Abstract: A stiff elongated hollow structural member has a double-walled structure in the shape of a tube made of an interior tube which has an annular cross-section and an exterior tube which has a different cross-section, in a concentric arrangement. The interior and exterior tubes bound a hollow space filled with an incompressible liquid. Responsive to pressure increases in the liquid, the exterior tube in the elastic material range is bendingly deformable in the direction of a ring-circular cross-section or to a ring-circular cross-section. This hollow structural member can be used as an adjustable stiffness torsion member such as a vehicle steering shaft. Another disclosed adjustable stiffness hollow structural member includes a flat plate facing a corrugated plate to form a hollow space for the incompressible liquid. Increases in pressure of the liquid forces the corrugated member to a more flattened configuration.