Theory | C3.3 Method of Sections

This way, we can consider the FBD of this cut truss and apply the equations of equilibrium to obtain the force in member AE directly. Shortcut for the win!

For the method of sections we would usually need to apply [ΣM = 0] in addition to [ΣF_x = 0] and [ΣF_y = 0] that the method of joints uses.

The same principle of determining tension/compression applies:

force pushing into the structure is compression, and
force pulling away from the structure represents tension

Let’s look at an example now.

This way, we can consider the FBD of this cut truss and apply the equations of equilibrium to obtain the force in member AE directly. Shortcut for the win!

For the method of sections we would usually need to apply [ΣM = 0] in addition to [ΣF_x = 0] and [ΣF_y = 0] that the method of joints uses.

The same principle of determining tension/compression applies:

force pushing into the structure is compression, and
force pulling away from the structure represents tension

Let’s look at an example now.