15.2 Project Schedules

LibraryVirginia Construction Law Deskbook (Virginia CLE) (2019 Ed.)

15.2 PROJECT SCHEDULES

15.201 Observations and Constraints. In their book, Construction Schedules, 1 Michael T. Callahan and H. Murray Hohns write:

The words "project schedule" have different meanings to the owner, contractor, subcontractor, design consultant, and most especially, to courts of law or other dispute resolution triers of fact involved in the construction industry. The project schedule may just mean the contractually stated final completion date and/or interim completion dates for phases of the work or it may refer to the process of sequencing and scheduling individual activities or tasks which are required to complete the project 2

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These authors also observe that construction project scheduling "is an attempt to look into the future, and any attempt to do that is subject to change and based on assumptions that do not play out in every case." 3 The uncertainty involved in construction scheduling is expressed in an often-quoted statement from Blake Construction Co. v. C.J. Coakley, Co.: 4

[E]xcept in the middle of a battlefield, nowhere must men coordinate the movement of other men and all materials in the midst of such chaos and with such limited certainty of present facts and future occurrences as in a huge construction project. . . . Even the most painstaking planning frequently turns out to be mere conjecture and accommodation to changes must be of the rough, quick and ad hoc sort, analogous to ever-changing commands on the battlefield. 5

15.202 Scheduling Techniques.

A. In General. In the construction industry today there are primarily two project scheduling techniques: the Bar Chart (or Gantt Chart) Technique 6 and the Critical Path Method (CPM).

Construction managers, general contractors, and trade contractors will often supplement their overall project schedule documents with short-interval (1-3 week durations) "look-ahead" schedules, which are employed at project site meetings to coordinate the trades and check progress. On a short-term basis, the look-ahead schedules will show from week to week the work accomplished or not accomplished, but will not show the effect of delayed work on the end date of the project.

B. Bar Chart Technique. A bar chart is a linear, horizontal graph portraying the major work activities that make up a project, the planned duration for each work activity, and the timing of each activity. The placement of the work activity bars on the horizontal timeline axis will show the planned sequence of the work activities but not the interrelationships among the work activities. For example, a bar chart can show the foundation

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work activity as a horizontal bar in time, and work activity on the walls atop the foundations as another horizontal bar. Because the foundation work activity is depicted as starting earlier in time, the bar chart shows the sequence between the foundations and walls, but not whether the completion, in part or whole, of the foundation work restrains or otherwise controls the start or completion of the wall work. We know that progress on the foundations will to some degree control the start and progress of the walls construction, but the bar chart, by itself, does not show the details of that relationship.

Another aspect of bar charts is that a work activity bar may represent a continuous day-for-day work schedule, or may simply show the start date and end date of that work activity without regard to whether the work will be a continuous, uninterrupted activity during that timeframe.

Bar charts do have advantages: they are easy to prepare, inexpensive, and readily understandable to jobsite personnel. Bar charts have been in use in the construction industry for many decades and likely will continue to be used as a scheduling technique.

C. Critical Path Method. The critical path method (CPM) of construction project scheduling is defined as a "method of planning and scheduling a construction project where activities are arranged based on activity relationships and network calculations determine when activities can be performed and the critical path of the project." 7 In George Sollitt Construction Co. v. United States, 8 the Court of Federal Claims described the critical path method and the "critical path":

Essentially, the critical path method is an efficient way of organizing and scheduling a complex project which consists of numerous interrelated separate small projects. Each subproject is identified and classified as to the duration and precedence of the work. . . . The data is then analyzed, usually by computer, to determine the most efficient schedule for the entire project. Many subprojects may be performed at any time within a given period without any effect on the completion of the entire project. However, some items of work are given

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no leeway and must be performed on schedule; otherwise, the entire project will be delayed. These latter items of work are on the "critical path." A delay, or acceleration, of work along the critical path will affect the entire project.9

There are many variations of CPM network schedule techniques. Three CPM network models typically used in the scheduling of construction projects are:

activity on arrow (AOA);
activity on node (AON); and
precedence diagramming method (PDM).

The use of the activity on arrow (AOA) technique seems to have diminished since 2015, but a description is included here because AOA still appears upon occasion. With the advent and popularity of the computer scheduling programs Primavera and Microsoft Project, both based on the precedence diagramming method (PDM), CPM network scheduling has evolved toward the PDM technique. The PDM technique is often the critical path scheduling method described but not identified by name in contract scheduling clauses and requirements.

1. Activity on Arrow (AOA) Method. The activity on arrow (AOA) network is now a less commonly used CPM scheduling method. This method is also referred to as arrow diagramming method (ADM) or "I-J."

In this method, an activity is represented by an arrow between two numbered nodes. Nodes represent the completion of one task and the start of another, and for identification are generally individually numbered. A simplified AOA network model is illustrated below:

In the diagram, the circles numbered 11, 12, 13, and 14 are nodes. The description below each arrow identifies the work task, such as obtaining a permit. The number above each arrow identifies the activity duration,

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usually in work days. In this network, the work activity "obtain permit" is a predecessor activity to the "sitework" work activity, and "sitework" is a successor activity to "obtain permit." In CPM scheduling, a predecessor activity is any activity that must be completed before a given activity can be started. A successor activity is any activity that cannot start until a given activity has been completed.

A major feature of the AOA diagram is the use of logic connectors called "dummies" or "restraints." A dummy or restraint is shown as a dashed arrow on the diagram. A dummy or restraint has no required time for performance; its function is to sequence particular work tasks or to indicate a restraint on the start or the progress of subsequent activities. An example of an AOA network with a restraint (dashed arrow) follows:

2. Activity on Node (AON) Method. In the activity on node (AON) method, work activities are generally represented by boxes, and arrows represent finish to start relationships between work activities. An AON network model is illustrated below:

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The activity identifiers are the numbers above the activity description and the activity durations are below.

3. Precedence Diagramming Method (PDM). The Precedence Diagramming Method (PDM) is an evolved version of the activity on node method. In the PDM method, as in the AON method, boxes represent the activities and arrows represent the relationships between activities. In contrast, however, to the finish to start relationships shown in the AON model, the precedence diagramming method is capable of showing overlapping relationships between activities. The PDM allows four types of logical relationships between activities to be shown in the network model:

Finish-to-Start (FS x): Successor activity can start x days after finish of predecessor activity;

Start-to-Start (SS x): Successor activity can start days after start of predecessor activity;

Finish-to-Finish (FF x): Successor activity can finish x days after finish of predecessor
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