CONSTRUCTION SCHEDULING: PROJECT MANAGEMENT AND DISPUTE RESOLUTION TOOLS, CHAPTER 1

The time available to engineer, design, procure, construct, start-up and turnover a facility determines when performance is required of the various project parties. Time is of the essence in many contracts and delays to the scheduled time of performance may cause financial impact to one or more of the parties involved. Thus, it is understood that time is important to all parties working on a construction project. To guide the time-related performance of the parties and mitigate the effect of delays on project progress, planning and scheduling has been used as a management tool for decades in the construction industry.

One of the primary purposes of planning and scheduling is to establish the sequence, duration and time of performance required to complete a project within a specified time frame. The various planning and scheduling methods that have been developed and implemented to achieve this purpose range from simple hand-drawn bar chart schedules with a dozen or so activities to complex and detailed computerized schedules with thousands of activities. The sophistication of planning and scheduling methods used by the construction industry has increased in recent years and computerized scheduling methods using CPM are now widely used and accepted in most industries.

CONSTRUCTION SCHEDULING: PROJECT MANAGEMENT AND DISPUTE RESOLUTION TOOLS

Introduction

Critical Path Method (CPM) scheduling methods have been used for planning, scheduling and monitoring of capital projects for over thirty years. CPM schedules are also applied extensively in retrospective claims analyses to demonstrate the causes of and responsibilities for schedule delay, disruption or acceleration. The purpose of this chapter is to present an overview of CPM scheduling concepts and fundamentals. The subjects of contract scheduling requirements and provisions, CPM methods most commonly used for the preparation of contemporaneous time extension requests associated with change orders or for retrospective delay analysis associated with claims, and case law affecting the preparation of change orders, schedule analyses and claims, will be discussed in future chapters.

Bar Chart Schedules can be very helpful in construction dispute resolution..

Prior to the advent of the CPM scheduling methods developed in the 1950s, the most common method used for planning and scheduling a construction project was the bar chart, which was developed in the early 1900s by Henry Gantt and Frederick Taylor. Hence, schedule bar charts are frequently referred to as "Gantt" charts. The bar chart method presents work activities as bars on a horizontal time scale. Work activities are listed vertically on the bar chart and the bar for each activity is drawn to graphically illustrate the time frame required to complete each activity. Bar charts are usually kept rather simple and uncomplicated, but can become very sophisticated by adding labor resource requirements and distributing budgeted costs or manhours to the various bars or activities. Predecessor and successor relationships can be added between activities, and an "S" curve may be used to illustrate the cumulative budgeted man-hour and percent complete projections over the duration of the project. A "bell curve" may be used to illustrate the schedule for daily or weekly manpower requirements.

The demonstration of physical project progress using a bar chart is normally simple and straightforward. A progress line indicating "time now" is drawn or overlaid on the bar chart and activity bars are filled in to represent the physical progress and completion status of activities. The physical representation of this progress in comparison with the progress time line identifies whether an activity is ahead, behind or on schedule. The bar chart can also be revised to reflect changes in activity sequencing and durations that vary from the original plan.

Although the bar chart has been used for many years and is still in use today, the method has fundamental weaknesses which limit their usefulness in both planning and scheduling large, complex projects and retrospective analyses. For example, bar charts cannot accurately distribute or control manpower and project cost. Data entered into a bar chart does not lend itself to computer analysis of scheduled starts and finishes, and is usually limited to the visual depiction of activities which, on a large scale, may be fine for determining completion dates, but is often inadequate for controlling the project or measuring performance.

A bar chart typically does not show the amount of float for an activity, which is the amount of time an activity or chain of activities may "slip" before downstream activities or the project completion date are impacted. The amount of slack or potential slippage is referred to as "float". A bar chart also does not usually display the interrelationships between activities necessary to accurately plan a project or measure the performance of the parties. The absence of activity logic and available float makes it difficult to assess the delay impact of one activity upon another. Also, because bar charts are prepared manually, the time and effort required to prepare and maintain bar charts for large, complex projects can be quite burdensome. Finally, detailed bar charts may be difficult to read, understand and maintain. For these reasons, bar charts have limited value and are most often used by field personnel as short-term planning tools.

Although the bar chart method contains inherent disadvantages, it was used for many years simply because other planning and scheduling methods had not been developed. As the size, complexity and cost of projects grew, the need for more sophisticated planning and scheduling methods grew. As a result, the Program Evaluation and Review Technique, or PERT, was a statistical planning and scheduling method developed for the U.S. Navy in the mid-1950s. PERT was used most effectively on research and development projects, but did not enjoy wide use in the construction industry.

The development of CPM planning and scheduling methods and techniques, which are used extensively today in both project management and disputes, began in the mid to late 1950s, when the birth of the computer age opened the door for the development of CPM scheduling. By the late 1950s, E. I. duPont de Nemours and Company had developed the fundamentals of a critical path scheduling method to control projects through the use of computers. This new management tool was developed to catalog and present the essential information necessary to plan, schedule and control duPont's construction and facility maintenance projects
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Arrow Diagram Method

As originally developed, the foundation of CPM was the Arrow Diagram Method (ADM). The arrow diagram is a model that depicts an arrangement of activities in a logical sequence of performance. Each arrow in the diagram is a separate activity that represents an item of work in the project. The tail of the arrow is the start of the activity and the head of the arrow is the finish of the activity. The arrow diagram identifies the activities that must be completed before a given activity can start and which activities can be performed during the same time period, or concurrently. An arrow diagram can be drawn with or without a time scale.

In ADM, the point at which two or more activities converge is designated as an event or node. All activity arrows leading into an event or node must be completed before any activities leading out of the event or node can be started. To facilitate the unique identification of activities in an arrow diagram, the events or nodes are numbered. An activity's identification is, therefore, comprised of two numbers - the starting node number and the ending node number. As the method of arrow diagramming evolved, the starting node was referred to as the "i" node and the ending node was referred to as the "j" node. This node numbering approach used with arrow diagrams is now commonly referred to as the "i-j" method.

I-J Networks

The development of the arrow diagramming, or i-j scheduling method, overcame the basic weaknesses of bar charts; that is, the logical interrelationships between activities are shown, the earliest and latest start and finish dates for each activity are calculated (through mathematical techniques called the "forward" and "backward" pass), and the amount of slip or float available for activities is determined. The "float" for an activity is calculated as the difference between either the early start and late start or early finish and late finish. The term "critical path" is defined as the longest path through a network. The early and late start dates and early and late finish dates for any single activity on the critical path are the same. The float on critical path activities is calculated to be zero and any delays to the critical path would theoretically extend the projected completion date. The concepts of critical path and float are the heart of the CPM scheduling method.

Further development of CPM scheduling methods in the 1960s produced the Precedence Diagram Method, or PDM. As previously stated, arrow diagrams or i-j networks are referred to as "activity on arrow" schedules because the activity is designated as the arrow between two nodes. On the other hand, PDM schedules are referred to as "activity on node" schedules because activities are identified as nodes in the network.
There are distinct differences between ADM and PDM networks, primarily in the type and representation of logical interrelationships in the networks. To add necessary logic into an ADM network that cannot be accomplished solely through the use of defined work activities, "dummy" activities are used. Dummy activities are not used in PDM scheduling but are replaced by additional types of logical relationships.