Steps in Construction of FAST Diagrams

The following steps are recommended in the construction of the FAST diagram:

1. Function listing. Prepare a list of all functions, by assembly or by system, using the verb-and-noun technique of identification of function. Do this by brainstorming the questions (a) “What does it do?” and (b) “What must it do?”

2. The function worksheet. Using lined paper, prepare a three-column function work­sheet in the format shown in Fig. 10.2. Insert the listed functions from above, one at a time, into the central column. Then, ask of each function the following questions:

a. How do I (verb) (noun)? Record the answer(s) in the right column.

b. Why do I (verb) (noun)? Record the answer(s) in the left column.

FIGURE 10.2 Function worksheet for FAST.

3. The diagram layout. Next, write each function separately on a small card in verb – and-noun terminology. Select a card with the function that you consider to be the basic function. Determine the position of the next higher and lower function cards by answering the following logic questions:

a. Perform the “how” test by asking of any function the question, “How do I (verb) (noun)?” The function answer should lie to the immediate right. Every function that has a function to its immediate right should logically answer the “how” test. If it does not, either the function is improperly described or a function is in the wrong place.

b. The second test, “why,” works in the same way, but in the opposite direction. Ask the question “Why do I (verb) (noun)?” The answer should be in the function to the immediate left and should read, “So that I can (verb) (noun).” The answer must make sense and be logical.

4. The critical path. To determine whether a function belongs on the critical path, test the functions with these questions:

a. How is (verb) (noun) actually accomplished, or how is it proposed to be accom­plished?

b. Why must (verb) (noun) be performed?

5. The support logic block. A support logic block is a block immediately underneath a given block at the same general level of activity. This contains functions that “happen at the same time as” and/or “are caused by” some other function. They can be deter­mined by answering these questions:

a. When is (verb) (noun) performed?

b. If (verb) (noun) is performed, what else must also happen?

6. Locating the scope lines. In determining where to place the scope lines, the choice is arbitrary. Actually, moving the left scope line from left to right lowers the level of activity of the problem to be studied. The basic function to be studied shifts, since it is always the function that lies to the immediate right of the left scope line. Locating the right scope line determines the assumptions and “givens” one is willing to accept before starting the study. Location of both scope lines is also subject to the point of view of the owner or user of the problem.

See NCHRP Synthesis 352 for an example of a FAST diagram for a highway application.

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