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Guidelines for constructing Free Body Diagrams (FBD's)

Key Concept: Free body diagrams are the starting point in the analysis of all equilibrium

applications. FBD's show the external force and/or moments acting on the isolated body or

bodies that make up the application you are analyzing. It is up to you to determine which

body or connected bodies will be used in each analysis.

Guidelines

Identify the type of supports, the restraints at each support, the restraint forces provided by each support, and the restraint moments provided by each support.

Identify the number of distinct bodies forming the structure to be analyzed.

It may be that you will have a FBD for each individual body, a FBD for a

combination of connected bodies, and a FBD for the entire system of bodies.

Your decision of the number and types of FBD's rests on the unknown forces

and/or moments you are asked to find. So you need to carefully examine the

best combination of FBD's that provide the equilibrium equations used to solve

for the desired unknowns.

One objective is to identify the fewest number of FBD's, one for each isolated

body or combination of bodies, that will most quickly allow you to solve for

the desired unknowns using the equilibrium equations.

Once you have completed the first four steps detailed above, write and solve

the equations of equilibrium for the desired unknowns. At each stage, it's a

good idea to check that your equations are dimensionally correct by using

symbols rather than specific numbers as you proceed with your solution of

the equations of equilibrium.

Finally, take a look at your results. Do they seem reasonable? You may also

check for special cases where perhaps you set one or more of the applied forces

to zero. Does your result seem reasonable in these cases? As an engineer you always need to review your calculations and results to validate they yield

reasonable results.

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