Part 2
THE network, given in the previous
the construction of a building up to over-site level will be used to illustrate the procedure for numbering events, ascribing durations, calculating earliest and latest event times and finding the critical path. (Fig. 8). For some activities, it will be assumed that there is some uncertainty in their durations. In these cases the PERT formula will be used.
Step 1
Having decided the degree of oper- ational breakdown, the activities are scheduled.
1. Set out.
2. Strip site.
3. Excavate to reduce levels.
4. 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. HC to ground floor slab. 12. Finish brickwork to D.P.C. 13. Formwork to ground beams. 14. Reinforcement to ground
beams.
15. Concrete ground beams, 16. Blinding to ground floor slab. 17. Start concreting to ground floor
slab.
Step 2
NETWORK ANALYSIS IN CONSTRUCTION
by P.A. Smith, AIOB
Part 1 of this series described the basic techniques for network analysis. Before proceeding to extension of these techniques, they are collated here in the form of a worked example.
ACTIVITY
Set out Strip site
Table 1
Build manholes
Concrete founds.
Start brickwork to D.P.C.
Hardcore to ground floor slab Finish brickwork to D.P.C. Formwork to ground beams Reinforcement to ground beams Concrete ground beams Blinding to ground floor slab Start concreting to ground floor slab
+
DURATION
3 day
2 days
Excavate to reduce levels
Start excavation to founds.
2 days
3 days
3.5 + 4 x 5.5 + 10.5
Excavate for drains
6 days
6
Finish excavation to founds.
4 days
Lay drains
5.5 + 4 X 7.5 + 12.5
6
8 days
6 days
6 days
7 days
5 days
7 days
3 days
2 days
1 day
1 day
4 days
Table 2
Event No.
Alternative Path
Durations
Earliest Time
20
3
3
30
3+2
5
40
5+2
7
50
7+3
10
60
7+6
13
70
10+0
10
80
13+0
13
90
10+6 or 10+4 16
These earliest times are entered on the network diagram and enclosed in squares.
100
13+6 or 13+8 or
16+7
23
Step 6
110
23+5
28
all
The latest event times are found by working backwards through the dia- gram and subtracting the duration of the job from the preceding event time. The latest time is the lowest of the values obtained.
120
23+3
26
130
26+2
28
140
28+1 or 23+7 or
28+1
30
150
30+4
34
By reference to drawings and con- sideration of all the factors involved. the network is drawn. At this stage. there should be no consideration of the duration of activities
a con-
siderable advantage over other plan- ning methods only considerations of logic. The questioning technique described earlier under "Establishing Sequence" is of considerable value in this respect.
Step 3
Number the events using Fulker- son's routine:
Starting at the first event number it 10.
Delete all emerging arrows creating a new initial event numbering it 20. Continue this process until events are numbered. Step 4
Establish activity duration times. using the PERT formula where there is uncertainty.
Having calculated the durations of all activities (Table 1) mark these on the diagram (Fig. 8).
Step 5
To calculate the earliest event times, one must examine the alternative paths leading to the various events and select the highest of the values obtained.
Starting at event 10, call this time zero: the earliest times of succeeding events are clearly seen by reference to the network diagram and the follow- ing table:-
54
Far East Architect & Builder July, 1967