Effect of flattening circular hollow sections in truss and dome structures

Circular hollow steel sections are normally specified in truss and dome structures to take advantage of their efficiency in compression and the ease in which they can be curved to match the various radii of different domes. However, it is not easy to connect these members together or to other members. In order to connect them together or to other members the ends are sometimes squashed or flattened. Although the process of flattening does not reduce the area of the section, it does reduce the flexural stiffness of the section. The aim of this paper is to study the behaviour of circular hollow members with flattened edges, in compression. Variables in the tests include the diameter, thickness and length of the sections, and number of bolts in the connection. Two failure modes were observed and these are overall flexural buckling (OFB) of the member and excessive deformation of the transition zone (DTZ). The results from these tests are compared with the flexural compressive resistance formula in the South African steel code (SANS10162-1) and the European steel code (EN 1993-1-1), with a view of determining a simple formula for designing such members.