Chapter 15 Construction Scheduling 259
Critical Path Method
Another scheduling method is the critical path method (CPM). In this
method, activities and events of a project are shown in the form of a flow chart,
or network. The network consists of paths, each of which indicates a series of
activities that are done in order. The path that takes the longest time to complete
is the critical path (CP). This path dictates the minimum length of time needed
to complete the project. If the critical path can be shortened, a second path may
become critical. More than one network can be made for a project.
A CPM network for a basketball hoop project is shown in Figure 15-4.
Each arrow is a task. The circles with numbers in them are events. Events
do not require time. They represent the start and end of tasks. The number
in an event circle at its beginning and end identifies a task. When two or
more arrows point to the same event, the next task cannot begin until all the
preceding tasks are completed. Dashed lines show relationships but do not
require time.
On major projects, many people can work at the same time without
interfering with each other. Working on multiple tasks simultaneously
reduces the total duration of a project. If only one task was worked on at a
time, the duration of the project would increase.
Use the following sequence to make a CPM network:
1. List individual activities.
2. Determine the sequence in which the activities can be completed.
3. Draw a network diagram.
4. Estimate the time needed to complete each activity.
5. Identify the critical path.
critical path method
(CPM): Scheduling
method that consists
of paths, each which
indicates a series of
events that are done
in order.
critical path (CP):
The path through a
CPM network that
takes the longest
time to complete.
*G and B = Goal and backboard
**C, P, and S = Concrete, pole, and steel
3
2
5 6 7 8 9 11 12 13 14
10 4
1
Critical path
Put goal and backboard on pole
15 min.
Assemble
G and B*
15 min.
Concrete
Steel
Buy lumber
30 min.
Pole
Buy goal and backboard
30 min.
Buy C, P, and S**
30 min.
Clean up site Strip forms Delay—concrete cures Finish concrete Mix and place concrete Position pole Set steel Build forms Dig hole Lay out hole
0
20 min. 30 min. 30 min. 30 min. 45 min. 1 hour 15 min. 20 hours 15 min. 45 min.
Figure 15-4. This CPM network shows that different workers can be completing different tasks at the
same time. Note the special symbol used to indicate the critical path.