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Project Scheduling: Lagging, Crashing and Activity Networks. Chapter 10. © 2007 Pearson Education. 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

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lags in precedence relationships
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

  • Finish to Start
  • Finish to Finish
  • Start to Start
  • Start to Finish
finish to start lag

0 A 6

Spec Design

6

6 B 11

Design Check

5

15 C 22

Blueprinting

7

Lag 4

Finish to Start Lag
  • Most common type of sequencing
  • Shown on the line joining the modes
    • Added during forward pass
    • Subtracted during backward pass

This lag is not the same as activity slack

finish to finish lag

15 D 21

Wiring

6

21 C 22

Inspection

1

10 A 16

Plumbing

6

16 B 21

HVAC

5

Finish to Finish Lag

Two activities share a similar completion point

  • The mechanical inspection cannot happen until wiring, plumbing, and HVAC installation are complete

Lag 3

start to start lag

30 D 36

Wiring

6

Lag 3

36 C 37

Inspection

1

33 B 36

HVAC

5

31 A 33

Plumbing

6

Start to Start Lag
  • Logic must be maintained by both forward and backward pass
start to finish lag

Lag 3

Start to Finish Lag
  • Least common type of lag relationship
  • Successor’s finish dependent on predecessor’s start

30 D 36

Wiring

6

33 C 34

Inspection

1

28 B 33

HVAC

5

22 A 28

Plumbing

6

gantt charts
Gantt Charts
  • Establish a time-phased network
  • Can be used as a tracking tool

Benefits of Gantt charts

  • Easy to create and comprehend
  • Identify the schedule baseline network
  • Allow for updating and control
  • Identify resource needs
crashing projects
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
managerial considerations
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
slide12
What is the lowest cost to complete this project in 53 weeks? Times are in weeks and costs in dollars.
activity on arrow networks
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
slide14
Use AOA to sketch the network that represents the project as described in the table.
  • Calculate early and late start and finish times for all activities.
controversies in the use of networks
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