Design and Construction Philosophy

The activity letters refer to the activities shown on the cross-section diagram of Figure 47.1, and on subsequent tables only these activity letters will be used. The total float column can, of course, only be completed when the network shown in Figure 47.2 has been analysed (see Table 47.1).

Table 47.2 shows the complete analysis of the network including TL_e (latest time end event), TE_e (earliest time beginning event), total float, and free float. It will be noted that none of the activities have free float. As mentioned in Chapter 21 free float is often confined to the dummy activities, which have been omitted from the table.

To enable the resource loading bar chart in Figure 47.4 to be drawn it helps to prepare a table of resources for each activity (Table 47.3). The resources are divided into two categories:

This is because tradesmen are more likely to be in short supply and could affect the programme.

The total labour histogram can now be drawn, together with the total labour curve (Figure 47.5). It will be seen that the histogram has been hatched to differentiate between labourers and tradesmen, and shows that the maximum demand for tradesmen is eight men in weeks 27 and 28. Unfortunately, it is only possible to employ six tradesmen due to possible site congestion. What is to be done?

The advantage of network analysis with its float calculation is now apparent. Examination of the network shows that in weeks 27 and 28 the following operations (or activities) have to be carried out:

The first step is to check which activities have float. Consulting Table 47.2 reveals that Q (Plastering) has 2 weeks float and R (Electrics) has 1 week float. By delaying Q (Plastering) by 2 weeks and accelerating R (Electrics) to be carried out in 2 weeks by 3 men per week, the maximum total in any week is reduced to 6. Alternatively, it may be possible to extend Q (Plumbing) to 4 weeks using 2 men per week for the first two weeks and 1 man per week for the next two weeks. At the same time, R (Electrics) can be extended by one week by employing 1 man per week for the first two weeks and 2 men per week for the next two weeks. Again, the maximum total for weeks 27–31 is 6 tradesmen.

The new partial disposition of resources and revised histograms after the two alternative smoothing operations are shown in Figures 47.6 and 47.7. It will be noted that:

If it is not possible to obtain the necessary smoothing by utilizing and absorbing floats the network logic may be amended, but this requires a careful reconsideration of the whole construction process.

The next operation is to use the EVA system to control the work on site. Multiplying for each activity the number of weeks required to do the work by the number of men employed on that activity yields the number of man weeks. If this is multiplied by 50 (the average number of working hours in a week), the manhours per activity are obtained. A table can now be drawn up listing the activities, durations, number of men, and budget hours (Table 47.4).

As the bank will advance the money to pay for the construction in four-weekly tranches, the measurement and control system will have to be set up to monitor the work every 4 weeks. The anticipated completion date is week 34, so that a measure in weeks 4, 8, 12, 16, 20, 24, 28, 32, and 36 will be required. By recording the actual hours worked each week and assessing the percentage complete for each activity each week the value hours for each activity can be quickly calculated. As described in Chapter 32, the overall percentage complete, efficiency, and predicted final hours can then be calculated. Table 47.5 shows a manual EVA analysis for four sample weeks (8, 16, 24, and 32).

In practice, this calculation will have to be carried out every week either manually as shown or by computer using a simple spreadsheet. It must be remembered that only the activities actually worked on during the week in question have to be computed. The remaining activities are entered as shown in the previous week’s analysis.

For purposes of progress payments, the value hours for every 4-week period must be multiplied by the average labour rate (≤5 per hour) and, when added to the material and plant costs, the total value for payment purposes is obtained. This is shown later in this chapter.

At this stage it is more important to control the job, and for this to be done effectively, a set of curves must be drawn on a time base to enable all the various parameters to be compared. The relationship between the actual hours and value hours gives a measure of the efficiency of the work, while that between the value hours and the planned hours gives a measure of progress. The actual and value hours are plotted straight from the EVA analysis, but the planned hours must be obtained from the labour expenditure curve (Figure 47.5) and multiplying the labour value (in men) by 50 (the number of working hours per week). For example, in week 16 the total labour used to date is 94 man weeks, giving 94 × 50 = 4700 manhours.

The complete set of curves (including the efficiency and percentage complete curves) is shown in Figure 47.8. In practice, it may be more convenient to draw the last two curves on a separate sheet, but provided the percentage scale is drawn on the opposite side to the manhour scale no confusion should arise. Again, a computer program can be written to plot these curves on a weekly basis as shown in Chapter 32.

Once the control system has been set up it is essential to draw up the cash flow curve to ascertain what additional funding arrangements are required over the life of the project. In most cases where project financing is required the cash flow curve will give an indication of how much will have to be obtained from the finance house or bank and when. In the case of this example, where the construction is financed by bank advances related to site progress, it is still necessary to check that the payments will, in fact, cover the outgoings. It can be seen from the curve in Figure 47.10 that virtually permanent overdraft arrangements will have to be made to enable the men and suppliers to be paid regularly.

When considering cash flow it is useful to produce a table showing the relationship between the usage of a resource, the payment date, and the receipt of cash from the bank to pay for it – even retrospectively. It can be seen in Table 47.6 that

The next step is to tabulate the labour costs and material and plant costs on a weekly basis (Table 47.7). The last column in the table shows the total material and plant cost for every activity, because all the materials and plant for an activity are being delivered one week before use and have to be paid for in one payment. For simplicity, no retentions are withheld (i.e., 100% payment is made to all suppliers when due).

A bar chart (Figure 47.9) can now be produced, which is similar to that shown in Figure 47.4. The main difference is that instead of drawing bars, the length of the activity is represented by the weekly resource. As there are two types of resources – men and materials and plant – each activity is represented by two lines. The top line represents the labour cost in ≤100 units and the lower line the material and plant cost in ≤100 units. When the chart has been completed the resources are added vertically for each week to give a weekly total of labour out (i.e., men being paid, line 1) and material and plant out (line 2). The total cash out and the cumulative outflow values can now be added in lines 3 and 4, respectively.

The chart also shows the measurements every 4 weeks, starting in week 4 (line 5), and the payments one week later. The cumulative total cash is shown in line 6. To enable the outflow of materials and plant to be shown separately on the graph in Figure 47.10, it was necessary to enter the cumulative outflow for material and plant in row 7. This figure shows the cash flow curves (i.e., cash in and cash out). The need for a more-or-less permanent overdraft of approximately ≤10,000 is apparent.