A panel structure for a vehicle, and especially for an aircraft or spacecraft, includes an area member, especially a skin member, that defines an areal expanse with a first surface and an opposite second surface and having a thickness between the first and second surfaces; and a plurality of elongate stiffener members which are attached to the area member and extend over at least one of the first and second surfaces; wherein at least one of the stiffener members is bifurcated at a bifurcation point into two or more branch stiffener members.