C3.2 Method of Joints

Now comes the fun bit. We will now look at the actual forces acting within each truss member using a technique called the method of joints.

This method works by first considering each joint in the truss structure. We isolate the joint by drawing its FBD, and replace all truss members connected to the joint by the forces that they exert on the joint. The magnitude of the forces is then determined by applying the equations of equilibrium ([ΣF_x = 0] and [ΣF_y = 0]) on each joint.

A common confusion that students face is determining whether each member is in tension or compression. There are many ways to go about doing this, but the trick is to stick to only one method consistently.

The way I would recommend is to treat compression as a pushing force and tension as a pulling force:

• If the force exerted by the member on the joint is pointing inwards towards the joint, it is pushing and therefore the member is in compression.
• The opposite, where the force is pointing outwards away from the joint, signifies that the member is in tension.