# Behaviour of Steel Tubular Knee Joint in Aluminium Frames with Tension-Tie Element
## 4 Conclusions  
Laboratory tests along with parametric numerical analysis were conducted to examine the steel knee joint flexural behaviour. Three identical steel knee joints with a tie element subjected to a bending moment were tested in a laboratory. Due to the limitations of the laboratory equipment, a steel tube was used as a tie element instead of high-strength steel wire rope, giving a much stiffer joint response that is quite different from the joints in real aluminium portal frame structures. To examine the influence of tie element stiffness on the overall behaviour of the knee joint, the numerical model was calibrated to the laboratory tests and additional numerical parametric analysis was conducted with various tie element stiffnesses. Based on the conducted research, the following conclusions can be drawn:  
1. Tested laboratory specimens have similar behaviour, all failing due to weld failure between the beam tube section and the tensile bolt nut.
2. In general, the stiffness of the tie element has very little influence on the bending moment-rotation relationship of the knee joint but has high influence on the force-piston displacement relationship.
3. The influence of the tie element stiffness on the bending moment-rotation and the force-piston displacement relationship is more pronounced in the knee range and especially in the postcritical region.
4. Knee joints with stiffer tie elements are much more prone to tensile bolt (nut) pull-out when reaching failure compared to knee joints with a softer tie element. When a softer tie element is used, web buckling at the beam to column intersection is much more pronounced, giving a more favourable ductile failure mode.  
The findings from the research performed lead to the conclusion that steel knee joints with tension tie elements are a good solution for aluminium portal frames. Ductile behaviour and the favourable effect of the soft tie element that is commonly used in practice ensure a safe way of connecting aluminium members without unfavourable welding. To get  
Figure 17: Comparison of failure modes for different tie element stiffnesses: (**a**) E\({}^{\prime}\) = 210,000 MPa; (**b**) E\({}^{\prime}\) = 610 MPa.  
an overall picture of the complex behaviour of aluminium portal frames, further research should be oriented to the behaviour of aluminium beam-to-column joints with steel inserts.  
This paper was accomplished based on collaborative work of the authors. Conceptualization and development of the main idea of the paper, D.S.; methodology, D.S. and I.C.; laboratory testing D.S. and I.G.; numerical simulation, I.C.; validation, all authors; writing--original draft preparation, D.S. and I.C., manuscript writing related to Section 2.3, I.G., D.S. and I.C.; literature review and writing--review and editing, all authors; supervision, F.M.M. All authors have read and agreed to the published version of the manuscript  
Laboratory specimens were provided by HALTOR Ltd. Croatia.  
In this section you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments).  
The authors declare that there is no conflict of interest regarding the publication of this paper.