By: Cordell Parvin
In several previous columns, I have advocated that
contractors and state DOTs use the linear scheduling method for
highway construction projects because it visually aids the
scheduler and the field personnel. As I said in my November 1991
column, "In preparing a linear schedule, the planner produces a
schematic diagram showing the location of the work and the
productivity of the crews. These features serve as an aid to the
visualization of the construction processes, sequences, and
resource allocation."
Because of the visual aspect of linear
scheduling, some contractors develop their plan by a linear
schedule and then input the data to develop a CPM (Critical Path
Method), schedule to meet the contract requirement. After the
CPM schedule is approved, these contractors use the data to
create a new linear schedule for use in the field.
In the
November 1991 column, I also noted that because of the growing
complexity of highway construction projects and the increasing
number of delay claims, several state DOTs had begun requiring
detailed CPM schedules for their projects, including specifying
the scheduling computer program, the maximum number of
activities, resource and cost loading requirements, and updating
requirements. Recently, I reviewed a contract with a scheduling
specification that was 23 pages long. One has to wonder when
reading such a specification, what the owner really hopes to
accomplish by it.
Reviewing the output CPM schedules is
similar to reading the Wall Street Journal. That is why it is so
difficult to get field superintendents and inspectors to use
them to build and monitor the construction of projects. Looking
at the output of a linear schedule is similar to reading USA
Today. It is a visual way of communicating the schedule.
A
linear schedule is a simple diagram showing location and time.
The movement of the crews through time and space (representing
productivity) is visually represented.
Preparation of a
linear schedule begins with the development of the "playing
field." The x-axis, or horizontal dimension, is used to measure
distance, usually by stations. The y-axis, or vertical
dimension, measures time, moving up the scale from earlier to
later dates. Enhancements can then be made to the playing field
by adding sight lines, the plan view or a profile of the
project.
Once the playing field has been established, the
planning process begins by tracking crews using three symbols:
bars, lines and blocks. A bar is a vertical line representing a
crew working in one place over a period of time. For example, a
bar would be used to represent a bridge crew. A line is used to
show a crew that moves through some or all of the project as
time progresses. A line would be used to represent a paving
crew.
A block is used to represent a crew which occupies
space on the project over a period of time. For example, a block
would be used to represent a grading crew that is grading an
area, but not necessarily progressing smoothly in one direction.
Recognizing that DOTs were likely to continue specifying CPM
schedules even on projects that did not justify their use, I
have been trying for several years to work with engineers to
develop software that will import CPM output and put it into a
linear schedule format for use in the field.
When I wrote
the 1991 column, Dr. Mike Vorster and his colleagues at Virginia
Tech had developed a computer interface between CPM and linear
scheduling. Essentially, the CPM schedule provided the activity
dates and information about the locations of the activities and
the type of activities and then the linear scheduling software
converted the CPM data into a linear schedule. Unfortunately,
the software developed by Virginia Tech ultimately had limited
usefulness.
Over the past 12 months, TransCon Consulting
Ltd., an engineering consulting firm I helped form, developed
more comprehensive linear scheduling software that will
interface with CPM schedules. In July, TransCon announced X
Position software to transfer data from a CPM schedule or
spreadsheet into a linear schedule format. Readers can preview
the software by downloading the preview information from the
company's website: http://www.transcon.net. I envision that this
software will help make CPM schedules, or schedules generally,
more useful in the field.
Parvin is a shareholder in the law
firm of Jenkins & Gilchrist, which has offices in Austin,
Dallas, Houston, and San Antonio, Texas, Washington, D.C., and
Los Angeles.
