Abstract | Introduction | Resource Loading | S-curves | What can be Learned?
Productivity Improvement | Learning vs. Experience | Original Theory | Two Approaches
Illustration | Issues | Conclusions | References | Appendix 1 | Appendix 2 

Resource Loading (input)

Figure 1 shows an example of manpower loading for a profitable civil contract which was predominantly formwork and concrete placing.[1] It displays a histogram of the make up and total numbers in the production work force, week by week over a 38 week period. It also shows the progressive cumulative total, or actual manpower loading S-curve. As noted earlier, the general profiles of these curves appear to be quite typical (Christian 1991) whether the observations refer to a whole construction project, a sub-contract, an individual trade or a continuous construction activity of significant duration.

Figure 1: Civil contract example of site manpower (predominantly) concrete work

The important points to note about the S-curve are that the initial part of the curve represents the "build-up", the central part of the curve is relatively "steady-state", or effectively a straight line slope, and the latter part of the curve represents a "run-down" which closely mirror-images the early part of the S-curve.

In order to understand the generality of the suggested rules of thumb that follow, it is instructive to recite the many practical reasons for the shapes of each of these three stages.

In Stage 1 there is an accelerating build up of manpower because

• Access to the work has to be opened up from a zero start, with the result that the work itself becomes progressively more available.
• Necessary preliminary preparatory activities, including planning and understanding local conditions, as well as ordering of materials, etc., often require fewer people but more intensive supervision.
• Key people may be brought in to start the work, but supporting labour is recruited locally. The recruiting and selection of local labour itself takes time.
• With productive efficiency in mind, crews are added only as experience builds and the work becomes available to be performed.
• Further crews are added only as pressure builds to get the job done within the required time frame.

Stage 2 achieves a steady state because

• The working environment has reached optimum conditions for balanced performance and repetition.
• Physical limitations to the capabilities of the men and equipment provided is reached.
• Adding more labour or separate crews would over-crowd the working area and reduce productivity.
• The number of repetitions available from which the benefits of "productivity improvement" can be derived would be reduced.
• In either case the costs would be higher.
• Alternatively, if the work force is held at a lower level, the elapsed time to accomplish the work will be prolonged, with consequent higher overheads and, possibly, contract penalties to be faced.

These obvious trade-offs require careful management and balance.

In stage 3 almost the reverse of Stage 1 is true. Manpower is progressively reduced because

• The work begins to run out.
• The remaining work space not occupied by following trades, or owner occupation, runs the danger of becoming over crowded.
• Morale sometimes deteriorates as the end of the work is in sight and people leave to join more active sites
• Less successful crews or individuals are let go first.
• The more difficult work may have been left to the end, may be more congested, or otherwise require only the skills of those brought in initially.
• Pressure to complete "the last few percent" dies down as management attention turns to more critical work.
• Latent defects may surface upon final inspection requiring re-work with no added measurable product to show for the effort.

A report issued by the National Electrical Contractors Association (NECA) in 1983 further illustrates the general shape of the resource loading S-curve. Data was collected from 40 different contractors on 54 building projects in 32 cities. The projects represented four broad types of public buildings competitively bid and which the contractors felt were typical of their business. The report includes the supporting data which show the ranges of variation.

Figure 2 shows the overall average manpower consumption rate S-curve for all the data collected (NECA 1983). The figure also shows the overall low and high values and it is interesting to note that the range of variation over a considerable number of projects is only 10% of the total time scale. It should also be noted that a particular condition typically prevails in electrical work on building construction. At the outset, only a small crew is required for installing conduit and other electrical hardware during the course of work by other trades. The bulk of the electrical work cannot be undertaken until those trades are substantially complete. In other words, the work takes longer to open up and accounts for a longer Stage 1 in this particular S-curve.

Figure 2: Cumulative manpower consumed for electrical systems installation
in new buildings -- NECA

Introduction

Progress S-curves (output)

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