Precedence Networks - Construction Project Scheduling and Control - page 92

DEFINITION AND INTRODUCTION
Precedence networks, also known as Precedence Diagramming Method , was originally
developed by Stanford University professor Johan Fondhal (1924-2008) in the
early 1960s. Precedence networks are node networks that allow for the use of four
types of relationships: finish to start (FS) , start to start (SS) , finish to finish
(FF) ,and start to finish (SF) . To understand the idea of precedence networks,
consider the simple project of laying 1,000 LF (linear feet) of a utility pipe. The
logic is simple: dig a trench, provide a 6-inch-thick gravel subbase (bedding), lay the
pipe, backfill, and compact—five consecutive activities. However, are they actually
“consecutive?” Do you need to finish excavating the entire 1,000 LF before you can
start the subbase? Do you need to finish the subbase completely before you start
laying the pipe? If the answer is yes, your bar chart may look like the one shown
in Figure 5.1.
Most likely, though, the answer to the preceding questions is no. Practically, once
you have dug a reasonable amount, say 100 LF, your second crew can start provid-
ing the subbase, while the first crew continues digging. Once digging reaches about
200 LF (say 20%), the subbase is about 10% complete, and your third crew can start
laying the pipe. As a result, the bar chart for the project looks like the one shown in
Figure 5.2. Clearly, relationships exist among these activities, but not the traditional FS
relationship. The activities are said to be overlapping . This project may be represented
Figure 5.1 Bar chart for five consecutive activities
Figure 5.2 Bar chart for five overlapping activities