Bar (Gantt) Charts - Construction Project Scheduling and Control - page 16

DEFINITION AND INTRODUCTION
A bar chart is a graphical representation of project activities that are shown in time-
scaled bar lines with no links shown between the bars (activities) (Popescu and
Charoenngam, 1995, p. 96). PMI (PMI, 2013) defines it as “a graphic display of
schedule-related information. In the typical bar chart, schedule activities or work
breakdown structure components are listed down the left side of the chart, dates are
shown across the top, and activity durations are shown as date-placed horizontal bars.”
The bar chart was originally developed by Henry L. Gantt, an American mechan-
ical engineer, in 1917 and is alternatively called a Gantt chart . 1 It quickly became
popular—especially in the construction industry—because of its ability to graphically
represent a project's activities in a clear, simple, and time-scaled manner.
Before a bar chart can be constructed for a project, the project must be broken
into smaller, usually homogeneous components, each of which is called an activity
or a task . None of the methods is a uniquely “correct” way to break down a project
into activities, nor can we describe other ways as incorrect. However, the scheduler
should take a balanced approach and break the project down into a reasonable number
of activities that are easily measured and controlled without being overly detailed.
(Project breakdown is discussed further in Chapter 4.)
An activity, or a task, may be as large as laying the foundation of a building, as
small as erecting the formwork of one footing, or anywhere in between. The duration
of each activity must be estimated. Bars are then drawn to show each activity: the
duration and the starting and ending points. As mentioned previously, links between
activities are not usually shown.
On a bar chart, the bar may not indicate continuous work from the start of the
activity until its end. For example, the activity Getting a Building Permit may be
represented by a 2-month-long bar. However, most of this time is a waiting period.
Likewise, a Concrete Foundation summary activity may include several days of waiting
for the concrete to cure. Noncontinuous (dashed) bars are sometimes used to distin-
guish between real work (solid lines) and inactive periods (gaps between solid lines)
(Callahan, Quackenbush, and Rowings, 1992).
Bar charts have become a vehicle for representing many pieces of a project's infor-
mation. Many variations of bar charts have evolved; some simply show the start and the
end of each activity (Figures 2.1 and 2.2), some are loaded with resource or budget
numbers (Figures 2.3 and 2.4), and others compare the as-planned schedule with
1 The notion that bar charts and Gantt charts are two different types of charts is baseless. There is a variety of
bar chart types, and any of them can be called Gantt chart. Even though the PMI when defining bar charts
says “also known as Gantt charts,” it defines a Gantt chart as “a bar chart of schedule information where
activities are listed on the vertical axis, dates are shown on the horizontal axis, and activity durations are
shown as horizontal bars placed according to start and finish dates.” This is not much different from the bar
chart definition given previously.