LESSON 5: Method of Joints


In using the method of joints you should begin by analyzing the truss as a whole. After analyzing the truss as a single structure, break each member and analyze each joint individually. The joints of the truss are the areas where the members meet. If given a truss that looks like this:


The proper way to break this truss down using the method of joints would look like this:



Notice how each member, or bar, within the truss has been broken and essentially turned into a vector that is pulling its respective joints. It is standard to have the vectors that represent each member of the truss to point outwards from the joint. This is because we assume that each member of the truss is in tension, meaning that each member is being stretched. Most metals naturally resist stretching, so if a bar is being stretched, it wants to return to its original unstretched state, so the bar is pulling the joints toward each other in order to return to its unstretched state. Pretend you are holding a rubber band and stretching it between your two hands. The rubber band is in tension, and it wants to pull your two hands toward each other. For trusses, the members are the rubber band, and the joints are your hands.

Next, analyze each joint individually. Take the sum of the forces in the x and y directions for each joint individually. Start with the simpler joints that you know the most information about and work your way around the truss. After analyzing each joint, you should be able to find the force of each member. If you get a negative number this means that the member with the negative value is in compression rather than in tension.

A link to an example problem is included here: click to view problem Hopefully this example problem will help clarify the process.