# Behaviour of Steel Tubular Knee Joint in Aluminium Frames with Tension-Tie Element
## 2 Experimental Program
### Mechanical Properties of Base Material and Welds  
To determine the material properties of the base material, a series of 15 coupon tensile tests were performed according to ISO 6892-1:2019 [18] on universal testing machine Zwick/Roell Z60 OE. Coupons were divided into two groups: 12 coupons taken from RHS 180 \(\times\) 80 \(\times\) 3, and the remaining 3 coupons taken from smaller RHS 30 \(\times\) 20 \(\times\) 2, as shown in Figure 3. SWR and SWL stand for steel web right and steel web left, respectively, while SFT and SFB stand for steel flange top and steel web bottom, respectively. Similarly, SST stands for smaller steel tube. All the steel elements were made from structural steel S235 JRH [19]. No coupons were taken from the tie element CHS 100 \(\times\) 4.75. The mechanical properties of RHS 180 \(\times\) 80 \(\times\) 3 were characterised by stress-strain curves taken from 12 coupons (Figure 4). Deviation of the results for the curves SWL1-SWL3 can be noted. Those coupons showed higher yield and ultimate strength but brittle behaviour as well. Such results are expected, since coupons for SWL were taken from the welded part of the cross section. Hence, those results were not taken into consideration when calculating mean values of mechanical properties for the RHS 180 \(\times\) 80 \(\times\) 3 for the purpose of numerical study. The mean value of yield strength (f\({}_{\text{y}}\)) was 369 MPa, with a standard deviation of 9.06 MPa. The mean value of ultimate tensile strength (f\({}_{\text{u}}\)) was 430 MPa, with a standard deviation of 6.64 MPa. It can be noted that values of f\({}_{\text{u}}\) of 9 tested samples show lower scatter compared to values of f\({}_{\text{y}}\) (6.64 \(<\) 9.06). Figure 3: Positions form where coupons were taken and their belonging labels. Figure 2: Geometrical properties of steel knee joint specimens S1–S3 (dimensions in mm). Mechanical properties of RHS 30 \(\times\) 20 \(\times\) 2 used as reinforcement are characterised by stress-strain curves SST1-SST3 (Figure 4). Deviation can also be noted for the SST1 coupon which was taken from the welded part of the smaller steel section RHS 30 \(\times\) 20 \(\times\) 2. That coupon failed prematurely as well; therefore, that result was omitted when calculating mean values of f\({}_{\text{y}}\) and f\({}_{\text{u}}\) for the smaller steel section. The mean value of f\({}_{\text{y}}\) was 435 MPa, with a standard deviation of 1.00 MPa. The mean value of f\({}_{\text{u}}\) was 465 MPa, with a standard deviation of 3.50 MPa. Although they are made from nominally the same steel grade, it can be noted that steel coupons taken from the smaller RHS 30 \(\times\) 20 \(\times\) 2 section show higher values of f\({}_{\text{y}}\) and f\({}_{\text{u}}\) strength but lower ductility. That kind of behaviour can be contributed to the higher effect of the residual stresses in the cross section introduced by welding and the cold forming process of the smaller tube. Detailed testing of butt welds between the beam and the column (Figure 5) was conducted at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb. The welding process MAG 135-D was used. It must be noted that test specimens were chosen by random sampling and they represent standard quality and work in the practice of the steelwork manufacturer. Figure 4: Stress-strain curves with main mechanical properties of tested coupons (RHS 180 \(\times\) 80 \(\times\) 3 and RHS 30 \(\times\) 20 \(\times\) 2). Figure 5: Performed testing procedures of butt welds between the beam and the column: (**a**) dye penetration test; (**b**) sample extraction; (**c**) bending test; (**d**) macrograph analysis; (**e**) static tension test. Based on the visual quality testing, welds are classified as category D (lowest quality of weld according to EN ISO 5817:2014, [20]) because of the uneven geometry, spatter on the base material and poor start-stop performance.