A hinge assembly for pivotally connecting a first frame section and a second frame section includes at least one pivot plate pivotally connected to the first mounting structure of the first frame section with a first pin and pivotally connected to the first mounting structure of the second frame section with a second pin. The hinge assembly further includes a linkage bar with a first end pivotally connected to the second mounting structure of the first frame section with a third pin and a second end pivotally connected to the second mounting structure of the second frame section with a fourth pin. The pivot plate and the linkage bar are configured such that the second frame section pivots about the fourth pin during a working state and about the third pin during a transition between the working state and a folding state.
