1. INTRODUCTION
Mankind has learned the application of hollow tubular members as structural elements from nature. Many examples in nature show not only the use of a hollow cylinder to transmit a fluid, but also the excellent properties of the tubular shape with regard to loading in compression, torsion, and bending in all directions. These advantages were quickly understood by our ancestors, when in their hands the bamboo pole became a light building component as well as a pipe for the supply of drinking water or for irrigation.
During the development of steel production and the manufacture of classical hot-rolled open sections such as I-, L-, and U-profiles, the first methods for the fabrication of tubes or circular hollow sections were developed in the 19th century. The production of rectangular hollow sections was not started, however, until 1952 (by Stewarts & Lloyds in the United Kingdom).
Circular shaped tubes are made either from a solid lump of steel, producing seamless tubes, or from a flat strip, giving welded tubes. There is no fundamental difference between the production process for a circular section tube intended for use as a pipe and that for a similar hollow section intended for a structural use.
The so-called "form" tubes - square, rectangular, hexagonal, or octagonal - are obtained by deforming, either hot or cold, a round tube as a blank. The circular blank tube is passed through forming roll cages working continuously and outwards only. This process gives the blank, usually after passing over several sets of rolls, the required shape, which is normally square and rectangular.
The selection of a particular profile in a steel structure is governed by many factors. It involves a comparison of the pros and cons with regard to mechanical properties; unit material costs; and the costs of fabrication, erection, and maintenance. The experiences of architects, designers, and fabricators also affect the choice. It is therefore very important that those involved should understand the behaviour of hollow sections and their connections.
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