In this method, after determining the reactions at the supports, the equilibrium of every is considered. This means the sum of all the vertical forces as well as horizontal forces acting on a is equal to zero. The ***** 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 to which it is connected whereas the force in the member will be tensile if the member pulls the 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 where only two members is unknown. 1- First select that on which three or less then three forces are acting. Then apply lami’ theorem on that ****.
Step-4: Draw free body diagram of selected since whole truss is in equilibrium therefore the selected 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 on which four forces, five forces etc are acting. On that apply resolution of forces method.
Note: If three forces act at a and two of them are along the same straight line, then for the equilibrium of the , the third force should be equal to zero.
