Stability Of Trusses
A stable truss is the one in which when loads are applied, truss maintains its position and configuration and when loads are removed it comes back to original position.
Unstable truss members undergoes rigid body motion and when loads are removed they dont come back to their original position. Furthermore it does not satisfy equilibrium position in each loading case.
How to Inspect Stability?
A stable truss must satisfy all three equilibrium conditions. In order to determine weather a structure is stable or not we must check all equilibrium conditions for possible loading cases. Conversely if a single equation is not satisfied we can conclude that truss is unstable.
This video tutorial by Dr. Structure will give a detail overview of stability and determinacy of trusses