Hysteretic behaviour of collar-plate-reinforced tubular K-joints under cyclic loading

This paper presents a numerical investigation into the hysteretic behaviour of tubular K-joints enhanced with collar plates under axial cyclic loading. While collar plates have been widely studied under monotonic loading, their cyclic performance in K-joints has not previously been addressed. A detailed finite element (FE) model is developed, including weld geometry, material and geometric nonlinearity and contact at the brace–chord intersection. The modelling approach is validated against cyclic tests on unenhanced and collar-plate-enhanced joints, showing good agreement in terms of stiffness, strength and deformation capacity. A parametric study is then carried out on 104 K-joint configurations, covering practical ranges of brace-to-chord diameter ratio β , chord slenderness γ , brace inclination θ , gap ratio ζ and collar plate dimensions (thickness ratio ρ and length ratio ꞷ). The results indicate that collar plates substantially modify and generally improve the cyclic response of tubular K-joints. Compared with the corresponding unreinforced joints, collar-plate-reinforced K-joints develop fuller and more stable hysteresis loops, higher peak strengths and delayed strength degradation. In joints with slender chords, the compressive ductility ratio µ c increases from 7.09 to 13.29 in one series and from 5.96 to 16.12 in another, while the tensile ductility µ t decreases, reflecting a shift towards controlled compressive plastification in the joint region. For representative gap configurations, the cumulative energy-dissipation index η total increases from 130.3 to 353.1 and from 75.2 to 235.1 for two benchmark cases. These gains correspond to a 2.5–3.0-fold increase in total hysteretic energy compared with the corresponding unreinforced joints. The findings provide a rational basis for using collar plates both in the design of new tubular structures and in the seismic strengthening of in-service CHS K-joints.