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Chapter 4 Standard CPM Smoke and Mirror Tricks (Manipulations)

A.Constraints

a) Date Constraints

Critical Path - Constraints

• •

Constraint

– A forced manual override of computed activity timing and/or float.

Constraints on activities, modify timing, reduce float and affect real and imagined critical paths. • • Early Start Constraint: “Start no earlier than” Early Finish Constraint: “Finish no earlier than” Copyright © 2009 T.L. Martin & Associates Inc.

Forward Pass 1

Early Start Constraint

Applied

ES CON DAY 11 4 4 11 14 4 FS=0 Backward Pass 11 4 14 7 10 4

Early Start Constraint

Not Applied

1 4 Forward Pass 4 ES CON DAY 3 5 8 4 Backward Pass 5 4 1 4 4

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Critical Path - Constraints

Forward Pass

Late Finish Constraint

Day 10 - Applicable

1 4 4 5 8 4 9 4 12 LF CON DAY 10 Backward Pass 7 4 10 3 6 4 -1 4 2

Late Finish Constraint

Day 15 - Not Applicable

1 4 4 Forward Pass 5 8 LF CON DAY 15 4 9 4 12 Backward Pass 9 4 12 5 8 4 1 4 4

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Critical Path - Constraints

Project Finish On Constraint

Day 15

1 4 4 Forward Pass PROJECT COMPLETION 5 8 4 9 4 12 Backward Pass 12 4 15 8 4 11 4 7

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Critical Path - Constraints

Start On Constraint

START ON DAY 9 Forward Pass 1 4 4 WOULD NOT ENGAGE IF ES IS PAST CONSTRAINT 9 4 12 13 4 16 PROJECT FINISH ON CONSTRAINT Backward Pass 5 9 12 4 TF=0 4 TF=4 8

17 4 TF=4 20

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Critical Path - Constraints

• • Mandatory Start Constraint: Overrides calculated dates to user imposed dates Mandatory Finish Constraint: Overrides calculated dates to user imposed dates Copyright © 2009 T.L. Martin & Associates Inc.

Mandatory Start Constraint

Forward Pass MANDATORY START 1 VIOLATES NETWORK LOGIC 4 3 4 4 6 7 4 10 PROJECT FINISH ON CONSTRAINT Backward Pass 3 6 4 TF=0 -2 4 TF=-2 2

10 4 TF=3 13

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Critical Path - Constraints

• Expected Finish Constraint – Revises the calculated early finish date to the imposed date and then computes the activities remaining duration as the difference between the calculated early start and the expected finish imposed date. Predecessor early dates are then modified as if an Early Start Constraint had been imposed including the revised remaining duration.

A.Constraints

b) Float Constraints

Critical Path - Constraints

• • Zero Total Float: Sets late dates to early dates, thus causing it and all predecessors to show zero (0) float. Successor activity will not be affected.

Zero Free Float – This constraint forces an activity to start as late as possible without delaying or reducing its successor’s float.

A.Constraints

c) Resource Constraints

Resources

• What the job is - labor, materials, equipment, small tools, money , expendable materials • Defined in dictionary for assignment to work activities • Budgeted quantities - CY of Concrete, Tons of Steel, men per day, # EA Cranes • Budgeted costs - 1 CY Concrete = $80.00

Resource Constraints

• Resource constraints imposed during leveling computations shift activities in time, revise float and criticality. • Primavera, can cause activities to: – split into segments, – stretch by adding duration reducing period resource use, and – shorten an activity’s duration by increasing period resources. • Primavera identifies the process as “splitting, stretching and crunching activities.” Copyright © 2009 T.L. Martin & Associates Inc.

Resource Constraints

• Activities are manipulated based on defined resource availability, which can vary over time depending on the assigned use curve. • The options, parameters and resources selected for leveling can generate many and varied results. • The direction, forward or backward, of the resource leveling calculation path can generate radically different activity dates and float. Copyright © 2009 T.L. Martin & Associates Inc.

Resource Constraints

• The forward leveling calculation starts with the first network activity proceeding to the last, obeying network logic and modifying early performance dates. • This mechanism is designed to ensure that sufficient resources are available to perform each activity. Copyright © 2009 T.L. Martin & Associates Inc.

Resource Constraints – Leveled

Constraints Summary

• • • • • • Constraints should only be incorporated into the CPM if they are required by the specifications Constraints should only be used to define project completion Constraints are often used in place of proper logic or duration Constraints can be used to reduce total float Constraints can be used to manipulate or generate false critical paths If at all possible, do not use constraints in the schedule Copyright © 2009 T.L. Martin & Associates Inc.

B. Software Calculation and Logic Overrides

Software Calculation Methods

• The calculation method selected affects forecast activity performance dates, float and many times the critical path.

Software Calculation Methods

• Primavera allows three calculation approaches to handle out-of-sequence progress.

–

Retained Logic

- This option maintains the integrity of network logic by requiring all predecessor requirements to be met prior to resumption of progress on the out-of-sequence activity.

–

Progress Override

– This option ignores network predecessor logic of activities that have started. Activities that have started are calculated to finish independent of its predecessors.

Software Calculation Methods

• Primavera allows three calculation approaches to handle out-of-sequence progress.

–

Actual Dates

– This option uses actual dates to compute activity float of uncompleted predecessors of out-of sequence activities. This option uses retained logic for incomplete predecessors linked to incomplete successors but progress override for incomplete successors and predecessors if an intermediate activity completes out-of-sequence. – Actual Dates computes float as the difference between the planned finish of the uncompleted predecessor and the actual start of the out-of sequence successor. Copyright © 2009 T.L. Martin & Associates Inc.

Software Calculation Methods

Note the Pour is after Formwork completes.

Note the Pour completes prior to Formwork.

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Software Calculation Methods

• • • Retained logic v. actual date schedule computation for uncompleted out-of-sequence work. The key difference is the computation of activity float. Actual Dates computes float as the difference between the planned finish of the uncompleted predecessor and the actual start of the out-of sequence successor. Copyright © 2009 T.L. Martin & Associates Inc.

Software Calculation Methods

Note Total Float.

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Software Calculation Methods

• • • Retained logic v. progress override v. actual date schedule computations for completed out-of sequence work. The key difference between progress override and actual dates is the computation of activity float. Actual dates computes float as difference between the planned finish of the uncompleted predecessor and the start of the not completed out-of-sequence successor. Copyright © 2009 T.L. Martin & Associates Inc.

Software Calculation Methods

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C. Float and Network Calendars

Multiple Calendars

• • • • Are necessary for certain special construction activities, i.e. paving or weather/temperature related work Should be defined directly or indirectly in the specifications Can generate a critical path (one continuous chain of activities) with multiple total float values Use of multiple calendars can generate project completion forecasts which are very misleading, if sequencing is inaccurate Copyright © 2009 T.L. Martin & Associates Inc.

Multiple Calendars

• • Different work calendars applied to a chain of activities will cause the float to vary along the chain. Different float amounts occur since float is calculated in workdays and the number of workdays available changes with different calendars. Copyright © 2009 T.L. Martin & Associates Inc.

Multiple Calendars

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Multiple Calendars

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D. Leads, Lags and Dummies

Dummies

• • The activity-on-arrow or i-j scheduling technique uses an arrow called a dummy, without duration or work description to represent logic ties between work tasks. Several tricks can be performed with the help of dummies: – The oldest trick is to include durations on the dummy arrows. – The second most popular trick for dummies is the creation of multiple smoking dummy links. The scheduler links several dummies in a chain in order to hide forced and illogical critical paths.

Leads and Lags

• • • The precedence diagramming method uses logical relationships or links between activity nodes. Primavera allows these links to contain a leading or a lagging time period prior to performance of its successor. These links, similar to dummies with duration, create a lot of smoke and odd reflections, which can disguise a networks true critical path and activity float. Copyright © 2009 T.L. Martin & Associates Inc.

Leads and Lags

• • • There are four basic relationship types. – FS – Finish-to-Start – An activity finishes and then its successor can start.

– SS – Start-to-Start – An activity starts allowing its successor to start. – FF – Finish-to-Finish – An activity finishes allowing its successor to finish.

– SF – Start-to-Finish – An activity starts allowing its successor to finish.

Each of these relationships can delay the successor activity by an imposed time period or a positive lag amount.

Finish to Start Relationship (“FS”) • • Completion of an activity allows the start of a successor activity Most common link: Default setting is the Finish to Start with zero duration lag Copyright © 2009 T.L. Martin & Associates Inc.

Finish to Start

Typical

1 3 3 Layout & Prep 4 8 5 Under-slab Plumbing

Finish to Start Relationship (“FS”) • FS with a positive lag is used to represent the passage of time between two activities. The lag contains a duration • Positive lag should not be used when an activity will perform the same function – concrete cure time.

Finish to Start with + Lag

Limited Usage

1 3 3 Pour Concrete FS 5 9 2 10 Strip Forms Lag should not be used in this case since an activity can be created to represent the cure time.

Finish to Start Relationship (“FS”) • FS with a negative lag is used to schedule activities with an overlap. The lag contains a negative duration.

Finish to Start

Overlap

1 6 Install Hangers 6 FS -2 5 6 Hang Ductwork 10

Start to Start Relationship (“SS”)

• • • • • • Another relationship used to schedule concurrent work Start of one activity allows the start of another activity Always have another successor activity What does the completion of this activity tie to?

Does this activity have to be finished for something else to finish?

Do not use lag that are larger than the preceding activity duration.

Start to Start

Good

10 1 10 Form 1st Lift S. Ret. Wall SS 3 4 10 Rebar 1st Lift S. Ret. Wall 13

Start to Start

Bad

1 3 3 Set Millwork SS 5 6 8 3 Set & Hookup Plumbing Fixtures

Finish to Finish Relationship (“FF”)

• • • • Allows an activity to finish after the completion of a predecessor activity Always have another predecessor activity Should typically used in tandem with the Start to Start relationship Lags should not be larger than successor activity Copyright © 2009 T.L. Martin & Associates Inc.

Finish to Finish

Good

1 5 5 Hang Drywall 3rd Flr Core FF 3 4 8 5 Finish Drywall 3rd Flr Core

Finish to Finish

Bad

1 3 3 Hang Drywall 3rd Flr Core FF 5 5 8 4 Finish Drywall 3rd Flr Core

Start to Finish Relationship (“SF”) • Allows an activity to finish after a predecessor activity has started • Lag should not be greater than the successor activity duration • Should be used with other relationships Copyright © 2009 T.L. Martin & Associates Inc.

Start to Finish

Good

5 5 Install Lights S. Wng 2nd Flr 9 SF 1 3 6 4 Temp. Lights S. Wng 2nd Flr

Start to Finish

Bad

5 7 3 Install Lights S. Wng 2nd Flr SF 4 6 8 3 Temp. Lights S. Wng 2nd Flr

E. Leveling resources

Resource Constraints

• Resource constraints imposed during leveling computations magically shift activities in time, revise float and criticality. • Primavera, can cause activities to: – split into segments, – stretch by adding duration reducing period resource use, and – shorten an activity’s duration by increasing period resources. • Primavera identifies the process as “splitting, stretching and crunching activities.” Copyright © 2009 T.L. Martin & Associates Inc.

Resource Constraints

• Critical paths created by different and varying resource constraints can differ greatly. • Real versus imagined resource issues should be accommodated. • This will require an understanding of the project: – its location, – space available, – labor and equipment requirements, – as well as contractor specific constraints. Copyright © 2009 T.L. Martin & Associates Inc.

Please continue with Chapter 5 Advanced Techniques of CPM Deception and Mystery

T.L. Martin & Associates Inc.

Slide 1

Transcript Slide 1

Chapter 4 Standard CPM Smoke and Mirror Tricks (Manipulations)

A.Constraints

a) Date Constraints

Critical Path - Constraints

Early Start Constraint

Early Start Constraint

Critical Path - Constraints

Late Finish Constraint

Late Finish Constraint

Critical Path - Constraints

Project Finish On Constraint

Critical Path - Constraints

Start On Constraint

Critical Path - Constraints

Mandatory Start Constraint

Critical Path - Constraints

A.Constraints

b) Float Constraints

Critical Path - Constraints

A.Constraints

c) Resource Constraints

Resources

Resource Constraints

Resource Constraints

Resource Constraints

Resource Constraints – Leveled

Resource Constraints – Leveled

Constraints Summary

B. Software Calculation and Logic Overrides

Software Calculation Methods

Software Calculation Methods

Software Calculation Methods

Software Calculation Methods

Software Calculation Methods

Software Calculation Methods

Software Calculation Methods

Software Calculation Methods

C. Float and Network Calendars

Multiple Calendars

Multiple Calendars

Multiple Calendars

Multiple Calendars

D. Leads, Lags and Dummies

Dummies

Leads and Lags

Leads and Lags

Finish to Start

Finish to Start with + Lag

Finish to Start

Start to Start Relationship (“SS”)

Start to Start

Start to Start

Finish to Finish Relationship (“FF”)

Finish to Finish

Finish to Finish

Start to Finish

Start to Finish

E. Leveling resources

Resource Constraints

Resource Constraints

Resource Constraints

Please continue with Chapter 5 Advanced Techniques of CPM Deception and Mystery

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