Effect of geometric parameters on ultimate strength of axially-loaded CFDST chord to CFST brace K-joints

The highest punch shear stress levels at concrete-filled double-skin steel tube (CFDST) K-joints form at chord-brace intersecting point. Increasing the section wall thickness of the outer tube at this point enhances the risk of fracture, thus endangers the entire joint. In order to eliminate such shortcoming, the present study proposes an alternative approach of implementing self-consolidating concrete (SCC) in the space between the inner and outer chords, as well as inside the circular hollow section (CHS) brace members, so as to improve load capacity and eliminate main chord plastification. A laboratory experiment followed by a nonlinear parametric finite element (FE) analysis were done to examine the effects of several mechanical and geometric features of an offshore jacket joint members, including core concrete compressive strength ($f_c^{\prime }$), chord effective length-to-diameter ratio ($\alpha =2L/D$), brace-to-chord diameter ratio ($\beta =d/D$), chord radius-to-wall thickness ratio ($\gamma =D/2T$), brace gap ratio ($\zeta =g/D$), chord-to-brace angle ($\theta$), and brace-to-chord wall thickness ratio ($\tau =t/T$) on the overall joint ductility and strength capacity. Findings show that K- joints with a higher chord-brace angle demonstrate lower ductility factors in general. Also, buckling occurs at brace elements while the chord-brace intersecting point remains intact when the CFDST K-joint members are filled with core concrete. Furthermore, joints with concrete infill had almost twice as much peak loads and greater initial stiffness compared to those with no infill concrete, because of the contribution of confined concrete in raising the overall capacity of the CFDST K-joint.