How to Calculate the Forces of Planar Trusses

Joints Method

• 1

  Assume that members of the truss are in a tension reaction. In a tension reaction, forces are experienced at pull forces at both ends of the bar and the force is denoted as a positive reaction.

• 2

  Make a virtual cut around a joint. Isolate the cut portion as a free body diagram. Analyze the joint using the following equations: Sum of F subset x = 0 and Sum of F subset y = 0.

• 3

  Repeat Step 2 for each joint in the truss. Work in order from those joints with the least number of unknown forces to those with the most. Tabulate all of the forces.

Sections Method

• 4

  Use the sections method to calculate the force in the section of the truss with the greatest force, and use this as an estimate of the force on the entire truss. This is a shortcut method that can be used when you need only a rough calculation instead of an exact number.

• 5

  Draw an imaginary cutting line called a section line through the truss that separates the truss into two separate parts. Calculate the force in the section of truss that experiences the greatest force.

• 6

  Determine the force by solving three equations of equilibrium. Solve Sum of F subset x = 0, Sum of F subset y = 0 and Sum of M = 0. In this case, "M = 0" is solving the member force at a joint where two members intersect.