Operation
Date
Labour Plant
Day
2
No.
1 Lab
Set out
Strip site
1 Lab
MC
Excavate to reduce levels
1 Lab
M.C
Start excavation of founds.
1 Lab
M.C
Excavale for drains
1 Lab
M C
Finish excavation to founds
I Lab
M.C
2 D'L
Lay drains
1
2 Labs
Build manholes
Concrete founds.
Start brickwork to OP.C.
2 B.L
2 Labs
6 Labs
48:L
2 Labs
—
Hardcore to ground floor slab
4 Labs
I
4 B/L
Finish brickwork to DP.C.
2 Labs
4 Carp
Formwork to ground beams.
2 Labs
Reinforcement to ground beams
2 S.F
2 Labs
Concrete ground beams
6 Labs
Blinding to ground floor slab
6 Labs
Start concreting to ground floor slab
6 Labs
LEGEND
Earliest Start
Earliest Finish
Latest Finish
Resource
FRA DURA
NOTAL FLOAT
Scheduled Start
Scheduled Finish
Labourers required
10- 9
for scheduled Start & Finish dates 5
يما
4
S
678
9
10
11
12
13 14
17 15 16
18 19 20
24 21 | 22 | 23
25 26 27 28 29 30 31 32 33 34
Fig. 11.
The initial information required for these computer-based systems is con- siderably more than for simple critical path systems. The more information given at the input stage, the greater the number of choices available to the computer and the better the sub- sequent schedule.
Most computers require a descrip- tion of each activity, i.e. its begin- ning and end event numbers, the de- sired completion date and any cost penalty incurred for each unit of time delay in completion. In place of activity duration, an estimate of the work content in man-hours is used, with three alternatives of the number of the resource, for example 4, 6, or 10 bricklayers.
The availability and number of each resource, period by period, must be specified. The availability can be either normal or abnormal, abnormal availability being the extra resources made available by overtime or addi- tional sub-contracting. The computer would level all normal resources and would consider abnormal resources only when it was economic to do so. The information coming from the computer usually consists of schedules in bar-chart form. The numbers of units of each resource in each time period are listed vertically over horizontal time scale. These schedules show the total numbers of each re- source allocated for both normal and abnormal availabilities.
a
The computer is normally faced with one of two possible situations either there will be more capacity than there is work to be done or there is more work to do than there is capacity to accomplish it.
In the first case, resources would be allocated to all activities and the priorities of a number of activities
Far East Architect & Builder August, 1967
competing for the limited resources available would have to be evaluated. This is done by a marking system, marks being awarded to each feasible allocation under various headings such as "Meeting the Project Completion Date", and "Obtaining Maximum Re- source Utilisation", with as many activities as possible running in parallel. The actual weighting of the marking is specified by the planner. The marks for each system are then added together, the highest mark in- dicating the best solution for the headings and weightings specified.
These programmes can cope with one or more projects but the best results come when the total work load and resources of a company are con- sidered.
In addition to allocating and sche- duling resources before the start of a project, the computer will control the running of a project by produc- ing revised schedules which show the possible future in the light of achieve- ment to date.
Least-Cost Scheduling
After drawing the network dia- gram, it is often necessary to reduce the project time or it may be required that a certain part of the project be completed by a fixed date. It is ob- viously desirable that these reductions be completed at the minimum extra cost.
For each activity on the diagram, the cost of completing it in the mini- mum possible time, i.e. its "Crash Cost" and the cost for the normal duration can be established.
It is often assumed that there is
an "inverse linear relationship" be- tween cost and time in the range be- tween "Crash" and "Normal Cost".
This is unlikely to be exactly correct and it is possible, by breaking activi- ties into elements, to plot a more exact cost/time relationship.
The methods of reducing project time at minimum cost are to examine the critical path and calculate the cost of reducing each activity by one unit of time. The activity which costs the least amount to reduce should then be reduced by one unit. The unit may be a day, an hour or a week. In building work, a day is the more realistic unit.
The process of reducing the activity which costs the least should be con- tinued until the required duration is obtained. Care should be taken when reducing durations that new critical paths, if created, are also con- sidered. For all but the very smallest jobs, this process is too laborious for manual application and a computer should be used.
Conclusion
The construction industry was among the first to see the potential of network analysis. This is borne out by a survey which was carried out in America four years ago into the types of industries using network analysis techniques. The survey gave the following results.
Research and development Construction programmes
25%
24%
Programming of computers Preparation of bids and
12%
12%
12%
8%
5%
5%
4%
proposals Maintenance planning Installation of computer
systems Distribution planning Cost reduction programmes Miscellaneous
57