Project Planning and Control Part 3

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Project Planning and Control Part 312Precedence or activity on node (AoN) diagrams Some planners prefer to show the interrelation- ship of activities by using the node as the activity box and interlinking them by lines. Because the durations are written in the activity box, dummy activities are eliminated. In a sense, each con- necting line is, of course, a dummy because it is timeless. The network produced in this manner is called variously a ‘precedence diagram’, a ‘circle and link diagram’ or an ‘activity on node diagram’. Precedence diagrams have a number of advan- tages over arrow diagrams in that 1 No dummies are necessary; 2 They may be easier to understand by people familiar with flow sheets; 3 Activities are identified by one number instead of two so that a new activity can be inserted between two existing activities without chang- ing the identifying node numbers of the existing activities; 4 Overlapping activities can be shown very easily without the need for the extra dummies shown in Figure 11.25.Project Planning and ControlAnalysis and float calculation (see Chapter 15) is identical to the methodsemployed for arrow diagrams and, if the box is large enough, the earliest andlatest start and finishing times can be written in. A typical precedence network is shown in Figure 12.1, where the letters inthe box represent the description or activity numbers. Durations are shownabove-centre and the earliest and latest starting and finish times are given inFigure 12.1the corners of the box, as explained in the key diagram. The top line of theactivity box gives the earliest start (ES), duration (D) and earliest finish (EF).Therefore: EF = ES + DThe bottom line gives the latest start and the latest finish. Therefore: LS = LF – DThe centre box is used to show the total float.ES is, of course, the highest EF of the previous activities leading into it, i.e.the ES of activity E is 8, taken from the EF of activity B.LF is the lowest LS of the previous activity working backwards, i.e. the LF ofA is 3, taken from the LS of activity B.The earliest start (ES) of activity F is 5 because it can start after activity D is50% complete, i.e.82 Precedence or activity on node (AoN) diagrams ES of activity D is 3 Duration of activity D is 4 Therefore 50% of duration is 2 Therefore ES of activity F is 3 + 2 = 5 Sometimes it is advantageous to add a percentage line on the bottom of theactivity box to show the stage of completion before the next activity can start(Figure 12.2). Each vertical line represents 10% completion. Apart fromshowing when the next activity starts, the percentage line can also be used toindicate the percentage completion of the activity as a statement of progressonce work has started, as in Figure 12.3.Figure 12.2 Figure 12.3 There are two other advantages of the precedence diagram over the arrowdiagram.1 The risk of making the logic errors is virtually eliminated. This is because each activity is separated by a link, so that the unintended dependency from another activity is just not possible. This is made clear by referring to Figure 12.4 which is the precedence representation of Figure 11.25. As can be seen, there is no way for an activity like ‘level bottom’ in Stage I to affect activity ‘Hand trim’ in Stage III, as is the case in Figure 11.24.2 In a precedence diagram all the important information of an activity is shown in a neat box. A close inspection of the precedence diagram (Figure 12.5), shows that in order to calculate the total float, it is necessary to carry out the forward and backward pass. Once this has been done, the total float of any activity is simply the difference between the latest finishing time (LF) obtained from the backward pass and the earliest finishing time (EF) obtained from the forward pass. 83Project Planning and ControlFigure 12.4 On the other hand, the free float can be calculated from the forward pass only, because it is simply the difference of the earliest start (ES) of a subsequent activity and the earliest finishing time (EF) of the activity in question. This is clearly shown in Figure 12.5.Figure 12.5 Despite the above-mentioned advantages, which are especially appreciatedby people familiar with flow diagrams as used in manufacturing industries,many prefer the arrow diagram because it resembles more closely a bar chart.Although the arrows are not drawn to scale, they do represent a forward-moving operation and, by thickening up the actual lin ...