Project Scheduling: Lagging, Crashing and Activity Networks

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 6 Spec Design 6 6 B 11 Design Check 5 Lag 4 15 C 22 Blueprinting 7 Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 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 21 C 22 Inspection 1 Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-4

Start to Finish Lag 30 D 36 Wiring 6 Lag 3 Least common type of lag relationship Successor s finish dependent on predecessor s start 22 A 28 Plumbing 6 28 B 33 HVAC 5 33 C 34 Inspection 1 Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 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 Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 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 Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-8

Crashing The process of accelerating a project Principal methods for crashing Improving existing resources productivity Changing work methods Increasing the quantity of resources Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 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 -- 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 Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 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 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 Copyright 2010 Pearson Education, Inc. Publishing as Prentice Hall 10-14

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