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 1.Finish to Start 2.Finish to Finish 3.Start to Start 4.Start to Finish

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 15 C 22 Blueprinting 7 Lag 4 This lag is not the same as activity slack

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

Start to Start Lag 31 A 33 Plumbing 6 33 B 36 HVAC 5 36 C 37 Inspection 1 30 D 36 Wiring 6 Lag 3 Logic must be maintained by both forward and backward pass

Start to Finish Lag Least common type of lag relationship Successors finish dependent on predecessors start 22 A 28 Plumbing 6 28 B 33 HVAC 5 33 C 34 Inspection 1 30 D 36 Wiring 6 Lag 3

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

Create a Gantt chart based on the activities listed in the table. TaskTimePredTaskTimePred Z8--U3W Y5ZT6V X8ZS7U,T W4Y,XR9S V5W

Gantt Chart With Resources in MS Project

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 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

What is the lowest cost to complete this project in 53 weeks? Times are in weeks and costs in dollars. ActivityPredNormal Time Min Time Normal Cost Crash Cost A BA CA DB, C ED FD GE, F HG

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

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. ActivityTimePredActivityTimePred A4--F4D B2AG15D C10AH7E,F,G D3B,CK11G E15B

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