ES120 Trusses  Method of Joints 
A truss is defined by these criteria: 

A simple truss is shown to the left. The method of joints is used to find forces in all members and to find all reactions. A step by step procedure will be outlined for the structure shown. 
1. FIND AS MANY REACTIONS AS POSSIBLE The pinned connected at A has two reactions, A_{x} and A_{y}, and the roller at C has only a vertical reaction. Taking moments about A yields F_{c}, and SF_{x} and SF_{x} yield A_{x} and A_{y} respectively. 

2. CHOOSE A JOINT
(preferably one with only two unknowns, since the moment equation is identically satisfied at each joint), DRAW A FBD AND SOLVE FOR UNKNOWNS. If we choose joint A, the following FBD results: Note: The forces are along members. has been assumed to in compression. (A wrong assumption will cause a negative answer for a force.) and has been assumed to be in tension. 
 The forces may be solved for. 
Lengths a and c are given in the original diagram. 
 A_{x} and A_{y} where found from the equations for the reactions, hence 
Another joint is now selected and the procedure repeated.
Note: If AB was in tension as found at A it will also be in tension at point B. 
The complete procedure is shown diagrammatically here... 
 A tabular method for listing solutions as they are found is suggested for convenience of bookkeeping. 
Note: A considerable amount of time may be saved in the case of a geometrically symmetric truss, symmetrically loaded. Members and reactions on either side of the line of symmetry will have identical forces. 