PSI - Issue 19

Jennifer Hrabowski et al. / Procedia Structural Integrity 19 (2019) 267–274 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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As illustrated in Fig. 3, the fatigue resistance is strongly dependent on the wall thickness or the wall thickness ratio = t i /t 0 , respectively. In EN 1993-1-9 (2010) and by Zhao et al (2001), the detail category (DC) for K-joints with gap made of RHS is 36 for a ratio t 0 /t i = 1.0 and 71 for t 0 /t i = 2.0. Values in between can be interpolated linear. So, for the series K45_100_6_80_4_g25_S700 a detail category 54 for t 0 /t i = 1.5 is relevant. Deviating from the usual slope m = 3.0 for fatigue strength curves, m = 5.0 is used for hollow section joints. This is also reflected in the test results graphed in Fig. 3, where the specimens of series K45_100_6_80_4_g25_S700 are sufficient for detail category 54 (green). The test results of series K45_130_4_80_4_g25_S500 and K45_130_4_80_4_g25_S700 are below and one point is even below the given class of 36 (blue).

Fig. 4. Weld execution for RHS-joints a) recommendation b) implementation.

In addition to load type, dimensions and wall thickness ratio, the weld execution has an influence on the fatigue resistance, too. This influence is not directly considered in the detail catalogs. The fatigue classes are based on test results. But it is difficult to understand in retrospect what kind of welds are implied there. For joints made of RHS, it is recommended not to start or end the weld in the corner areas, since the maximum stress concentration occurs here (Fig. 4a). However, this is often common practice, as can be seen at the welds of the investigated specimens in Fig. 4b). The start and end points of the welds in the corner areas of the braces are seen as the main reason for the scatter of the test results and the partial underrun of detail category 36. 2.3. Fatigue Tests on CHS K-joints Fourteen fatigue tests on K-joints with circular hollow sections made of steel grade S355 and S700 have been carried out, see Table 2. Herein, the influence of the steel grade and the weld seam execution is examined. For the evaluation of CHS K-joints, the nominal stress range is determined for axially loaded braces and the magnification factor of MF = 1.3 is used according to EN 1993-1-9 (2010) and by Zhao et al. (2001).

Table 2. Dimensions of CHS K-joints for fatigue testing.

Series

Number

Chord [mm]

Brace [mm]

Gap

Brace Material

of tests

Diameter d 0

Thickness t 0

Diameter di

Thickness t i

[mm] g

Angle 

K45_193.7_10_114.3_6.3_g25_S355 K45_193.7_10_114.3_6.3_g25_S700

6 8

193.7 193.7

10 10

114.3 114.3

6.3 6.3

25 25

45 45

S355 S700

As already noted for RHS K-joints, the fatigue resistance of CHS K-joints is also dependent on the wall thickness or the wall thickness ratio = t i /t 0 , respectively, where t 0 is the wall thickness of the chord and t i is the wall thickness of the corresponding brace. In EN 1993-1-9 (2010) and by Zhao et al (2001), the detail category (DC) for K-joints