# Circular hollow sections

$$Failure mode methodIn this chapter, component-based finite element method (CBFEM) for design of uniplanar welded Circular Hollow Sections (CHS) is verified to Failure Mode Method (FMM): T, X, and K-joints. In CBFEM, the design resistance is limited by reaching 5 % of strain or a force corresponding to 3% d0 joint deformation, where d0 is chord diameter. The resistance in FMM is generally determined by peak load or 3% d0 deformation limit, see (Lu et al. 1994). FMM is based on the principle of identifying modes that may cause joint failure. From the practical experience and experiments accomplished during the 70s and 80s, two modes of failure were identified for the CHS joints: chord plastification and chord punching shear. This calculation method is always limited to a probed geometry of joints. This means that different formulas always apply for each geometry. In the following studies, the welds are designed according to EN 1993‑1‑8:2006 not to be the weakest components in the joint.Chord plastificationThe design resistance of a CHS chord face can be determined using the method given by FMM model in Ch. 9 of prEN 1993-1-8:2020; see Fig. 7.1.1. The method is also given in ISO/FDIS 14346 and is described in more detail in (Wardenier et al. 2010). The design resistance of the axially loaded welded CHS joint is: for T and Y joint $N_{1,Rd} = C_f \frac{f_{y0} t_0^2}{\sin{\theta_1}} (2.6+17.7 \beta^2) \gamma^{0.2} Q_f / \gamma_{M5}$ X joint $N_{1,Rd} = C_f \frac{f_{y0} t_0^2}{\sin{\theta_1}} \left ( \frac{2.6+2.6 \beta}{1-0.7 \beta} \right ) \gamma^{0.15} Q_f / \gamma_{M5}$ and for K gap joint $N_{1,Rd} = C_f \frac{f_{y0} t_0^2}{\sin{\theta_1}} (1.65+13.2 \beta^{1.6}) \gamma^{0.3} \left [ 1+ \frac{1}{1.2+(g/t_0)^{0.8}} \right ] Q_f / \gamma_{M5}$where: di – an overall diameter of CHS member i (i = 0, 1, 2 or 3) fyi – yield strength of member i (i = 0, 1, 2 or 3) g – gap between braces of K joint ti – thickness of the wall of CHS member i (i = 0, 1, 2 or 3) $$\theta_i$$ – included angle between brace member i and the chord (i =1, 2 or 3) $$\beta$$ – ratio of the mean diameter or width of brace members, to that of the chord $$\gamma$$ – ratio of a chord width or diameter to twice its wall thickness Qf – chord stress factor Cf – material factor $$\gamma_{M5}$$ – partial safety factor for resistance of joints in hollow section lattice girders Ni,Rd – design resistance of a joint expressed in terms of the internal axial force in member i (i = 0, 1, 2 or 3) Fig. 7.1.1 Examined failure mode – chord plastificationChord punching shear(for $$d_i \le d_0 - 2 t_0$$)The design resistance of the axially loaded T, Y, X, and K joint of welded circular hollow sections for chord punching shear (Fig. 7.1.2) is:$N_{1,Rd} = C_f \frac{f_{y0}}{\sqrt{3}} t_0 \pi d_i \frac{1+\sin{\theta_1}}{2 \sin^2{\theta_1}} / \gamma_{M5}$where: di – overall diameter of CHS member i (i = 0,1,2 or 3) ti – thickness of the wall of CHS member i (i = 0,1,2 or 3) fy,i – yield strength of member i (i = 0,1,2 or 3) $$\theta_i$$ – included angle between brace member i and the chord (i = 1,2 or 3) Cf – material factor Ni,Rd – design resistance of a joint expressed in terms of the internal axial force in member i (i = 0, 1, 2 or 3) Fig. 7.1.2 Examined failure mode – chord punching shearChord shear(for X joints, only if $$\cos{\theta_1} > \beta$$)The design resistance of the axially loaded X joint of welded circular hollow sections for chord shear, see Fig. 7.1.3, is:$N_{1,Rd} = \frac{f_{y0}}{\sqrt{3}} \frac{(2/\pi A_0)}{\sin{\theta_1}} / \gamma_{M5}$where: Ai – area of cross-section i (i = 0,1,2 or 3) fy,i – yield strength of member i (i = 0,1,2 or 3) $$\theta_i$$ – included angle between brace member i and the chord (i = 1,2 or 3) Ni,Rd – design resistance of a joint expressed in terms of the internal axial force in member i (i = 0, 1, 2 or 3) Fig. 7.1.3 Examined failure mode - Chord shearRange of validityCBFEM was verified for typical joints of the welded circular hollow sections. Range of validity for these joints is defined in Table 7.1.8 of prEN 1993-1-8:2020; see Tab 7.1.2. The same range of validity is applied to CBFEM model. Outside the range of validity of FMM, an experiment should be prepared for validation or verification performed for verification according to a validated research model.Tab. 7.1.2 Range of validity for method of failure modes General$$0.2 \le \frac{d_i}{d_0} \le 1.0$$$$\theta_i \ge 30^{\circ}$$$$-0.55 \le \frac{e}{d_0} \le 0.25$$ $$g \ge t_1+t_2$$$$f_{yi} \le f_{y0}$$$$t_i \le t_0$$ ChordCompressionClass 1 or 2 and $$10 \le d_0 / t_0 \le 50$$ (but for X joints: $$d_0/t_0 \le 40$$) Tension$$10 \le d_0 / t_0 \le 50$$ (but for X joints: $$d_0/t_0 \le 40$$) CHS bracesCompressionClass 1 or 2 and $$d_i / t_i \le 50$$ Tension$$d_i / t_i \le 50$$ Uniplanar T and Y-CHS jointOverview of the considered examples in the study is given in Tab. 7.1.3. Selected cases cover a wide range of joint geometric ratios. Geometry of the joints with dimensions is shown in Fig. 7.1.2. In the selected cases, the joints failed according to the FMM by the chord plastification or punching shear. Fig. 7.1.4 Dimensions of T/Y jointTab. 7.1.3 Examples overview ExampleChordBraceAnglesMaterial SectionSection$$\theta$$fyfuE [°][MPa][MPa][GPa] 1CHS219.1/5.0CHS48.3/5.090355490210 2CHS219.1/5.0CHS114.3/6.390355490210 3CHS219.1/6.3CHS114.3/6.390355490210 4CHS219.1/10.0CHS60.3/5.090355490210 5CHS219.1/18.0CHS159/14,090355490210 6CHS219.1/8.0CHS48.3/5.090355490210 Verification of resistanceThe results of the method based on FMM are compared with the results of CBFEM. The comparison is focused on the resistance and design failure mode. The results are presented in Tab. 7.1.4.The study shows a good agreement for the applied load cases. The results are summarized in a diagram comparing CBFEM’s and FMM’s design resistances; see Fig. 7.1.5. The results show that the difference between the two calculation methods is in all cases less than 14%. Fig. 7.1.5 Verification of CBFEM to FMM for the uniplanar CHS T and Y-jointTab. 7.1.4 Comparison of design resistances for loading in tension/compression: prediction by CBFEM and FMM ExampleFMM Mode of failureCBFEMMode of failurediff. [kN] [kN] [%] 156.9Chord plastification60.2Chord plastification5.5 2122.0Chord plastification117.2Chord plastification4.1 3185.0Chord plastification193.8Chord plastification4.5 4225.8Chord plastification240.8Chord plastification6.2 51842.8Chord punching shear1618.0Chord punching shear13.9 6132.7Chord plastification142.0Chord plastification6.5 Benchmark exampleInputsChord Steel S355 Section CHS219.1/5.0 Brace Steel S355 Sections CHS48.3/5.0 Angle between the brace member and the chord 90° Weld Butt weld around the brace Loaded By force to brace in compression Mesh size 64 elements along surface of the circular hollow member Outputs The design resistance in compression is NRd = 60.2 kN The design failure mode is chord plastification Sample files Open in Viewer Download Uniplanar X-CHS jointOverview of the considered examples in the study is given in Tab. 7.1.5. Selected cases cover a wide range of joint geometric ratios. Geometry of the joints with dimensions is shown in Fig. 7.1.6. In the selected cases, the joints failed according to the FMM by the chord plastification or punching shear. Fig. 7.1.6 Dimensions of X jointTab. 7.1.5 Examples overview ExampleChordBraceAngles Material SectionSection$$\theta$$fyfuE [°][MPa][MPa][GPa] 1CHS219.1/6.3CHS60.3/5.090355490210 2CHS219.1/8.0CHS76.1/5.090355490210 3CHS219.1/10.0CHS139.7/10.090355490210 4CHS219.1/12.5CHS114.3/6.390355490210 5CHS219.1/10.0CHS76.1/5.090355490210 6CHS219.1/8.0CHS114.3/6.390355490210 7CHS219.1/6.3CHS48.3/5.060355490210 8CHS219.1/6.3CHS114.3/6.360355490210 9CHS219.1/8.0CHS60.3/5.060355490210 10CHS219.1/10.0CHS114.3/6.360355490210 11CHS219.1/12.5CHS139.7/10.060355490210 12CHS219.1/8.0CHS139.7/10.060355490210 13CHS219.1/6.3CHS48.3/5.030355490210 14CHS219.1/6.3CHS193.7/16.030355490210 15CHS219.1/6.3CHS219.1/20.030355490210 16CHS219.1/8.0CHS76.1/5.030355490210 17CHS219.1/8.0CHS168.3/12.530355490210 18CHS219.1/12.5CHS168.3/12.530355490210 Verification of resistanceThe results of CBFEM are compared with the results of FMM. The comparison is focused on the resistance and design failure mode. The results are presented in Tab. 7.1.6.Tab. 7.1.6 Comparison of results of prediction by CBFEM and FMM ExampleFMMMode of failureCBFEMMode of failurediff. [kN][kN][%] 188,8Chord plastification93,4Chord plastification5,2 2155,7Chord plastification152,0Chord plastification2,4 3390,9Chord plastification395,0Chord plastification1,0 4478,7Chord plastification461,0Chord plastification3,8 5235,3Chord plastification219,0Chord plastification7,4 6209,7Chord plastification219,0Chord plastification4,2 793,7Chord plastification85,2Chord plastification10,0 8155,6Chord plastification164,0Chord plastification5,1 9159,5Chord plastification148,5Chord plastification7,4 10365,8Chord plastification383,0Chord plastification4,5 11682,1Chord plastification703,0Chord plastification3,0 12298,7Chord plastification336,0Chord plastification11,1 13162,3Chord plastification181,5Chord plastification10,6 14555,8Chord plastification582,0Chord plastification4,5 15749,7Chord plastification727,0Chord plastification3,1 16669,1Chord plastification766,0Chord plastification12,7 171527,7Chord plastification1578,0Chord plastification3,2 The study shows a good agreement for most of the applied load cases. The results are summarized in a diagram comparing CBFEM’s and FMM’s design resistances; see Fig. 7.1.7. The results show that the difference between the two calculation methods is in most cases less than 13%. Fig. 7.1.7 Verification of CBFEM to FMM for the uniplanar CHS T and Y-jointBenchmark exampleInputsChord Steel S355 Section CHS219.1/6,3 Brace Steel S355 Sections CHS60,3/5,0 Angle between the brace member and the chord 90° Weld Butt weld around the brace Loaded By force to brace in compression Mesh size 64 elements along surface of the circular hollow member Outputs The design resistance in compression is NRd = 93,4 kN The design failure mode is chord plastification Sample files Open in Viewer Download Uniplanar K-CHS jointOverview of the considered examples in the study is given in Tab. 7.1.7. Selected cases cover a wide range of joint geometric ratios. Geometry of the joints with dimensions is shown in Fig. 7.1.8. In the selected cases, the joints failed according to the method based on the failure modes (FMM) by the chord plastification or punching shear.Tab. 7.1.7 Examples overview ExampleChordBraceGapAnglesMaterial SectionSectiong$$\theta$$fyfuE [mm][°][MPa][MPa][GPa] 1CHS219.1/4,5CHS139.7/10,020.160355490210 2CHS219.1/4,5CHS219.1/20,039.960355490210 3CHS219.1/5,0CHS88.9/5,09.960355490210 4CHS219.1/5,0CHS219.1/20,039.960355490210 5CHS219.1/6,3CHS48.3/5,09.960355490210 6CHS219.1/6,3CHS60.3/5,09.960355490210 7CHS219.1/8,0CHS114.3/6,312.560355490210 8CHS219.1/8,0CHS139.7/10,020.160355490210 9CHS219.1/6.3CHS114.3/6.312.560355490210 10CHS219.1/6.3CHS139.7/10,020.160355490210 Fig. 7.1.8 Dimensions of K jointVerification of resistanceThe results of the method based on failure modes (FMM) are compared with the results of CBFEM. The comparison is focused on the resistance and design failure mode. The results are presented in Tab. 7.1.8 and in Fig. 7.1.9.Tab. 7.1.8 Comparison of results of design resistances by CBFEM and FMM ExampleFMM Mode of failureCBFEMMode of failurediff. [kN] [kN] [%] 1213.3Chord plastification237.9Chord plastification11.5 2367.5Chord plastification344Chord plastification6.8 3165.4Chord plastification170Chord plastification2.7 4443.6Chord plastification398Chord plastification11.5 5149.2Chord plastification150.5Chord plastification0.9 6175.7Chord plastification177.5Chord plastification1 7501.1Chord plastification469Chord plastification6.8 8605.6Chord plastification551Chord plastification9.9 9324.4Chord plastification332Chord plastification2.3 10392.4Chord plastification363Chord plastification8.1 The study shows a good agreement for the applied load cases. The results are summarized in a diagram comparing CBFEM’s and FMM’s design resistances; see Fig. 7.1.6. The results show that the difference between the two calculation methods is in all cases less than 12 %. Fig. 7.1.9 Verification of CBFEM to FMM for the uniplanar CHS K-jointBenchmark exampleInputsChord Steel S355 Section CHS 219.1/4.5 Brace Steel S355 Sections CHS 139.7/10 Angle between the brace member and the chord 60° Gap between braces g = 20.1 mm Weld Butt weld around the brace Loaded By force to brace in compression Mesh size 64 elements along surface of the circular hollow member Outputs The design resistance in compression is NRd = 237.9 kN The design failure mode is chord plastification Sample files Open in Viewer Download$$