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?
7
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
