Structural behaviour of T-joints between high strength S690 steel cold-formed circular hollow sections

This paper reports an experimental investigation into structural behaviour of T-joints between S690 cold-formed circular hollow sections (CFCHS). A total of twelve T-joints of both S355 and S690 CFCHS under i) brace axial compression, and ii) brace in-plane moments are tested to failure in order to examine their deformation characteristics, in particular, their joint resistances and failure modes. These tests are conducted with extensive instrumentation on both overall deformations of the T-joints and local deformations of their welded brace/chord junctions. Welding of all these T-joints between S690 CFCHS is carefully performed so that the heat input energy during welding varies narrowly between 1.2 and 1.5 kJ/mm. Standard tensile tests are also conducted on both flat and curved coupons extracted from the sections to provide mechanical properties for subsequent strength analysis. All the tests have been successfully conducted. For those T-joints between S690 CFCHS under brace axial compression, chord plastification is always found to be a critical failure mode while for those T-joints between S690 CFCHS under brace in-plane moments, both brace fracture and chord punching shear failure are found to be critical at the end of the tests. All the measured joint resistances of the T-joints between S690 CFCHS are found to be larger than those design resistances obtained with EN 1993-1-8 and Design Guide 1 of CIDECT. Consequently, this investigation provides structural insights and scientific data for possible improvements onto prediction capabilities of the design methods.