project scheduling: lagging, crashing and activity networks chapter 10 copyright © 2010 pearson...
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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
Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall 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 6Spec Design
6
6 B 11Design Check
5
15 C 22Blueprinting
7
Lag 4
This lag is not
the same as
activity slack
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Finish to Finish Lag
Two activities share a similar completion point– The mechanical inspection cannot happen until
wiring, plumbing, and HVAC installation are complete
10 A 16Plumbing
6
16 B 21HVAC
5
21 C 22Inspection
1
15 D 21Wiring
6
Lag 3
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Start to Start Lag
Logic must be maintained by both forward and backward pass
Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-5
31 A 33Plumbing
6
33 B 36HVAC
5
36 C 37Inspection
1
30 D 36Wiring
6Lag 3
Start to Finish Lag
• Least common type of lag relationship
• Successor’s finish dependent on predecessor’s start
22 A 28Plumbing
6
28 B 33HVAC
5
33 C 34Inspection
1
30 D 36Wiring
6 Lag 3
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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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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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Gantt Chart With Resources in MS Project
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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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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
TimeMin
TimeNormal
CostCrash Cost
A -- 14 9 500 1500
B A 5 2 1000 1600
C A 10 8 2000 2900
D B, C 8 5 1000 2500
E D 6 5 1600 1900
F D 9 6 1500 3000
G E, F 7 4 600 1800
H G 15 11 1600 3600
Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall 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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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 Time Pred Activity Time Pred
A 4 -- F 4 D
B 2 A G 15 D
C 10 A H 7 E,F,G
D 3 B,C K 11 G
E 15 B
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Activity on Arrow Network
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A H
F
E
D
C
B
KG
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
Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-16
Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-17