Performance analysis and design of demountable concrete-filled steel tubular K-joints by advanced numerical modelling

The concept of demountable composite steel-concrete connections enables easy erection and dismantlement of reusable structural members during the designed service life. It diminishes large quantities of energy consumption and global greenhouse gases (GHG) emission in contrast with recycling and remanufacturing the composite members. This thesis proposed innovative designs on demountable square and circular concrete-filled steel tubular (CFST) K-joints using semi-enclosed flush endplate and blind bolts. The assembly of key components is achieved by welding the brace members to the flush endplate and bolting the flush endplate to the chord in the design. A clear load transferring mechanism is merged with ensured construction quality by conducting welds in workshop and installing bolts on-site. At the end of the first-stage design life, the disassembly includes removing the blind bolts and detaching the flush endplate. The structural integrity of steel, composite components and blind bolts should be assessed and ensured before reusing. Comprehensive analysis on the structural behaviour of the newly proposed joints is conducted using finite element modelling. The effect of significant parameters such as the design of flush endplate, the joint eccentricity, the geometry of the brace and chord, and the types of blind bolts on joint capacity and stiffness are investigated. The brace buckling is chosen as the desired failure mode among the captured modes such as flush endplate failure, chord tube failure and bolt failure. The critical reasons are listed: the failure of the brace member is relatively independent on other structural components; the replacement of the potentially damaged brace-endplate component is practical; the integrity and reusability of the composite chord member are ensured. To precisely evaluate and compare the demountable K-joints with traditional welded K-joints, the effect of residual distortions and stresses on structural characteristics due to the welding process in welded K-joints is considered using a prescribed temperature approach. In the model, two-dimensional shell and three-dimensional solid modelling techniques are adopted for heat affected and buckling sensitive zones, respectively, to develop an appropriate balance between the computational accuracy and efficiency. The results indicate that the initial and service-level stiffnesses are significantly reduced due to heat effects on the welding zone, while the failure modes and joint capacities are hardly affected. Full range analysis on the demountable K-joints reveals that the joint strength remains at the same level with enhanced tensile stiffness in contrast to the welded joints. Recommendations on the design of the newly proposed joint are given as follows: Appropriate spacings and detailing of the blind bolts should be evaluated to ensure the practical installation; The single endplate design has advantages over the separated design in terms of minimising the relative displacement between components; The utilization of through-bolts in the demountable square joints makes full use of the composite materials and helps strengthen the connectivity; The flush endplate to chord thickness ratio is suggested to be no smaller than 1.25 and 2.5 in circular and square demountable joints, respectively, to prevent the undesired endplate failure