What jobs must precede it?
What job can run concurrently? What job follows?
What controls the start?
What controls the finish?
Dummy Activities
In certain circumstances, it will be found necessary to introduce artificial or dummy activities into a network diagram. This occurs where one wishes to specify that a particular event cannot occur before some other event although no specific job occurs between them.
The other occasion where a dum- my should be used occurs when a number of activities are in parallel and would share the same beginning and end event numbers.
A unique numbering system for events is essential if the network is to be analysed on a computer, as the computer identifies each activity by its beginning and end event numbers. It has become standard practice to illustrate a dummy by a dashed ar- row. Figures 2(a) and 2(b) illustrate the situations where dummies are re- quired.
10
Concrete to Drains and Manholes
20
Deliver Bricks
Fig 2 (a)
10
20
Concrete to Drains and Manholes
Deliver Bricks
Fig 2 (b)
Although Figure 2(a) expresses the correct sequence, it is incorrect as it implies that before the operation "lay pipes" is carried out the operation "deliver bricks" must be completed.
Figure 2(b) expresses the sequence correctly and is also logically correct. The statement this diagram makes in network form is that as soon as the activity "Concrete to Drains and Manholes" is complete, activity "Lay pipes" can commence. The dummy arrow indicates that before the ac- tivity "Build Manholes" can start the activity "Concrete to Drains and Manholes" as well as "Deliver Bricks" must be complete.
Figure 3(a) shows sequence and logic correctly but activities "Setting Out" and "D/L Excavator" have the
Far East Architect & Builder June, 1967
Set Out
Sign Contract
10
Deliver Excavator
Fig 3 (a)
Set Out
Sign Contract
Deliver Excavator
Fig 3 (b)
same beginning and end event num- bers.
Figure 3(b) still expresses both se- quence and logic correctly and in addition to this the numbering system is unique.
Example
It is convenient at this stage to illustrate the preceding notes with an
Lay Pipes
30
Build Manholes
40
50
Lay Pipes
30
40
Build Manholes
50
25
example. The network is a simplified version of that required for the con- struction of a building up to over site level. The diagram is built up from the following schedule of activities.
1. Set out.
2. Strip site.
3.
4.
Excavate to reduce levels. Start excavation to founds. 5. Excavate for drains.
6. Finish excavation to founds, 7. Lay drains.
8. Build manholes 9. Concrete founds. 10. Start brickwork to D.P.C. 11. H/C to ground floor slab. 12. Finish brickwork to D.P.C. 13. Formwork to ground beams. 14. Reinforcement to ground
beams.
Excavation
20
30
Excavation
40
15. Concrete ground beams. 16. Blinding to ground floor slab. 17. Start concreting to ground
floor slab.
As can be seen in Figure 4 certain jobs have been broken down into two operations-the degree of such break- down is one of the decisions to be made by the person drawing the dia- gram. But the practical considerations of space and access on site should be constantly borne in mind. Also the use to which the plan is to be put a preliminary over-all programme for top management would not re- quire the detail required by the resi- dent "ngineer on site.
Drawing The Network Diagram
The drawing of a neat diagram which is logically correct and easy to follow is almost an impossibility at the first attempt, and in order to pro- duce a reasonably legible diagram the following suggestions are offered:- (a) Start the arrows as soon as it is logical to do so. i.e. as indicated by the questioning technique in paragraph "Establishing Sequen- ce", but leave the heads of the arrows off until sure at which event the arrow should close. (b) Write the names of the activities on the arrows rather than try to use code letters. This will slight- ly increase the size of the dia- gram but will reduce errors which arise from the need of cross referencing which occurs when a coding system is used. (c) Draw the arrows in straight lines
running from left to right across the paper trying to avoid arrows crossing each other. This avoid- ance of crossing will not always be possible, in these cases the arrow should be “jumped" where they cross.
Even if these suggestions are fol- lowed alterations in the diagram will almost certainly be necessary. Errors of Logic
Apart from technological errors - specifying that a particular operation should follow another which would be there are impossible in practice two other types of error:
Looping This occurs when acti-
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