Copyright © 2009 T.L. Martin & Associates Inc. Chapter 4 Standard CPM Smoke and Mirror Tricks...

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Chapter 4Chapter 4Standard CPM Smoke and Standard CPM Smoke and

Mirror TricksMirror Tricks(Manipulations)(Manipulations)

A.A.ConstraintsConstraintsa) Date Constraints a) Date Constraints

Critical Path - ConstraintsCritical 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”

Early Start ConstraintEarly Start ConstraintAppliedApplied

ForwardPass

BackwardPass

ES CONDAY 11

Early Start ConstraintEarly Start ConstraintNotNot Applied Applied

ForwardPass

BackwardPass

ES CONDAY 3

T.L. Martin & Associates, Inc.

Critical Path - ConstraintsCritical Path - Constraints

• Late Start Constraint: “Start no later than”• Late Finish Constraint: “Finish no later than”

Late Finish ConstraintLate Finish ConstraintDay 10 - ApplicableDay 10 - Applicable

ForwardPass

BackwardPass

LF CONDAY 10

Late Finish ConstraintLate Finish ConstraintDay 15 - Day 15 - NotNot Applicable Applicable

LF CONDAY 15

ForwardPass

BackwardPass

1 4 T.L. Martin & Associates, Inc.

• Project Finish On Constraint

Project Finish On ConstraintProject Finish On ConstraintDay 15Day 15

ForwardPass

BackwardPass

PROJECTCOMPLETION

4 7T.L. Martin & Associates, Inc.

• Start On Constraint: Sets early and late to same date

Start On ConstraintStart On Constraint

PROJECT FINISH ON CONSTRAINT

ForwardPass

BackwardPass

START ON DAY 9

WOULD NOT ENGAGE IF ES

IS PAST CONSTRAINT

• Mandatory Start Constraint: Overrides calculated dates to user imposed dates

• Mandatory Finish Constraint: Overrides calculated dates to user imposed dates

Mandatory Start ConstraintMandatory Start Constraint

PROJECT FINISH ON CONSTRAINT

ForwardPass

BackwardPass

VIOLATES NETWORK

MANDATORY START

• 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.A.ConstraintsConstraintsb) Float Constraints b) Float 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.A.ConstraintsConstraintsc) Resource Constraints c) Resource Constraints

ResourcesResources

• 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 ConstraintsResource 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.”

Resource ConstraintsResource 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.

• 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.

Resource Constraints – Not LeveledResource Constraints – Not Leveled

Resource Constraints – LeveledResource Constraints – Leveled

Constraints SummaryConstraints 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

B. Software Calculation B. Software Calculation and Logic Overridesand Logic Overrides

Software Calculation MethodsSoftware Calculation Methods

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

• 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 MethodsSoftware 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.

Software Calculation Methods Software Calculation Methods

Note the Pour is after Formwork completes.

Note the Pour completes prior to Formwork.

• 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.

Note Total Float.

Software Calculation MethodsSoftware 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.

C. Float and Network C. Float and Network CalendarsCalendars

Multiple CalendarsMultiple 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

Multiple CalendarsMultiple 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.

D. Leads, Lags and D. Leads, Lags and DummiesDummies

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 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.

Leads and Lags 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.

• Additionally, negative lag time periods can also be used causing the linked activities to overlap.

Finish to Start Relationship (“FS”)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

Finish to StartFinish to StartTypicalTypical

Layout & Prep

Under-slab Plumbing

• 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 Finish to Start with + Lag Limited UsageLimited Usage

3 21 3 9 10

Pour Concrete Strip Forms

Lag should not be used in this case since an activity can be created to represent the cure time.

• FS with a negative lag is used to schedule activities with an overlap. The lag contains a negative duration.

Finish to StartFinish to StartOverlapOverlap

Install Hangers

Hang Ductwork

Start to Start Relationship (“SS”)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 StartStart to StartGoodGood

Form 1st Lift S. Ret. Wall

Rebar 1st Lift S. Ret. Wall

Start to StartStart to StartBadBad

Set Millwork

Set & Hookup Plumbing Fixtures

Finish to Finish Relationship Finish to Finish Relationship (“FF”)(“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

Finish to FinishFinish to FinishGoodGood

Hang Drywall 3rd Flr Core

Finish Drywall 3rd Flr Core

Finish to FinishFinish to FinishBadBad

Hang Drywall 3rd Flr Core

Finish Drywall 3rd Flr Core

Start to Finish Relationship (“SF”)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

Start to FinishStart to FinishGoodGood

Install Lights S. Wng 2nd Flr

Temp. Lights S. Wng 2nd Flr

Start to FinishStart to FinishBadBad

Install Lights S. Wng 2nd Flr

Temp. Lights S. Wng 2nd Flr

E. Leveling resourcesE. Leveling resources

• 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.”

Resource ConstraintsResource 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.

Resource ConstraintsResource 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.

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

Copyright © 2009 T.L. Martin & Associates Inc. Chapter 4 Standard CPM Smoke and Mirror Tricks...

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