ABSTRACT
Cold-formed structural hollow sections are most commonly manufactured internationally by a “continuous forming” or “round-to-square” method, whereby steel plate or coil strip is progressively bent into a circular shape by passage through a series of pressure rolls, then welded into a tube by means of an induction heating process. Further shaping of the tube into square or Rectangular Hollow Sections (RHS) is then achieved by additional flattening and cold-working, using still more rolls. The final square or rectangular hollow section contains high levels of residual stress throughout its entire cross-section.An alternative production process for cold-formed square and rectangular hollow sections, practiced in some parts of the world, is the so-called “direct forming”method,whereby steel plate is bent only at four discrete points to form the corners of the section and the shape is again closed by a continuous longitudinal seam weld. The cold-working in this case is concentrated at the four corners.
