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Scheduling systems and their selection

Scheduling systems

Now we are ready to discuss the various methods for scheduling capital construction

projects that are available. The two basic methods that will be discussed are bar charts and

logic-diagram-based schedules. Both methods are used extensively, and sometimes

interchangeably, in project and construction work. Each method has its advantages and

disadvantages. Knowing when to select the correct method is half the battle in successfully

making and controlling your project schedule.

Bat charts

The forerunner to the bar chart was developed by two industrial engineers, Frederick

W. Taylor and Henry L. Gantt, for scheduling production operations during World War I.

the name “Gantt chart” is still in use today to designate certain types of bar charts. It was

sometime after World War I that bar-charting was adapted to the scheduling of construction

projects.

Bar charts are the simplest from of scheduling and have been in use the longest of any

of the systems we have available. They offer the advantage of being cheap and simple to

prepare; they are easy to read and update, and they are readily understood by anyone with a

basic knowledge of the capital projects business. They are still in wide use today, even as a

final product of the computerized CPM scheduling system. The main disadvantage of the

bar chart is its inability to show enough detail to cover all the activities on larger, complex

projects. On large projects, the number of pages required to bar- chart the project becomes

cumbersome, and interrelation of work activities becomes difficult to follow from page to

page.

As the size and complexity of projects grew in the late fifties and sixties in the 20th

century, finishing projects late became the rule rather than the exception. Late finishes,

along with their associated cost overruns, caused increased pressure on owners and

contractors to develop improved scheduling techniques. Now when we try to schedule a

larger project in that sort of detail with bar charts, we quickly lose most of the advantages

that we listed earlier. The schedule becomes unwieldy and difficult to interpret, and we run

the risk of losing control of the project time plan.

Logic- based schedules

Fortunately, on the same time, the network schedule and the computer came on the

- - 2 capital projects scene. We now had a tool available to make the many repetitive

calculations for the earl and late start dates, and a place to store and sort the data needed to

control a large number of work activities.

In the late 1950s the U.S. Navy and the Du Pont Company concurrently developed

two different logic-diagram-based scheduling systems at about the same time. The Naty’s

system was called PERT, for Program Evaluation and Review Technique. Its first

successful application was on the Polaris Missile Program. At about the same time, Du

Pont first successfully used their critical path method (CPM) of logic diagram scheduling

on several new petrochemical plants.

Other owners and contractors lost no time in adapting the new scheduling methods to

their projects in order to improve their timely completion performance. The CPM system

was somewhat simpler than the PERT method, so it soon became the system favored for

use on commercial and industrial capital projects. The KISS principle triumphed again!

The basic logic-diagraming principles developed in the 1960s are still and graphical output.

In the 1980s, the development of the relatively low-cost PC made the use of the CPM

system possible for even the smallest companies.

Shortly after the introduction of the PERT/CPM systems in the early sixties, the

pendulum swung from simple bar charting to the side of overly detailed, computerized

schedules. That didn’t work out as well as the early success with the systems had seemed

to indicate it would. If a little bit of CPM was good, more had to be better! Everyone

promptly defied the kiss principle and started to schedule in too much detail on each

activity. The result was reams and reams of computer output that virtually inundated many

untrained people.

Most of the construction managers and field schedulers of that period were

entrepreneurial craft people who had worked themselves up through the ranks. In many

cases they were literally untrainable in the new technology of computerized CPM