in structural mechanics, a cylindrical or prismatic bar whose three dimensions (length and largest and smallest cross sections) are of different orders of magnitude. Thin-walled bars include most rolled sections of steel and aluminum, built-up and bent sections, and thin-walled elements of reinforced concrete, asbestos cement, plastic, and other materials.
Thin-walled bars may have an open or closed cross section. The first group includes I-beams, channels, and angles, and the second includes box girders. Thin-walled bars are used in construction, machine building, and aircraft construction.
Unlike ordinary, solid bars, thin-walled bars do not retain their plane cross sections upon deformation. This rules out the application of planar cross-section theory in design calculations. In exact calculations, thin-walled bars are regarded as shells. In practice, however, the calculations are usually based on the assumption that the cross section is nondeformable and that there are no displacements in the median surface of the bar.