Ch08 Crashing Essay

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Pages: 5

Project Management

Chapter 8 (Crashing)

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Prentice Hall

8-1

Project Crashing
Basic Concept
In last lecture, we studied on how to use
CPM and PERT to identify critical path for a project problem
Now, the question is:
Question:
Can we cut short its project completion time? If so, how!
Chapter 8 - Project Management

2
8-2

Project Crashing
Solution!
Yes, the project duration can be reduced by assigning more resources to project activities. But, doing this would somehow increase our project cost!
How do we strike a balance?
■ Project

crashing is a method for shortening project duration by reducing one or more critical activities to a time less than normal activity time.
3
8-3

Trade-off concept
Here, we adopt the “Trade-off” concept
 We attempt to “crash” some “critical” events by allocating more resources to them, so that the time of one or more critical activities is reduced to a time that is less than the normal activity time.  How to do that:
 Question: What criteria should it be based on when deciding to crashing critical times?
4
8-4

Example – crashing (1)
Max weeks can be crashed
Normal weeks
1

5 (1)

2

6(3)
3

The critical path is5(0)
1-2-3, the completion time =11
How? Path: 1-2-3 = 5+6=11 weeks
Path: 1-3 = 5 weeks
Now, how many days can we “crash” it? 5
8-5

Example – crashing (1)
5 (1)
1

2

6(3)
3

5(0)

The maximum time that can be crashed for:
Path 1-2-3 = 1 + 3 = 4
Path 1-3 = 0
Should we use up all these 4 weeks?
6
8-6

Example – crashing (1)
4(0)

5 (1)
1

3(0)
2

6(3)
3

5(0)

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?
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8-7

Example – crashing (1)
Such as
5 (1)
1

2

6(3)
3

8(0)

Now, we cannot use all 4 days (Why?)
Because path 1-2-3 will not be critical path anymore as 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?)
8
8-8

Example – crashing (1)
Solution:
5 (1)
1

2

6(3)
3

8(0)

We can only reduce total time for path 1-2-3 = path 1-3, that is 8 weeks
If the cost for path 1-2 and path 2-3 is the same then
We can random pick them to crash so that its completion
Time is 8 weeks
9
8-9

Example – crashing (1)
Solution:

4(0)
5 (1)

3

1

OR

8(0)
5 (1)

1

2

4(1)
6(3)

2

3(0)
6(3)
3

8(0)
Now, paths 1-2-3 and 1-3 are both critical paths 10
8-10

The Project Network
AOA Network for House Building
Project

Figure 8.6
Expanded Network for
Building a
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House
Prentice
Hall Showing Concurrent

8-11

Project Crashing and Time-Cost
Trade-Off
Example Problem (1 of 5)

Figure 8.19 The Project Network for
Copyright
© 2010 Pearson a
Education,
Inc. Publishing as
Building
House
Prentice Hall

8-12

Project Crashing and Time-Cost
Trade-Off
Example Problem (3 of 5)

Copyright © 2010 Pearson Education, Inc. Publishing as
Prentice Hall

Table 8.4

8-13

Project Crashing and Time-Cost
Trade-Off
Example Problem
(2&of
5) time have a
Crash cost crash linear relationship:

Total Crash Cost
$2000

Total Crash Time 5 weeks
 $400 / wk

Figure
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Education, Inc. Publishing as
8.20
Prentice Hall

8-14

Project Crashing and Time-Cost
Trade-Off
General Relationship of Time and
Cost (2 of 2)

Copyright © 2010 Pearson Education, Inc. Publishing as
Prentice Hall

Figure 8.23
The Time-Cost
8-15

Project Crashing and Time-Cost
Trade-Off
Example Problem (4 of 5)

Figure 8.21 Network with Normal Activity Times and Weekly
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Crashing
Costs
Prentice Hall
8-16

Project Crashing and Time-Cost