Fundamentals of Scheduling

(CRITICAL PATH METHOD [CPMJ OF SCHEDULING)

CONTENTS

Objectives of CPM 33

Terms and Definitions 34

Arrow Versus Precedence Networks 35

Simple Network Format 35

Network Development Example 37

Logic Ties 38

Calculation Process for Early Start (ES) 39

Calculation Process for Late Finish (LF) 41

Tabulating Data — EF and LS 43

Float and Its Application 43

Total Float (Definition and Calculation) 44

Free Float (Definition and Calculation) 47

Conclusions Drawn from Network Analysis 48

Precedence Diagrams Terminology 48

Major Piping Modification Network 48

Revised Network Logic 50

Critical Path Evaluation 51

Forward Pass 52

a. Terminology b. Time Analysis

Float Calculations and the Critical Path 55

a. Float Terminology b. Network Float Calculations

Resources 55

a. Terminology b. Unscheduled Resources Requirements c. Scheduled Resource Requirements

Summary of Precedence Network Development 57

Schedule Development and Evaluation 57

a. General b. Personal Judgment c. Historical Data Base d. Construction Duration — Trapezoidal/Labor Density Method

Duration of Determination 60

a. Logical Critical Path b. Manning and Peak Labor c. Working Hours

Practical Scheduling — Basic Scope Appreciation 62 "Fast Track" Scheduling Relationship 62 Construction Duration Trapezoidal Technique 64 Worked Example — Trapezoidal Method 66 Sclieduling Systems (Cost Integration) 66 Typical Project Master Schedule 70 Summary Schedule 70

HISTORICAL BACKGROUND

CPM scheduling was developed in the late 1950's. It was introduced to the industry as a tool to improve planning and scheduling of construction programs. Concurrent with industrial development of CPM, the U.S. Navy introduced a similar method of scheduling called PERT. PERT is an acronym for Program Evaluation and Review Technique. The Navy developed this method to evaluate and monitor progress of the Polaris Missile Program. The major difference between CPM and PERT is that PERT is a more probabilistic approach that lends itself to activities for which there is little or no historical experience, whereas CPM uses historical information for establishing durations. Subsequent development led to a considerable amalgamation of the two methods.

It was not until 1967 that James Kelly developed the techniques of CPM as used today. He used digital computer techniques developed by Rand Corporation and applied them to a complex construction project for DuPont Corporation. This resulted in completion of a project well ahead of schedule.

OBJECTIVES OF CPM

Figure 3-1 lists the objectives of CPM scheduling. As seen from the figure, CPM can be used as a logic tool for decision-making. It provides a means for planning, scheduling, controlling and presenting alternate courses of action. It also provides a visual means of communication to Project Management and an organized approach to implement a schedule program. CPM scheduling can be carried out manually or with a computer program.

A major problem with the CPM computer programs can be the number of activities. Very large networks became the norm during the 1960's. Size, not quality, became a dominant factor and computer scheduling methods became more important than the scheduling program itself. Theory replaced practicality and, as a result, quality of scheduling deteriorated.

It was not until the mid-1970's that a proper balance of computer method and size of networks was achieved. Experience has shown than 10,000/20,000 activity networks are costly, unmanageable and inefficient. Careful prior evaluation of criticallity and networks with a maximum of 5,000 activities have proven effective.

Figure 3-1

Objectives of CPM

Plan

Communicate

• Schedule

• Organize

• Control

• Implement

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