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.
