9.6 Estimating Resource Requirements for Work Activities
In addition to precedence relationships and time durations, resource requirements are usually estimated for each activity. Since the work activities defined for a project are comprehensive, the total resources required for the project are the sum of the resources required for the various activities. By making resource requirement estimates for each activity, the requirements for particular resources during the course of the project can be identified. Potential bottlenecks can thus be identified, and schedule, resource allocation or technology changes made to avoid problems.
Many formal scheduling procedures can incorporate constraints imposed by the availability of particular resources. For example, the unavailability of a specific piece of equipment or crew may prohibit activities from being undertaken at a particular time. Another type of resource is space. A planner typically will schedule only one activity in the same location at the same time. While activities requiring the same space may have no necessary technical precedence, simultaneous work might not be possible. Computational procedures for these various scheduling problems will be described in Chapters 10 and 11. In this section, we shall discuss the estimation of required resources.
The initial problem in estimating resource requirements is to decide the extent and number of resources that might be defined. At a very aggregate level, resources categories might be limited to the amount of labor (measured in man-hours or in dollars), the amount of materials required for an activity, and the total cost of the activity. At this aggregate level, the resource estimates may be useful for purposes of project monitoring and cash flow planning. For example, actual expenditures on an activity can be compared with the estimated required resources to reveal any problems that are being encountered during the course of a project. Monitoring procedures of this sort are described in Chapter 12. However, this aggregate definition of resource use would not reveal bottlenecks associated with particular types of equipment or workers.
More detailed definitions of required resources would include the number and type of both workers and equipment required by an activity as well as the amount and types of materials. Standard resource requirements for particular activities can be recorded and adjusted for the special conditions of particular projects. As a result, the resources types required for particular activities may already be defined. Reliance on historical or standard activity definitions of this type requires a standard coding system for activities.
In making adjustments for the resources required by a particular activity, most of the problems encountered in forming duration estimations described in the previous section are also present. In particular, resources such as labor requirements will vary in proportion to the work productivity, Pij, used to estimate activity durations in Equation (9.1). Mathematically, a typical estimating equation would be:
where Rkij are the resources of type k required by activity ij, Dij is the duration of activity ij, Nij is the number of standard crews allocated to activity ij, and Ukij is the amount of resource type k used per standard crew. For example, if an activity required eight hours with two crews assigned and each crew required three workers, the effort would be R = 8*2*3 = 48 labor-hours.
From the planning perspective, the important decisions in estimating resource requirements are to determine the type of technology and equipment to employ and the number of crews to allocate to each task. Clearly, assigning additional crews might result in faster completion of a particular activity. However, additional crews might result in congestion and coordination problems, so that work productivity might decline. Further, completing a particular activity earlier might not result in earlier completion of the entire project, as discussed in Chapter 10.
Example 9-5: Resource Requirements for Block Foundations
In placing concrete block foundation walls, a typical crew would consist of three bricklayers and two bricklayer helpers. If sufficient space was available on the site, several crews could work on the same job at the same time, thereby speeding up completion of the activity in proportion to the number of crews. In more restricted sites, multiple crews might interfere with one another. For special considerations such as complicated scaffolding or large blocks (such as twelve inch block), a bricklayer helper for each bricklayer might be required to insure smooth and productive work. In general, standard crew composition depends upon the specific construction task and the equipment or technology employed. These standard crews are then adjusted in response to special characteristics of a particular site.
Example 9-6: Pouring Concrete Slabs
For large concrete pours on horizontal slabs, it is important to plan the activity so that the slab for a full block can be completed continuously in a single day. Resources required for pouring the concrete depend upon the technology used. For example, a standard crew for pumping concrete to the slab might include a foreman, five laborers, one finisher, and one equipment operator. Related equipment would be vibrators and the concrete pump itself. For delivering concrete with a chute directly from the delivery truck, the standard crew might consist of a foreman, four laborers and a finisher. The number of crews would be chosen to insure that the desired amount of concrete could be placed in a single day. In addition to the resources involved in the actual placement, it would also be necessary to insure a sufficient number of delivery trucks and availability of the concrete itself.
9.7 Coding Systems
One objective in many construction planning efforts is to define the plan within the constraints of a universal coding system for identifying activities. Each activity defined for a project would be identified by a pre-defined code specific to that activity. The use of a common nomenclature or identification system is basically motivated by the desire for better integration of organizational efforts and improved information flow. In particular, coding systems are adopted to provide a numbering system to replace verbal descriptions of items. These codes reduce the length or complexity of the information to be recorded. A common coding system within an organization also aids consistency in definitions and categories between projects and among the various parties involved in a project. Common coding systems also aid in the retrieval of historical records of cost, productivity and duration on particular activities. Finally, electronic data storage and retrieval operations are much more efficient with standard coding systems, as described in Chapter 14.
In North America, the most widely used standard coding system for constructed facilities is the MASTERFORMAT system developed by the Construction Specifications Institute (CSI) of the United States and Construction Specifications of Canada. After development of separate systems, this combined system was originally introduced as the Uniform Construction Index (UCI) in 1972 and was subsequently adopted for use by numerous firms, information providers, professional societies and trade organizations. The term MASTERFORMAT was introduced with the 1978 revision of the UCI codes. MASTERFORMAT provides a standard identification code for nearly all the elements associated with building construction.
MASTERFORMAT involves a hierarchical coding system with multiple levels plus keyword text descriptions of each item. In the numerical coding system, the first two digits represent one of the sixteen divisions for work; a seventeenth division is used to code conditions of the contract for a constructor. In the latest version of the MASTERFORMAT, a third digit is added to indicate a subdivision within each division. Each division is further specified by a three digit extension indicating another level of subdivisions. In many cases, these subdivisions are further divided with an additional three digits to identify more specific work items or materials. For example, the code 16-950-960, "Electrical Equipment Testing" are defined as within Division 16 (Electrical) and Sub-Division 950 (Testing). The keywords "Electrical Equipment Testing" is a standard description of the activity. The seventeen major divisions in the UCI/CSI MASTERFORMAT system are shown in Table 9-6. As an example, site work second level divisions are shown in Table 9-7.
TABLE 9-6 Major Divisions in the Uniform Construction Index
While MASTERFORMAT provides a very useful means of organizing and communicating information, it has some obvious limitations as a complete project coding system. First, more specific information such as location of work or responsible organization might be required for project cost control. Code extensions are then added in addition to the digits in the basic MASTERFORMAT codes. For example, a typical extended code might have the following elements:
The first four digits indicate the project for this activity; this code refers to an activity on project number 0534. The next five digits refer to the MASTERFORMAT secondary division; referring to Table 9-7, this activity would be 02220 "Excavating, Backfilling and Compacting." The next two digits refer to specific activities defined within this MASTERFORMAT code; the digits 21 in this example might refer to excavation of column footings. The next character refers to the block or general area on the site that the activity will take place; in this case, block A is indicated. The digits 00 could be replaced by a code to indicate the responsible organization for the activity. Finally, the characters cf34 refer to the particular design element number for which this excavation is intended; in this case, column footing number 34 is intended. Thus, this activity is to perform the excavation for column footing number 34 in block A on the site. Note that a number of additional activities would be associated with column footing 34, including formwork and concreting. Additional fields in the coding systems might also be added to indicate the responsible crew for this activity or to identify the specific location of the activity on the site (defined, for example, as x, y and z coordinates with respect to a base point).
As a second problem, the MASTERFORMAT system was originally designed for building construction activities, so it is difficult to include various construction activities for other types of facilities or activities associated with planning or design. Different coding systems have been provided by other organizations in particular sub-fields such as power plants or roadways. Nevertheless, MASTERFORMAT provides a useful starting point for organizing information in different construction domains.
In devising organizational codes for project activities, there is a continual tension between adopting systems that are convenient or expedient for one project or for one project manager and systems appropriate for an entire organization. As a general rule, the record keeping and communication advantages of standard systems are excellent arguments for their adoption. Even in small projects, however, ad hoc or haphazard coding systems can lead to problems as the system is revised and extended over time.
TABLE 9-7 Secondary Divisions in MASTERFORMAT for Site Work
- Baracco-Miller, E., "Planning for Construction," Unpublished MS Thesis, Dept. of Civil Engineering, Carnegie Mellon University, 1987.
- Construction Specifications Institute, MASTERFORMAT - Master List of Section Titles and Numbers, Releasing Industry Group, Alexandria, VA, 1983.
- Jackson, M.J. Computers in Construction Planning and Control, Allen & Unwin, London, 1986.
- Sacerdoti, E.D. A Structure for Plans and Behavior, Elsevier North-Holland, New York, 1977.
- Zozaya-Gorostiza, C., "An Expert System for Construction Project Planning," Unpublished PhD Dissertation, Dept. of Civil Engineering, Carnegie Mellon University, 1988.