In the PERT/CPM scheduling techniques (see “The Program Evaluation and Review Technique (PERT): Incorporating activity time variability in a project schedule” and “The Critical Path Method (CPM): Incorporating activity time/cost trade-offs in a project schedule”), project activities are characterized by their estimated duration(s) and project networks are constructed by adding precedence relations between these activities. It is therefore implicitly assumed that these activities do not require resources during their execution (or alternatively, the assumption is that the resources are unlimited in availability). In practice, activities need resources during their execution that are often limited in availability. These resources have been classified in two basic categories, as follows:
-
Renewable resources
-
Consumable resources
After the presentation of the two commonly used resource types, some special cases are briefly presented.
Renewable resources
Renewable resources are available on a period-by-period basis, i.e. the available amount is renewed from period to period (e.g. per hour, per day, ...). Only the total resource use at every time instant is constrained. Typical examples are manpower, machines, tools, equipment and space. As an example, skilled laborers are available to work each day on a project, although their availability is limited each day and may vary over time due to absence, sickness, vacations, etc.. Consequently, there is no general constraint placed on the number of days skilled labor may be used but instead they are renewed each period of the project.
Consumable resources
Consumable resources (or non-renewable) are not constrained on a periodic basis but rather have a limited consumption availability for the entire project. Typical examples are money, raw materials and energy. Usually, the overall project costs are limited and pre-defined in a total contract price. Their consumption is not renewed as is the case with renewable resources, but instead, these resources are consumed when used.
Special cases
Renewable and consumable resources are the two commonly used resource classes in software tools, although many other extensions have been proposed in the academic literature. These special cases can often not be incorporated in software tools, although they can sometimes be modeled as a combination of renewable and consumable resources. A non-exhaustive list is given along the following lines.
Spatial resources are required by a group of activities, rather than a single activity as renewable resources. The spatial resource is occupied from the first moment an activity from the group starts until the finish of all activities from that group. Examples are dry docks in a ship yard or a freezing machine in the Westerscheldetunnel (see
www.westerscheldetunnel.nl).
Doubly constrained resources are constrained on a periodic basis, similar to renewable resources, as well as for the total project duration, as with the consumable resources. An example is a total budget with an extra restriction of a maximum limit per period.
Partially renewable resources assume for each resource a capacity restriction on a subset of periods. A resource type is characterized by a number of subsets of periods and a certain total availability. A typical example is a worker who is allowed to work during all days of the week and only one day (i.e. Saturday or Sunday) during the weekend.
