Fatigue strength and stress concentration factors of CHS-to-RHS T-joints

Abstract The static and fatigue behaviour of eight different hollow section T-joints subjected to both axial and in-plane bending loads were investigated experimentally. The joints had circular brace members and rectangular chords. Hot spot stresses and the stress concentration factors (SCFs) were determined experimentally. Fatigue testing was carried out under constant amplitude loading in air. The test results have been statistically evaluated, and show that the experimental SCF values for circular-to-rectangular (CHS-to-RHS) hollow section joints were found to be between those of circular-to-circular (CHS-to-CHS) and rectangular-to-rectangular (RHSto-RHS) hollow section joints. The fatigue strength, referred to experimental hot spot stress, was in reasonably good agreement with referred fatigue design codes for tubular joints. 2002 Elsevier Science Ltd. All rights reserved.
Li-Chun Bian a, Jae-Kyoo Lim a,∗
a
Department of Mechanical Design, Automobile Hi-Te Chonbuk National University, Chonju 561-756, South Korea Received 14 March 2002; received in revised form 29 June 2002; accepted 1 July 2002
Journal of Constructional Steel Research 59 (2003) 627–640 /locate/jcsr
Fatigue strength and stress concentration factors of CHS-to-RHS T-joints
∗
Corresponding author. Tel.: +82-63-2702321; fax: +82-63-2702460. E-mail address: jklim@moak.chonbuk.ac.kr (J.-K. Lim).
0143-974X/03/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0143-974X(02)00048-2
L.-C. Bian, J.-K. Lim / Journal of Constructional Steel Research 59 (2003) 627–640
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Fig. 1.
Geometry of a CHS-to-RHS joint.
is a very little information and virtually no design guidance available for the fatigue design of CHS-to-RHS joints. The current lack of design recommendations has been a major obstacle to the use of this hybrid connection for fatigue-loaded structures. According to a previous study [6], the stress concentration factors (SCFs) for this hybrid joint (CHS-to-RHS) were sometimes between those of the RHS-to-RHS joint and the CHS-to-CHS joint, and sometimes even below the CHS-to-CHS joint. The mean fatigue life correlated well with the proposed design curve for 16 mm plate thickness [7]. Therefore, it appeared worthwhile doing more investigations on CHSto-RHS joints. Tagoe [8] presented some experimental results for CHS-to-RHS joints with axial loading, which indicated that joints with a large b-ratio would fail from the brace side and with an anomalously low fatigue strength. In the present study, the fatigue behaviour of CHS-to-RHS T-joints subjected to both axial and in-plane bending loadings was investigated, and the results obtained were compared with design guidance for RHS-to-RHS and CHS-to-CHS joints. Most of the specimens were joints with a large b-ratio. The work was undertaken in order to provide a data base which could be developed into design guidance for fatigue design of CHS-toRHS joints. This study is a basic, and also a necessary, step for the practical application of CHS-to-RHS joints and forms a foundation for future studies. 2. Specimens The material of the brace and chord members was steel to Standard DIN 17100, which is commonly used in offshore structures and has the following nominal tensile properties,
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L.-C. Bian, J.-K. Lim / Journal of Constructional Steel Research 59 (2003) 627–640
Nomenclature d l t A B DEn DnV E IPB L N R SCF Smax Smin SHS Snom T a, b, g, ⌬SHS ⌬Snom eHS SYS outside diameter of brace length of brace thickness of brace axial loading outside width of chord Department of Energy, UK Det norske Veritas Young’s modulus of elasticity in-plane bending loading length of chord cycle number (fatigue life) stress ratio, Smin/Smax stress concentration factor maximum nominal stress in a constant amplitude cycle minimum nominal stress in a constant amplitude cycle hot spot stress brace nominal stress (according to beam theory) thickness of chord t non-dimensional geometrical parameters, see Fig. 1 and Table 2 hot spot stress range brace nominal stress range extrapolated hot spot strain minimum specified yield stress
offshore structures, which are subjected to fatigue by wave loading, circular hollow sections are used almost exclusively due to their relatively smaller hydrodynamic loads and stress concentrations in joints. However, in CHS-to-CHS joints, the hollow section members are joined together by welding the profiled ends of secondary members, the braces, onto the circumference of the main member, the chord. Therefore, compared with RHS-to-RHS joints, the fabrication of CHS-to-CHS joints is more costly due to their complicated joint geometry. Also, the behaviour of these welded tubular connections, even in their simplest configuration, is complex and their analysis is difficult. The stress concentrations in RHS-to-RHS joints are generally larger, and they have correspondingly lower fatigue lives. However, rectangular hollow sections provide a simpler joint geometry for fabrication of the joints (lower joint fabrication cost) and are frequently used for onshore structures and structures which are predominantly subjected to static loads. Joints with circular braces and rectangular chords may in many applications provide an optimum solution with a simple joint geometry for fabrication and a relatively smaller stress concentration factor compared to RHS-to-RHS joints. However, a study of the available literature reveals that there