The combination with a spring assembly of pairs of structures designed to be applied transversely to the top and bottom supporting surfaces at one or more longitudinally spaced intervals to enhance the resistance to sinking of the supporting surfaces, bowing at the longitudinal sides and collapse between the surfaces at the longitudinal sides comprising pairs of elongate parts which extend transversely across the assembly at the top and bottom sides from one longitudinal side to the other, said parts being transversely inextensible and resistant to flexing and at one end thereof a polygonal structure disposed between the border wires in the plane of the sides, two sides of which, the top and bottom, are parallel to the border wires and the other two sides of which embody oppositely disposed spring arms.