In this method, after determining the reactions at the supports, the equilibrium of every joint is considered. This means the sum of all the vertical forces as well as horizontal forces acting on a joint is equal to zero. The joint should be selected in such a way that at any time there are only two members, in which the forces are unknown.
The force in the member will be compressive if the member pushes the joint to which it is connected whereas the force in the member will be tensile if the member pulls the joint to which it is connected.
Steps for Method of Joint
To find out force in member of the truss by this method, following three Steps are followed.
Step-1: Calculate reaction at the support.
Step-2: Make the direction of force in the entire member; you make the entire member as tensile. If on solving the problems, any value of force comes to negative that means the assumed direction is wrong, and that force is compressive.
Step-3: Select a joint where only two members is unknown. 1- First select that joint on which three or less then three forces are acting. Then apply lami’s theorem on that joint.
Step-4: Draw free body diagram of selected joint since whole truss is in equilibrium therefore the selected joint will be in equilibrium and it must satisfy the equilibrium conditions of coplanar concurrent force system. ∑V = 0 and ∑H = 0
Step-5: Now select that joint on which four forces, five forces etc are acting. On that joint apply resolution of forces method.
Note: If three forces act at a joint and two of them are along the same straight line, then for the equilibrium of the joint, the third force should be equal to zero.
