activity sequencing

8/3/2019 Activity Sequencing

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Project Scheduling: Lagging,

Crashing and Activity Networks

Chapter 10


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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 Finish3. 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


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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 16

Plumbing

6

16 B 21

HVAC

5

21 C 22

Inspection

1

15 D 21

Wiring

6

Lag 3


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Start to Start Lag

31 A 33

Plumbing

6

33 B 36

HVAC5

36 C 37

Inspection1

30 D 36Wiring

6

Lag 3


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Start to Finish Lag

Least common type of lagrelationship

Successors finish dependent onpredecessors start

22 A 28

Plumbing

6

28 B 33

HVAC

5

33 C 34

Inspection

1

30 D 36

Wiring

6

Lag 3


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Gantt Charts

Establish a time-phased network

Can be used as a tracking tool

Benefits ofGantt charts

1. Easy to create and comprehend

2. Identify the schedule baseline network

3. Allow forupdating 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 Projects

Accelerating a project by committing more

resources than initially planned

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 crash cost exceeds the penalty cost


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What is the lowest cost to complete thisproject in 53 weeks? Times are in weeks

and costs in dollars.Activity Pred NormalTime

Min

Time

Normal

Cost

Crash

Cost

A -- 14 9 500 1500

B A 5 2 1000 1600C 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 allactivities.

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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Controversies in the Use of Networks

Networks can be too complex

Poornetwork 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

activity sequencing

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