Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-1

Project Scheduling: Lagging, Crashing and Activity

Networks

Chapter 10

Copyright © 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-2

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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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

31 A 33Plumbing

6

33 B 36HVAC

5

36 C 37Inspection

1

30 D 36Wiring

6Lag 3

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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 10-12

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

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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

A H

F

E

D

C

B

KG

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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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