See attachment, this is what I mean by a discrete system with N particles :) The upper cantilever is a continuous structure, while the bottom cantiliver is a discrete system with N = 4 particles, in which the actual continuous stiffness is concentrated in those 4 nodes. If the number of N goes...
I always thought that the principle of virtual work (PVW) is valid for all structures, including continuous structures (like bars, beams, plates, etc.). However, in his book 'Fundamentals of Structural Mechanics', Hjelmstad states that the PVW is only valid for discrete systems with N particles...