# Behaviour of Steel Tubular Knee Joint in Aluminium Frames with Tension-Tie Element
## 1 Introduction  
So far, aluminium structures have mostly been used in cases where the lowest possible weight of the structure and the longest durability are essential [1; 2]. A typical example of the use of aluminium in civil engineering is for temporary roofing structures. Today, when sustainable construction is of utmost importance, there is an obvious need for aluminium application in permanent structures and, consequently, for reliable aluminium design methods [3]. Prefabricated aluminium halls are mostly designed as portal frames with added high-grade steel tension tie elements, where larger spans are required. Since welding in some types of aluminium alloy causes a significant reduction in resistance [4], typical joints of such structures are formed using bolts in combination with steel reinforcement embedded inside hollow aluminium sections. Such joints transfer the load partially through bolts and partially through a complex contact between the steel insert and aluminium tubes [3]. Expressions for reliable assessment of the behaviour for such joints are lacking, thus inhibiting the usage of aluminium prefabricated halls. Although the behaviour of joints between steel hollow sections is generally well known, steel knee joints that are usually used in aluminium frames are not investigated as often. [PERSON] and [PERSON] conducted numerous tests to assess the ability of knee joints in portal frames to form plastic hinges [5; 6; 7]. Cold-formed rectangular hollow sections (RHS) made from steel grade C350 and C450 (nominal yield strength of 350 MPa and 450 MPa according to Australian Standard [8]) were used in tests. Experimental research included welded unstiffened and stiffened knee joints, end-plated bolted knee joints, and knee joints with a fabricated internal sleeve [7]. The joints were tested under both opening and closing moments. Knee joints under an opening moment failed at low rotational values, mostly by fracture in the heat-affected zone of the steel hollow section, while under a closing moment, web buckling was observed. Unstiffened and stiffened welded knee joints complied with the strength interaction requirements given in the CIDECT design guide for rectangular sections [9], but the joints were unsuitable for plastic design. Unstiffened joints were incapable of reaching the plastic moment both under opening and closing moments. Stiffened joints under an opening moment were also considered unsuitable for plastic design. Only stiffened joints under a closing moment were considered suitable to form plastic hinges, and only for steel grade C350. The bolted plate connection exhibited plasticity; however, the problem of weld failure in the heat-affected zone under tensile stresses was not fully overcome [7]. Joints with an internal sleeve sustained the plastic moment for large rotations, proving them to be suitable for plastic design. An additional benefit of the welded sleeve joint was the formation of the plastic hinge away from the junction of the two steel tubes [7]. Based on the research given in [5; 6; 7], [PERSON] [10] investigated the influence of a new welding-based solution on the rotation behaviour of stiffened steel knee joints under an opening moment. The solution implies the use of additional layers of weld on the flange in tension of a stiffened welded knee joint. The conducted research proved that the proposed welding-based solution allows for a significant increase in the rotation capacity of stiffened welded knee joints made from rectangular hollow sections. The influence of bending moment and axial force interaction on the strength and stiffness of beam-to-beam and beam-to-column knee joints using component method extension was examined in [11; 12] by [PERSON] and [PERSON]. The obtained results show that tensile axial force reduces the stiffness of such joints. Tensile load equivalent to 10% and 25% of the joint load-bearing capacity can lead to stiffness reduction in the joint of up to 15% and 50%, respectively [11].