Critical Path Scheduling

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Critical Path Scheduling
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Planning Scheduling Process

• Identify project activities
• Estimate activity durations
• Develop the project plan
• Schedule project activities
• Review and analyze the schedule
• Implement the schedule

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CPM Basics
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Only Three Inputs Required

• Break scope into separate activities
• Estimate durations for each activity
• Define relationships between activities

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Example Build a Spot Footing

• Break into separate activities
• Excavate to required grade
• Place forms
• Tie rebar
• Place concrete

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Example Build a (Big) Spot Footing

• Estimate activity durations
• Excavate to required grade 1day
• Place forms 2 days
• Tie rebar 3 days
• Place concrete 1 day

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Example Build a Spot Footing

• Define relationships

Excavate
Place forms
Place concrete
Start
Tie rebar
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Now for some
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Forward Backward Pass
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• The critical path determines PROJECT duration
• It is the longest path through the project time
  wise.
• The early times equal the late times on the
  critical path.
• The difference between the early times and late
  times is float

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ACTIVITY IDENTIFICATION
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Activity Definition

• A discrete part of the process with the following
  characteristics
• Usually consumes time
• Usually consumes resources
• Has definable start and finish
• Are assignable to a single supervisor
• Are measurable

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Activity Categories

• Administration
• Mobilization / Demobilization
• Procurement
• Construction
• Start-up and Testing

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Data for Activity Identification

• Plans and specs
• Quantity take-off
• Estimate
• Contract
• Pre-construction meetings

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Criteria for Sub Division of the Work

• What work has to be done?
• Where will it be done?
• Who will be responsible?
• When will it be performed?
• How will you do it?

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Level of Detail

• Purpose
• Complexity of Work
• Time frame
• Owners requirements
• Draws based on completed activities?

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Activity Description

• Should stand alone
• Clear
• Concise
• Verb (location) Object i.e. Paint west wall

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Work Breakdown Structure

• Hierarchical organization
• Allows consolidation at various levels
• Outline format

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Estimating Durations
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Time Unit

• Years, Months, Weeks, Days, Hours
• Use whole numbers and be consistent

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Determining Duration

• Duration Work Quantity/ Production Rate
• Work Quantity - The amount of work to be done
  expressed in units that can be measured in the
  field to report progress.
• Production Rate - The amount of work (same units
  as work quantity) that a certain crew averages
  per unit of time.

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Resource Availability

• Assume material will be available
• Assume equipment will be available
• Choose most efficient crew mix
• Avoid planned overtime

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Factors Affecting Productivity

• Nature of the work
• Labor and equipment productivity
• Management skill
• Material and equipment availability
• Seasonal conditions
• Work restrictions
• Quality of work
• Concurrent activities

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Sources of Quantity Productivity Data

• QTO
• Estimate
• Experience
• Outside information
• Estimating guides (Walkers, Means)
• Industry and trade organizations
• Manufacturers, fabricators, and suppliers
• Specialty Contractors

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Bar Charts

• Also called Gantt Charts

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Advantages

• Traditional Method
• Easy to construct and read
• No computer needed
• Easily produced by all scheduling software

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Disadvantages

• Difficult to depict precedence relationships
• Difficult to show effects of change

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Process

• Identify Activities
• Estimate Durations
• Develop Activity Sequence
• Construct Bar Chart

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Types of Activity Relationships

• Physical
• Safety
• Resource
• Preferential

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Additional Information

• Cash Flow
• Resource Loading

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Fenced Bar Charts

• Show relationships between activities
• Limited to a few activities

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ACTIVITY NETWORK DEVELOPMENT
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Network Models

• AON
• PDM
• AOA
• PERT

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Network Model Characteristics

• Discrete Activities
• Deterministic Durations
• Deterministic Logic
• Activity Flow

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Network Elements

• Activities
• Milestones
• Relationships
• Physical
• Safety
• Resource
• Preferential

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AON

• Unique Activity Number
• Unique Description
• Estimated Duration
• Relationships to other Activities
• Beginning and Ending Activities

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Logic
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Logic Errors

• Incorrect Logic
• Redundant Logic
• Logic Loops
• Hangers

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Activity Numbers

• Used to identify the activity
• Can have a structure used to categorize
  activities (WBS)
• Possible Categories
• CSI Division
• Responsibility
• Location
• Activity type (NIC, Change order)

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Other Codes

• Group and sort activities
• Coordination with estimating and accounting

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Network Schedule Calculations

• Early Start
• Early Finish
• Late Start
• Late Finish
• Total Float
• Free Float
• Critical Path

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Forward Pass

• Calculates the earliest each activity can start
  and finish
• Calculates the minimum overall duration of the
  project

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Backward Pass

• Calculates the latest that each activity can
  start and finish without delaying the project

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Total Float

• Late start minus early start or LF - LS
• The amount of time an activity can be delayed
  without delaying the project
• Assumes activity started on ES date

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Free Float

• The amount of time an activity can be delayed
  without delaying any other activity
• Assumes activity started on ES date

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Network Report

• Activity Description
• Activity Number
• Planned Duration
• Early Start
• Early Finish

• Late Start
• Late Finish
• Total Float
• Free Float

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Critical Path

• Longest path through network
• Establishes overall duration
• Continuous chain of activities with zero float

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Activity Start / Finish Conventions

• End-of-day
• Beginning-of-day
• Combination

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Project Calendar

• Assigns job days (ordinal days) to calendar days

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Calendar Information

• Project start date
• Rest days (week end)
• Holidays
• Work days
• Project calendar (for manual conversions)

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Multiple Calendars

• Normal (five, 8 hour days)
• Elapsed Time ( seven, 24 hour days)
• Four, 10 hour days

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Reports and Graphics
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Communication

• Who will use information?
• What format should be used?
• What information should be included?
• How should information be organized?

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Who is the Report for?

• Construction firms upper management
• Summary info on progress cost cash flow
• Owner and designer
• Summary info on progress cost cash flow
• Detailed info on key systems

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Who is the Report for?

• Project Management
• Detailed info on whole job for near term
• Resource information
• Summary info on progress cost

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Who is the Report for?

• Superintendents
• Detailed info on whole job for near term
• Resource information
• Might be better done with spreadsheet
• Foremen
• Detailed information on specific systems

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Format

• Tabular
• Narrative
• Calendar
• Bar Chart
• Network diagram
• Graphs

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What Information

• ONLY that which is needed
• Could a picture be worth 999 words?

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How Organized

• Sorts
• Activity Number
• Activity Codes
• Early Start
• Total Float
• Etc..

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Data Transfer

• Information can be organized for data transfer as
  well as for reporting information
• To from CAD
• To from estimating programs
• To from spreadsheets
• To from data bases
• To word processing

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Specifying Scheduling Requirements

• State format, content, frequency
• Limit requirements to only data needed
• Handouts

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Reviewing and Analyzing the Schedule
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Reviewing the Schedule

• Is the schedule complete?
• Are durations reasonable?
• Are relationships complete?
• Are the calendars correct?
• Does the scheduled finish meet contract
  requirements?
• Are all contractual requirements met?

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Analyzing the Schedule

• Is the critical path(s) reasonable?
• What activities are near critical?
• How does the work flow?
• Are there conflicts between concurrent
  activities?
• Are resources over or under allocated?
• Can subs and suppliers meet their scheduled
  dates?
• Are any activities scheduled out of season?

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Short Interval Scheduling
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Two general types of scheduling

• Overall Project (what we have been talking about)
• Day to day
• Look ahead scheduling
• Production scheduling
• Differentiated by whos doing it and why

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Look Ahead Scheduling

• Coordinate wide array of work over a short time
• Greater attention to balancing resources

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Production Scheduling

• Detailed schedule for a particular process
• Often uses a smaller time unit for durations
  (hour)
• Matrix schedules (ie. resource usage view in MP)

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Updating the Schedule
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Purpose Of Updating The Schedule

• Evaluate Project Status
• Predict Completion Date
• Create A Historical Record

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Baseline Schedule
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Frequency of Updates

• Project complexity
• When an unexpected event occurs
• When Specified

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Step By Step Procedure

• Gather activity status information
• Determine schedule status
• Analyze schedule status
• Modify and revise the schedule
• Analyze the revised schedule
• Implement the updated schedule

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Activity Status

• Actual start date
• Actual finish date
• Percent complete
• Work in place
• Time expended
• Cost incurred
• Resources used
• Remaining duration

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Gathering Status Info

• Direct observation
• Update meetings
• Field progress reports
• Turnaround reports
• Photographs videotapes

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Analysis of Updated Schedule

• Against what baseline is this schedule compared?
• What is the planned completion date?
• Has the critical path shifted?
• Where have the floats changed?
• How will weather affect future activities?
• Are there any trends apparent?
• Have there been any scope changes since last
  update?

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Modify Revise Schedule

• Revise the network logic
• Revise activity durations
• Adding or deleting activities

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Record Keeping

• Code notes and calculations to activities
• Keep both hard and electronic copies of each
  iteration of the schedule

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Using the Schedule to analyze the Impact of Change

• Isolate change
• Calculate schedule with without change

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Out of Sequence Work

• Only happens on non-physical logic relationships
• Two choices
• Preserve Schedule Logic (usually preferred)
• Eliminate Schedule Logic