Experimental Investigation of Cold-Formed High Strength Steel K Joints

One of the main challenges in applying high strength steel to tubular structures is appropriate design of the welded joints between high strength steel tubular members, i.e., high strength steel tubular joints. Although certain design standards have provided design guidance for tubular joints with steel grades up to S460 or S700, it is generally agreed that current design provisions for high strength steel tubular joints recommended by the design standards need further validation based on experimental evidence. Results of a recent testing program on cold-formed high strength steel K joints are presented in this paper to augment the test database of high strength steel tubular joints and evaluate related design provisions in the revised draft of EN 1993-1-8 (prEN 1993-1-8) including the material factor and the yield ratio limit ($f_{\mathrm{y}}\le 0.8f_{\mathrm{u}}$). The material factor 0.9 for grade 460 steel is found to be appropriate for the ductile chord failure (plastification) of circular hollow section (CHS) K gap joints. However, it is observed that brace fracture can also be involved in the failure of CHS K gap joints when high strength steel is used, although this kind of limit state is currently not covered by prEN 1993-1-8. For CHS K overlap joints and box section K gap joints fabricated from grade 460 steels, the prEN 1993-1-8 equations provide unconservative design resistances or design resistances with insufficient margin of safety. Particularly, it is pointed out that the widely accepted rules to determine the effective width properties may be unconservative for high strength steel joints.