【正文】 s to conduct a physical or puter based simulation of the construction process and observe any problems. Finally, it is important to realize that different types of precedence relationships can be defined and that each has different implications for the schedule of activities: Some activities have a necessary technical or physical relationship that cannot be superseded. For example, concrete pours cannot proceed before formwork and reinforcement are in place. Some activities have a necessary precedence relationship over a continuous space rather than as discrete work task relationships. For example, formwork may be placed in the first part of an excavation trench even as the excavation equipment continues to work further along in the trench. Formwork placement cannot proceed further than the excavation, but the two activities can be started and stopped independently within this constraint. Some precedence relationships are not technically necessary but are imposed due to implicit decisions within the construction plan. For example, two activities may require the same piece of equipment so a precedence relationship might be defined between the two to insure that they are not scheduled for the same time period. Which activity is scheduled first is arbitrary. As a second example, reversing the sequence of two activities may be technically possible but more expensive. In this case, the precedence relationship is not physically necessary but only applied to reduce costs as perceived at the time of scheduling. In revising schedules as work proceeds, it is important to realize that different types of precedence relationships have quite different implications for the flexibility and cost of changing the construction plan. Unfortunately, many formal scheduling systems do not possess the capability of indicating this type of flexibility. As a result, the burden is placed upon the manager of making such decisions and insuring realistic and effective schedules. With all the other responsibilities of a project manager, it is no surprise that preparing or revising the formal, puter based construction plan is a low priority to a manager in such cases. Nevertheless, formal construction plans may be essential for good management of plicated projects. Example 94: Precedence Definition for Site Preparation and Foundation Work Suppose that a site preparation and concrete slab foundation construction project consists of nine different activities: A. Site clearing (of brush and minor debris),B. Removal of trees,C. General excavation,D. Grading general area,E. Excavation for utility trenches,F. Placing formwork and reinforcement for concrete,G. Installing sewer lines,H. Installing other utilities,I. Pouring concrete.Activities A (site clearing) and B (tree removal) do not have preceding activities since they depend on none of the other activities. We assume that activities C (general excavation) and D (general grading) are preceded by activity A (site clearing). It might also be the case that the planner wished to delay any excavation until trees were removed, so that B (tree removal) would be a precedent activity to C (general excavation) and D (general grading). Activities E (trench excavation) and F (concrete preparation) cannot begin until the pletion of general excavation and tree removal, since they involve subsequent excavation and trench preparation. Activities G (install lines) and H (install utilities) represent installation in the utility trenches and cannot be attempted until the trenches are prepared, so that activity E (trench excavation) is a preceding activity. We also assume that the utilities should not be installed until grading is pleted to avoid equipment conflicts, so activity D (general grading) is also preceding activities G (install sewers) and H (install utilities). Finally, activity I (pour concrete) cannot begin until the sewer line is installed and formwork and reinforcement are ready, so activities F and G are preceding. Other utilities may be routed over the slab foundation, so activity H (install utilities) is not necessarily a preceding activity for activity I (pour concrete). The result of our planning are the immediate precedences shown in Table 91. TABLE 91 Precedence Relations for a NineActivity Project ExampleActivityDescriptionPredecessorsABCDEFGHISite clearing Removal of treesGeneral excavationGrading general areaExcavation for utility trenchesPlacing formwork and reinforcement for concreteInstalling sewer linesInstalling other utilitiesPouring concreteAAB,CB,CD,ED,EF,GWith this information, the next problem is to represent the activities in a network diagram and to determine all the precedence relationships among the activities. One network representation of these nine activities is shown in Figure 95, in which the activities appear as branches or links between nodes. The nodes represent milestones of possible beginning and starting times. This representation is called an activityonbranch diagram. Note that an initial event beginning activity is defined (Node 0 in Figure 95), while node 5 represents the pletion of all activities. Figure 95 ActivityonBranch Representation of a Nine Activity ProjectAlternatively, the nine activities could be represented by nodes and predecessor relationships by branches or links, as in Figure 96. The result is an activityonnode diagram. In Figure 96, new activity nodes representing the beginning and the end of construction have been added to mark these important milestones. These network representations of activities can be very helpful in visualizing the various activities and their relationships for a project. Whether activities are represented as branches (as in Figure 95) or as nodes (as in Figure 95) is largely a matter of organizational or personal choice. Some considerations in cho