The options by management are usually quite wide, provided sufficient time is taken to think them out. It is in such situations that the ‘what if’ scenarios are a useful facility on a computer. However, the real implication can only be seen by ‘plugging’ the various alternatives into the network on paper and examining the downstream effects in company with the various specialists, who, after all, have to do the actual work. There is no effective substitute for good teamwork.
The Alternative Approach
Resource smoothing can, of course, be done very effectively without a computer – especially if the program is not very large. Once a network has been prepared, it is very easy to convert it into a bar chart, since all the ‘thinking’ has already been completed. Using the earliest starting and finishing times, the bars can be added to the gridded paper in minutes. Indeed, the longest operation in drawing a bar chart (once a network has been completed) is writing down the activity descriptions on the left-hand side of the paper. By leaving sufficient vertical space between the bars and dividing the grid into week (or day) columns, the resource levels for each activity can be added. Generally, there is no need to examine more than two types of resources per chart, since only the potentially restrictive or quantitatively limited ones are of concern. When all the activity bars have been marked with the resource value, each time period is added up vertically and the total entered in the appropriate space. The next step is to draw a histogram to show the graphical distribution of the resources. This will immediately highlight the peaks and troughs, and trigger off the next step – resource smoothing.
Manual resource smoothing is probably the most practical method, since unprogrammable factors such as access, working space, hard-standing for cranes, personality traits of foremen, etc., can only be considered by a human when the smoothing is carried out. Nevertheless, the smoothing operation must still follow the logical pattern given below:
1. Advantage should be taken of float. In theory, activities with free float should be the first to be extended, so that a limited resource can be spread over a longer time period. In practice, however, such opportunities are comparatively rare, and for all normal operations, all activities with total float can be used for the purpose of smoothing. The floats can be indicated on the bars by dotted line extensions, again read straight off the network by subtracting the earliest from the latest times of the beginning node of the activity.
2. When the floats have been absorbed and the resources are distributed over the longer activity durations, another vertical addition is carried out from which a new histogram can be drawn. A typical network, bar chart and histogram are shown in Fig. 30.1.
3. If the peaks still exceed the available resources for any time period, logic changes will be required. These changes are usually carried out on the network, but it may be possible to make some of them by ‘sliding’ the bars on the bar chart. For example, a common problem when commissioning a process or steam-raising plant is a shortage of suitably qualified commissioning engineers. If the bars of the bar chart are cut out and pasted onto cardboard, with the resources written against each time period on the activity bar, the various operations can be moved on the time-scaled bar chart until an acceptable resource level is obtained. The reason it is not always necessary to use the network is that in a commissioning operation there is often considerable flexibility as to which machine is commissioned first. Whether pump A is commissioned before or after compressor B is often a matter of personal choice rather than logical necessity. When an acceptable solution has been found, the strips of bar can be held on to the backing sheet with an adhesive putty (Blu-Tack) and (provided the format is of the necessary size) photocopied for distribution to interested parties.
4. If the weekly (or daily) aggregates are totalled cumulatively it is sometimes desirable to draw the cumulative curve (usually known as the S-curve, because it frequently takes the shape of an elongated letter S), which gives a picture of the build-up (and run-down) of the resources over the period of the project. This curve is also useful for showing the cumulative cash flow, which, after all, is only another resource. An example of such a cash-flow curve is given in Chapter 45.
The following example shows the above steps in relation to a small construction project where there is a resource limitation. Fig. 30.2 shows the activity on arrow configuration and Fig. 30.3 shows the same network in activity on node configuration. Fig. 30.4 shows their translation into a bar (or Gantt) chart where the bars are in fact a string of resource numbers. For simplicity, all the resources shown are of the same type (e.g., welders). By adding up the resources of each week, a totals table can be drawn, from which it can be seen that in week 9 the resource requirement is 14. This amount exceeds the availability, which is only 11 welders, and an adjustment is therefore necessary. Closer examination of the bar chart reveals a low resource requirement of only 6 in week 12. A check on the network (Fig. 30.2) shows that there is 15 − 9 = 6 weeks’ float on activity K. This activity can therefore be used to smooth the resources. By delaying activity K by three weeks, the resource requirement is now:
Figure 30.2
Figure 30.3
Week 9, –10
Week 12, –10
A histogram and a cumulative resource curve are drawn from the revised totals table. The latter can also be used as a planned performance curve, as the resources (if men) are directly proportional to man-hours. It is interesting to note that any ‘dip’ or ‘peak’ in the cumulative resource curve indicates a change of resource requirement, which should be investigated. A well-planned project should have a smooth resource curve following approximately the shape of letter S.
The method described may appear to be lengthy and time-consuming, but the example given by Figs. 30.2 (or 30.3) and 30.4, including the resource smoothing and curve plotting, took exactly 6 minutes. Once the activities and resources have been listed on graph paper, the bar chart drafting and resource smoothing of a practical network of approximately 200 activities can usually be carried out in about 1 hour.
Most of the modern computers’ project-management programs have resource smoothing facilities, which enable the base to be re-positioned on the screen to give the required resource total for any time period.
However, it is advisable not to do this automatically as the machine cannot make allowances for congestion of work area, special skills of operators, clients’ preferences and other factors only apparent to the people on the job.
Fig. 30.5 shows the relationship between the networks, bar chart, histogram and cumulative S-curve.
It should be noted that the term used for redistributing the resources was ‘resource smoothing’. Some authorities also use the term ‘resource levelling’, by which they mean flattening out the histogram to keep the resource usage within the resource availability for a particular time period. However, to do this without moving the position of some of the activities is just about impossible. Whether the resources are ‘levelled’ to reduce the unacceptable peaks due to resource restraints or ‘smoothed’ to produce a more even resource usage pattern, activities have to be readjusted along the time scale by utilizing the available float. To ascribe different meanings to the terms smoothing and levelling is therefore somewhat hair-splitting, since in both cases the operations to be carried out are identical. If the resource levels are so restricted that even the critical activities have to be extended, the project completion will inevitably be delayed.
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