A hinge assembly is provided for pivotally connecting a first frame section (140, 150) and a second frame section (160, 170). The hinge assembly includes at least one pivot plate (260) pivotally connected to the first mounting structure (212) of the first frame section (140, 150) with a first pin (300) and pivotally connected to the first mounting structure (232) of the second frame section (160, 170) with a second pin (310). The hinge assembly further includes a linkage bar (280) with a first end (282) pivotally connected to the second mounting structure (214) of the first frame section (140, 150) with a third pin (320) and a second end (284) pivotally connected to the second mounting structure (237) of the second frame section (160, 170) with a fourth pin (330). The pivot plate (260) and the linkage rod (280) are configured such that the second frame section (160, 170) pivots about the fourth pin (330) during a working state and about the third pin (320) during a transition between the working state and a folding state.
