Critical Path
Method (CPM): The Critical Path Method (CPM)
was first developed in USA by the E.I DuPont Nemours & Co. in 1956 for
doing periodic overhauling and maintenance of a chemical plant.
The CPM differentiates between planning and scheduling of the project. While planning refers to determination of activities to be accomplished, scheduling refers to the introduction of time schedule for each activity of the project. The duration of different activities in CPM is deterministic. There is a precise known time that each activity in the project will take.
Scheduling by bar charts
A construction project consists of a sequence
of various activities like preparation of the site, foundation, substructure,
super structure, fittings, finishing, and other activities. Some of the
activities may be of critical nature and if the activity is not completed in
estimated time, it delays the entire project. The sequence of activities in the
construction of a building is given below.
[1]
Earth work in excavation.
[2] P.C.C bed and BW in foundation.
[3] D.P.C 11.Flooring
[4] Precasting RCC lintels.
[5] Casting RCC columns.
[6] BW in super structure.
[7] Sanitary work.
[8] RCC roof slab.
[9] Plasteering.
[10] Door Panels.
[11] White washing.
[12] Electrica lworks.
[13] Sanitary works.
[14] White washing.
[15] Sanitary fittings.
[16] Clearing up site.
The bar chart lists various activities
involved in a construction project and the period of time that each activity
takes for completion. Indicate in the form of a horizontal bar plotted to a
suitable time scale against each activity. The conventions commonly used in bar
chart are shown in the fig.
The planned program is shown by a thick or
double line with the planned start and finish by short vertical lines. The
float is shown by a hyphenated horizontal line and indicates that, although the
item is scheduled to be completed by a certain date, yet subsequent items are
not likely to be held up in case of delay up to the period represented by the
hyphenated line. A hyphenated vertical line, connecting the start of the item
with the completion of the item on which it is dependant, indicates the
dependency of one work on another. In fig, item 3 is dependent on item 1 and
cannot be started till item 1 is completed. The actual progress shown in single
line drawn below the line showing planned program. Bar charts are suitable for
determining the resources, such as materials, labour, machinery and finance,
required for construction work.
Figure shows the resource aggregation chart for RCC work. Bar chart is simple, easily understandable and widely used method of scheduling. However it has certain limitations, firstly it is difficult to depict complicated interdependencies of various items of work. It does not give actual progress of the work. It is not possible to know the peak rate of work necessary for timely completion of a project. The bar chart, there fore, is a static representation and does not respond to the dynamic happenings on the construction site of a complex project
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