【文章內(nèi)容簡介】 o perform an activity is called the duration of the activity. The beginning and the end of activities are signposts or milestones, indicating the progress of the project. Occasionally, it is useful to define activities which have no duration to mark important events. For example, receipt of equipment on the construction site may be defined as an activity since other activities would depend upon the equipment availability and the project manager might appreciate formal notice of the arrival. Similarly, receipt of regulatory approvals would also be specially marked in the project plan. The extent of work involved in any one activity can vary tremendously in construction project plans. Indeed, it is mon to begin with fairly coarse definitions of activities and then to further subdivide tasks as the plan bees better defined. As a result, the definition of activities evolves during the preparation of the plan. A result of this process is a natural hierarchy of activities with large, abstract functional activities repeatedly subdivided into more and more specific subtasks. For example, the problem of placing concrete on site would have subactivities associated with placing forms, installing reinforcing steel, pouring concrete, finishing the concrete, removing forms and others. Even more specifically, subtasks such as removal and cleaning of forms after concrete placement can be defined. Even further, the subtask clean concrete forms could be subdivided into the various operations: Transport forms from onsite storage and unload onto the cleaning station. Position forms on the cleaning station. Wash forms with water. Clean concrete debris from the form39。s surface. Coat the form surface with an oil release agent for the next use. Unload the form from the cleaning station and transport to the storage location. This detailed task breakdown of the activity clean concrete forms would not generally be done in standard construction planning, but it is essential in the process of programming or designing a robot to undertake this activity since the various specific tasks must be well defined for a robot implementation. [5] It is generally advantageous to introduce an explicit hierarchy of work activities for the purpose of simplifying the presentation and development of a schedule. For example, the initial plan might define a single activity associated with site clearance. Later, this single activity might be subdivided into relocating utilities, removing vegetation, grading, etc. However, these activities could continue to be identified as subactivities under the general activity of site clearance. This hierarchical structure also facilitates the preparation of summary charts and reports in which detailed operations are bined into aggregate or superactivities. More formally, a hierarchical approach to work task definition deposes the work activity into ponent parts in the form of a tree. Higher levels in the tree represent decision nodes or summary activities, while branches in the tree lead to smaller ponents and work activities. A variety of constraints among the various nodes may be defined or imposed, including precedence relationships among different tasks as defined below. Technology choices may be deposed to decisions made at particular nodes in the tree. For example, choices on plumbing technology might be made without reference to choices for other functional activities. Of course, numerous different activity hierarchies can be defined for each construction plan. For example, upper level activities might be related to facility ponents such as foundation elements, and then lower level activity divisions into the required construction operations might be made. Alternatively, upper level divisions might represent general types of activities such as electrical work, while lower work divisions represent the application of these operations to specific facility ponents. As a third alternative, initial divisions might represent different spatial locations in the planned facility. The choice of a hierarchy depends upon the desired scheme for summarizing work information and on the convenience of the planner. In puterized databases, multiple hierarchies can be stored so that different aggregations or views of the work breakdown structure can be obtained. The number and detail of the activities in a construction plan is a matter of judgment or convention. Construction plans can easily range between less than a hundred to many thousand defined tasks, depending on the planner39。s decisions and the scope of the project. If subdivided activities are too refined, the size of the network bees unwieldy and the cost of planning excessive. Subdivision yields no benefit if reasonably accurate estimates of activity durations and the required resources cannot be made at the detailed work breakdown level. On the other hand, if the specified activities are too coarse, it is impossible to develop realistic schedules and details of resource requirements during the project. More detailed task definitions permit better control and more realistic scheduling. It is useful to define separate work tasks for: those activities which involve different resources, or those activities which do not require continuous performance. For example, the activity prepare and check shop drawings should be divided into a task for preparation and a task for checking since different individuals are involved in the two tasks and there may be a time lag between preparation and checking. In practice, the proper level of detail will depend upon the size, importance and difficulty of the project as well as the specific scheduling and accounting procedures which are adopted. However, it is generally the case that most schedules are prepared with too little detail than too much. It is important to keep in mind that task definition will serve as the basis for scheduling, for