Transcript Lagging, Crashing and Activity Networks

Project Scheduling: Lagging,
Crashing and Activity Networks
Chapter 10
Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall
10-1
Lags in Precedence Relationships
The logical relationship between the start
and finish of one activity and the start and
finish of another activity.
Four logical relationships between tasks
1. Finish to Start
2. Finish to Finish
3. Start to Start
4. Start to Finish
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10-2
Finish to Start Lag
•
•
Most common type of sequencing
Shown on the line joining the modes
– Added during forward pass
– Subtracted during backward pass
0
A
6
Spec Design
6
6
B
11
Design Check
5
Lag 4
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15
C
22
Blueprinting
7
10-3
Finish to Finish Lag
Two activities share a similar completion point
– The mechanical inspection cannot happen until
wiring, plumbing, and HVAC installation are complete
15
D
21
Wiring
6
10
A
16
Plumbing
6
Lag 3
16
B
21
HVAC
5
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21
C
22
Inspection
1
10-4
Start to Start Lag
30
D
36
Wiring
6
Lag 3
33
31
A
33
Plumbing
6
B
36
HVAC
5
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36
C
37
Inspection
1
10-5
Start to Finish Lag
•
•
30
D
36
Wiring
6
22
A
28
Plumbing
6
Least common type of lag
relationship
Successor’s finish dependent on
predecessor’s start
Lag 3
28
B
33
HVAC
5
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33
C
34
Inspection
1
10-6
Gantt Charts
 Establish a time-phased network
 Can be used as a tracking tool
Benefits of Gantt charts
1. Easy to create and comprehend
2. Identify the schedule baseline network
3. Allow for updating and control
4. Identify resource needs
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10-7
Create a Gantt chart based on
the activities listed in the table.
Task Time Pred Task Time Pred
Z
8
--
U
3
W
Y
5
Z
T
6
V
X
8
Z
S
7
U,T
W
4
Y,X
R
9
S
V
5
W
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10-8
Gantt Chart With Resources in MS Project
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10-9
Crashing
The process of accelerating a project
Principal methods for crashing
 Improving existing resources’ productivity
 Changing work methods
 Increasing the quantity of resources
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10-10
Managerial Considerations
• Determine activity fixed and variable costs
• The crash point is the fully expedited activity
• Optimize time-cost tradeoffs
• Shorten activities on the critical path
• Cease crashing when
– the target completion time is reached
– the crashing cost exceeds the penalty cost
Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall
10-11
What is the lowest cost to complete this
project in 53 weeks? Times are in weeks
and costs in dollars.
Activity
Pred
Normal
Time
Min
Time
Normal
Cost
Crash
Cost
A
B
C
D
-A
A
B, C
14
5
10
8
9
2
8
5
500
1000
2000
1000
1500
1600
2900
2500
E
F
G
D
D
E, F
6
9
7
5
6
4
1600
1500
600
1900
3000
1800
H
G
15
11
1600
3600
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10-12
Activity on Arrow Networks
 Activities represented by arrows
 Widely used in construction
 Event nodes easy to flag
 Forward and backward pass logic similar to AON
 Two activities may not begin and end at
common nodes
 Dummy activities may be required
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10-13
1. Use AOA to sketch the network that represents the
project as described in the table.
2. Calculate early and late start and finish times for all
activities.
Activity
A
Time
4
Pred
--
Activity
F
Time
4
Pred
D
B
C
D
2
10
3
A
A
B,C
G
H
K
15
7
11
D
E,F,G
G
E
15
B
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10-14
Activity on Arrow Network
E
B
H
A
F
C
D
K
G
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10-15
Controversies in the Use of Networks
Networks can be too complex
Poor network construction creates problems
Networks may be used inappropriately
When employing subcontractors
– The master network must be available to them
– All sub-networks must use common methods
Positive bias exists in PERT networks
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10-16
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10-17