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Spring 2013-Lecture 02-CPM Time-Scaled Network
Citation preview
2/1/2013
1
CIVE 585, CIVE 685, ENMG 698 Construction Planning & Scheduling
Instructor: Farook Hamzeh, Ph.D.
2- Critical Path Method
Overview
CPM History
CPM Methodology
Network calculations
Forward Pass
Backward Pass
Float
Time-Scaled Network Diagrams
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CPM History
The Critical Path Method (CPM) emerged in the mid-1950s.
US Navy- Polaris Missile System
DuPont & Remington Rand
Benefits of Scheduling & Evaluation
Without some kind of periodic and consistent measurement system--it's close to impossible to make critical judgments with confidence that the facts are in hand.
Better decision making
Better resource management
Better time management
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Managing Small Projects And Why Scheduling Still Matters
Multiple projects to manage
Higher risk
Limited resources
Short duration
Overtime?
Activity Priority
Requested Project Duration
Estimated Project Duration
Q: How long will it take?
We often neglect consideration of such factors as:
Mobilization
Paper Work
Material and Equipment Deliveries
Subcontractors Availability
A: We can handle it Attitude:
We typically respond with the time we think
it would take us to do the work, when
actually it will be done by others.
The Most Common Q&A:
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The Benefits: Decision Making
The Benefits: Decision Making
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The Benefits: Resource Management
The Benefits: Multi Project Management
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Advantages
Shows the Construction Logic Identifies the Critical Activities (80 20 Rule) Allows Management to set Priorities Shows the Effects of a Change Order or Delay Is Self-adjusting when updated Easy to Follow Visually See the Big Picture Potentially it is the most effective scheduling system Helps to Analyze Different Methods or Sequences Useful for Court Cases Keeps You Out of Court Serves as a Basis for Systems Control Forces the Management Team to Plan and Think in Detail Communicates the Process to Every Team Member Promotes Teamwork
Disadvantages
Can Become Complex - It is the Most Sophisticated Scheduling System
Requires Training to Become an Effective User
Due to Not Fully Understanding CPM and Its Complexity, It is Often Abandoned
88
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Network
Activity on Node (AON)
Activity A
Activity
B
Relationship
Successor Predecessor
Critical Path Method (CPM) Provides a timetable for performing the tasks which comprise
a project
Acts as a tool for examining activity relationships and resource utilization
Supplies data for reports and graphics that aid in decision making
The longest continuous path of activities through a project determines the project end date.
A delay in a critical activity delays other activities and the project as a whole
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A CPM Network
Name
ES EF
LS LF
TF FF
Duration
(TF, FF, IF, INDF)
Construction Flow Chart
Shows Preceding Activities
Shows Succeeding Activities
Shows Concurrent Activities
Shows the Critical Path Activities
Shows the Amount of Float
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How to Create a CPM Schedule
Review the Plans and Specifications
Get the Project into Your Head
Brain Storm the Project with the Project Team
Make Sure the Project Manager and Supervisors are involved Check with the Estimator
Discuss the Overall Job Approach
Discuss Problem Areas Anticipated
Any Special Conditions or Features?
Steps to Create A CPM Schedule
1) Breakdown the project into activities
2) Construct the Logic Diagram
3) Add the Durations
4) Calculate the Early Dates (forward pass)
5) Calculate the Late Dates (backward pass)
6) Calculate the Total Float
7) Mark the Critical Path
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Break the Project into Activities
Could use a Work Breakdown Structure (WBS)
Keep trades and contractual divisions separate
May even want to separate crews
Keep field work separate from shop or office work
Determine the level of Detail
Include submittal process?
Include procurement of material?
Include curing time for concrete?
Break the Project into Activities
Could Use a Work Breakdown Structure
WBS example:
1. Preconstruction Phase
1.1 Obtain Permits
1.2 Obtain Bonds
1.3 Obtain Insurance
2. Engineering Phase
2.1 Shop drawings
2.2 Review shop drawings
2.3 Fabrication
3. Job Mobilization Phase
3.1 Set up field office
3.1.1Locate Job Office Trailer
3.1.2 Construct Project Bulletin Board
3.1.3 Construct Deck and Roof Cover
3.2 Install temporary Power
3.3 Set up security Fence
3.4 Telephone Hookup
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Make Sure the Logic is Correct
Review Logic Diagram with ALL Team Members
Ask question #1 of every activity - Does this activity really have to be done before each of the following activities can start?
Ask question #2 What other activities need to be done before this activity can start?
How Do You Determine the Durations of the Scheduled Activities?
From Subs
Estimating or Productivity Manuals
Company Records of Productivity Rates
Flat out Guess
What was it we were trying to manage?
Add the Durations to the Logic Diagram
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Exercise
Activity
Immediate
Predecessor Duration
A H 2
B C 4
C H 1
D A,C 3
E A 1
F B 4
G E,D 2
H ~ 0
I F,G 0
Activity On Node Notation (typical)
ES = Early Start
EF = Early Finish
LS = Late Start
LF = Late Finish
TF = Total Float
FF = Free Float
Name
ES EF
LS LF
TF FF
Duration
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CPM -Definitions
Activity: The performance of a task required to complete the project, such as, design of foundations, review of design, procure steel contracts, or form concrete columns. An activity requires time, cost, or both time and cost.
Duration (D): estimated time required to perform an activity.
Early Start (ES): earliest time an activity can be started. Early Finish (EF): earliest time an activity can be finished.
Equal to the early start plus the duration. EF=ES+D Late Finish (LF): latest time an activity can be finished. Late Start (LS): latest time an activity can be started without
delaying the projects completion date. LS=LF-D
Early Start Calculation (Forward Pass)
ES = MAX {EFpredecessors}
A
B
C
D
EF=2
EF=6
EF=4
ES= 6
Name
ES EF
LS LF
TF FF
Duration
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Early Finish Calculation (Forward Pass)
EF = ES + Activity Duration
Framing
ES = 3 EF =
LS LF
TF FF
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9
Late Finish Calculation (Backward Pass)
Successors
B
C
D 7
LF=
10
7
LS=
LS=
LF = MINIMUM {LSsuccessors}
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Late Start Calculation (Backward Pass)
Set Anchor Bolts
ES EF
4
LS= LF=16 12
LS = LF - Activity Duration
Total Float (TF): the amount of time an activity may be delayed without delaying the projects completion date.
TF=LF-EF=LS-ES
Free Float (FF): amount of time an activity may be delayed without delaying the ES time of the following activity or the projects completion date .
EFi=ESj-EFi,where the subscript i represents the preceding activity and the subscript j represents the following activity.
Critical Path: a series of interconnected activities through the network diagram, with each activity having zero, free and total float time. The critical path determines the minimum time to complete the project.
CPM -Definitions
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Total Float
0
LF EF ES
TF
TF = LS ES TF = LF - EF
LS
3 7 10
TF
Total Float
TF = LS ES
TF = LF EF
Activity
3
5
6
8 14
9
TF =
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Free Float
Minimum ES of all Successors
ES
FF
EF
FF = MIN {ES successors} EF activity
Free Float
FF = MIN {ES successors} EF activity
B
C
D
EF= 1
10
11
12
FF = Min{11,12} 10 = 1
FF=
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Time-scaled Network
From: Verzuh, E., (1999). The Fast Forward MBA in Project Management. John Wiley & Sons, Inc., New York, NY.
Types of Float
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Calculating Interfering & Independent Float
Managing the Float
Which Type of Float Would You Share with Subs?
Using Float to Hide Days in the Schedule
Sharing Shared Float in Order to Make the Schedule More Manageable
Use the Float to Manage Resources