construction planning, project management and economics

1. Project crashingProject crashing

2. Project crashingProject crashing
• It is common in project management that
additional resources are used to either speed up
some activities to get the project back on
schedule or to reduce the project completion
time.
• Crashing an activity refers to the speeding up or
shortening of the duration of an activity by using
additional resources. These include overtime,
hiring temporary staff, renting more efficient
equipment, and other measures.

3. Project crashingProject crashing
• Project crashing refers to the process of
shortening the duration of the project by
crashing the duration of a number of activities.
• Since it generally results in an increase of the
overall project costs, the challenge faced by the
project manager is to identify the activities to
crash and the duration reduction for each
activity such that as the project crashing is done
in the least expensive manner possible.

4. Project crashingProject crashing
• The normal time refers to the estimated activity
duration used with CPM or PERT in the
computation of earliest (latest) start or finish
times.
• The normal cost refers to the activity cost under
the normal activity time.
•

5. Project crashingProject crashing
• The crash time refers to the shortest possible time to
complete an activity with additional resources.
• The crashing cost refers to the activity cost under the
crashing activity time. This relationship is assumed to
be linear. Hence, for each activity a crash cost per
period (e.g.,per week) can be derived as follows:
• crash cost per period = (Crash cost - Normal Cost)/
(Normal time - Crash time)

6. Project crashingProject crashing
• the cost to crash per period assumes that the
relationship between adding more money to
the activity and reducing the time is linear.
Spend half of the money, and get half the time
reduction, spend _ of the money and get _
time reduction. This is not always true in
practice, but works alright for a rough
planning technique

7. THE GENERAL RELATIONSHIP OF TIMETHE GENERAL RELATIONSHIP OF TIME
AND COSTAND COST
• Crashing costs increase as project durationCrashing costs increase as project duration
decreases.decreases.
• Indirect costs increase as productionIndirect costs increase as production
duration increases.duration increases.
• Reduce project length as long as crashingReduce project length as long as crashing
costs are less than indirect costs.costs are less than indirect costs.

8. Conti….Conti….

9. Conti….Conti….
• The objective of crashing was to reduce theThe objective of crashing was to reduce the
scheduled completion time to reap thescheduled completion time to reap the
results of the projects sooner.results of the projects sooner.
• However, there may be other reasons forHowever, there may be other reasons for
reducing project time.reducing project time.
• There also may be direct financial penaltiesThere also may be direct financial penalties
for not completing a project on time.for not completing a project on time.

10. What criteria should it be based onWhat criteria should it be based on
when deciding to crashing criticalwhen deciding to crashing critical
times?times?
The critical path is 1-2-3, the completion time =11The critical path is 1-2-3, the completion time =11
How? Path: 1-2-3 = 5+6=11 weeksHow? Path: 1-2-3 = 5+6=11 weeks
Path: 1-3 = 5 weeksPath: 1-3 = 5 weeks
Now, how many days can we “crash” it?Now, how many days can we “crash” it?
1
3
25 (1) 6(3)

11. Conti…Conti…
• The maximum time that can be crashed for:The maximum time that can be crashed for:
Path 1-2-3 = 1 + 3 = 4Path 1-2-3 = 1 + 3 = 4
Path 1-3 = 0Path 1-3 = 0
• ShouldShould we use up all these 4 weeks?we use up all these 4 weeks?
1
3
25 (1) 6(3)
5(0)

12. • If we used all 4 days, then path 1-2-3 has
(5-1) + (6-3) = 7 completion weeks
Now, we need to check if the completion time for path 1-3
has lesser than 7 weeks (why?)
Now, path 1-3 has (5-0) = 5 weeks
Since path 1-3 still shorter than 7 weeks, we used up all 4
crashed weeks
• Question: What if path 1-3 has, say 8 weeks completion
time?
1
3
2
5 (1)
6(3)
5(0)
4(0) 3(0)

13. • Now, we cannot use all 4 days (Why?)
Because path 1-2-3 will not be critical path anymore
path 1-3 would now has longest hour to finish
• Rule: When a path is a critical path, it will not stay as
a critical path
• So, we can only reduce the path 1-2-3 completion
time to the same time
• as path 1-3. (HOW?)
1 3
25 (1) 6(3)
8(0)

14. • We can only reduce total time for path 1-2-3 =We can only reduce total time for path 1-2-3 =
path 1-path 1-33,,
that is 8 weeksthat is 8 weeks
If the cost for path 1-2 and path 2-3 is the sameIf the cost for path 1-2 and path 2-3 is the same
thenthen
We can random pick them to crash so that itsWe can random pick them to crash so that its
completioncompletion
Time is 8 weeksTime is 8 weeks
1
3
25 (1) 6(3)
8(0)

15. 1
3
25 (1)5 (1) 6(3)6(3)
8(0)8(0)
1
2
3
5 (1)5 (1)
6(3)6(3)
8(0)8(0)
OR
4(0)4(0) 4(1)4(1)
3(0)3(0)