is that time which allows all out- going activity chains to be completed inside the project duration already determined. It is found by working backwards through the arrow dia- gram using similar calculations to those above. The Critical Path then passes through those events whose earliest and latest times are equal. Critical activities associated with these events are indicated by heavy arrows in Fig. 3.
Fig. 3.
(35)
©
(47)
(20)
(set
E
(17)
A complete project might contain thousands of arrows but the method of analysis is just the same. Critical activities form only a small propor- tion of the total in a complex arrow diagram.
The extent to which non-critical activities can be delayed without affecting the projection is found by comparing earliest and latest times for each event, as in Table 2.
The maximum time available for activity 1, 2 is the difference between
examined. Re-analysis of sequence the amended diagram will indicate the effects of any changes made.
When an acceptable project dura- tion has been obtained the resources The be examined. required can estimated times for activities will be related to their individual require- ments for manpower and plant. A total resource requirement (with each activity starting as early as it can) can then be calculated.
If jobs in parallel on the diagram require the same resource the total resources required may exceed those available. By delaying non-critical jobs up to the limit of their float the need for extra resources can be minimized. As a result, a schedule each is produced defining when activity will be performed.
Stage 5-Project control
As work proceeds, CPM can help to control the project by:
1. indicating where management attention should be concentrat- ed--on the critical activities, 2. determining the effects of what has happened to date by re- analysis of the diagram,
3. minimizing the effects of any delays that may have occurred by replanning the remaining activities,
4. carrying out (2) and (3) at re- gular intervals throughout the project. The examples quoted. though simple, are sufficient to illustrate the basic method and indicate what it
can achieve, without going into the varia- tions and refinements appro- priate to the nature or size of the particular projects and needs of the planning team.
Conclusion
The basis of CPM is simple to understand and its use for small- scale projects requires no apparatus more complex or costly than pencil and paper.
For the architect it has an impor- tant role to play because the entire sequence of operations has its origin in the design stage. The method has a further attraction to the archi- tect: normal drawing board practice should enable him to appreciate readily the diagrammatic presenta- tion of the project plan which is central to CPM.
This explanation was prepared by the Building Research Station. Garston. Herts., UK (BRS Diggest (2nd Series) - 53). Crown Copyright.
BRICKWORK (ground floor)
(4)
„First Floor
JPISTS
ARCANCAK „[forat daar)
(+)
BACKWORK (Topping out)
(1)
12
TILING
(2)
CARCASE
CARPENTER'S
ELECTRICAL
CARCASE
(5)
PLATITIONS
(1)
GAS
(2)
75
PREWORK
(3)
AXIMO
(†) = Activity duration
L - Earliest tvtal time
AA:NWATER
PLUMBING
Fig. 4. Structure and finishing of a house.) ས
FLASHINGS
(7)
PLASTERBOARD
AND FLASTER
CARPENTER'S
55
PLUMBERS
FITTINGS
(2)
(2)
SPDECORATION Ş
INTERNAL
AS
ELECTRICAL
BITTINGS
EXTERNAL
PAINTING
FLOOR FINIS34
45
CLEAN & MANÖÖVER
A modest example, showing the structure and finishing stages of a house, is shown in Fig. 4. Activity durations are given in brackets against the arrows, as in Figs. 2 and 3, but earliest times for each event are now given where events were previously numbered and circled. For the labour force assumed, the earliest completion date is 45 working days from d.p.c. level, this time having been determined by the arrowed sequence of critical activities drawn with heavy lines. A different labour force or design may well modify the outline of this diagram without, of course, changing the basic C.P.M. approach in any way.
the earliest time for event 1 and the latest time for event 2, that is 70 hours. But activity 1,2 is estimated to take 47 hours, hence the total float for the activity is 23 hours. Total float times for each activity are shown in squares in Fig. 3.
Table 2.
Event
No.
Earliest
time
Latest time
1
35
35
2
82
105
3
105
105
4
105
105
155
155
WN
Stage 4--Final scheduling
If the project duration given by the analysis is excessive then either one or more of the critical activities be shortened or an alternative
can
56
Books
Your Architect by Derek Senior (Hodder, Ltd., London, 5 shillings).
THIS is a book about the architectural profession written by a journalist at the request of the Royal Institute of British Architects. It discusses the situations and the problems of the architect and is aimed at the clients who employ him and the committee he has to deal with.
As a book for clients, it is good at pedestrian level; the architect is portrayed as well-meaning and com- petent, and we are told that he has the means to control cost and under- stand the material needs of the user. The chapter on private houses is ex- cellent.
However, the book has a short- coming. In the first chapter we are
told that architects are so trained that they automatically design pleasing buildings. Mr. Senior must be blind, or it would seem he has never walked down a post-war street!
On reading further, it appears that he is not blind but indifferent to the art of architecture: he says "Far more significant than any recent change in the external appearance of buildings has been the great increase in func- tional efficiency and value for money produced." This materialism per- meates the entire book, and any architect who passionately wants to build fine buildings is dismissed in a few asides.
Perhaps it is because of its RIBA parentage that the book reflects the current establishment line, that all we need is a bit more research, a few more consortia, a better information service, a management course and all will be well with the profession. J.W.
Far East Architect & Builder March, 1965
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