Ultimate Strength of Internal Ring-Reinforced KT Joints Under Brace Axial Compression

Adnan Rasul, Saravanan Karuppanan, Veeradasan Perumal, Mark Ovinis, Mohsin Iqbal


Internal ring stiffeners are frequently used to improve the ultimate strength of tubular joints in offshore structures. However, there is a noticeable absence of specific design guidance regarding the assessment of their ultimate strengths in prominent offshore codes and design guides. No equations are available to determine the ultimate strength of internal ring-reinforced KT joints. This work developed equations to determine the ultimate strength and the strength ratio of internal ring-reinforced KT joints based on numerical models and parametric studies comprising ring parameters and joint parameters. Specifically, a finite element model and a response surface approach with eight parameters (λ, δ, ψ, ζ, θ, τ, γ, and β) as inputs and two outputs (ultimate strength and the strength ratio) were evaluated since efficient response surface methodology has been proven to give precise and comprehensive predictions. KT-joint with parameters λ=0.9111, δ=0.2, ψ=0.7030, ζ=0.3, θ=45°, τ=0.90, γ=16.25, and β=0.6 has the maximum ultimate strength, and the KT-joint with parameters: λ=1, δ=0.2, ψ=0.8, ζ=0.5697, θ=45°, τ=0.61, γ=24, and β=0.41 has the maximum strength ratio. The KT-joints with the optimized parameters were validated through finite element analysis. The percentage difference was less than 1.7%, indicating the applicability and high accuracy of the response surface methodology.


Doi: 10.28991/CEJ-2024-010-05-012

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KT-Joint; Response Surface Methodology; Ultimate Strength; Ring-Stiffeners; Initial Stiffness; Optimization; Finite Element Analysis; Strength Ratio.


Yang, K., Zhu, L., Bai, Y., Sun, H., & Wang, M. (2018). Strength of external-ring-stiffened tubular X-joints subjected to brace axial compressive loading. Thin-Walled Structures, 133, 17-26. doi:10.1016/j.tws.2018.09.030.

Dehghani, A., & Aslani, F. (2019). A review on defects in steel offshore structures and developed strengthening techniques. Structures, 20, 635–657. doi:10.1016/j.istruc.2019.06.002.

Iqbal, M., Karuppanan, S., Perumal, V., Ovinis, M., & Rasul, A. (2023). Rehabilitation Techniques for Offshore Tubular Joints. Journal of Marine Science and Engineering, 11(2). doi:10.3390/jmse11020461.

Choo, Y. S., Liang, J. X., Van Der Vegte, G. J., & Liew, J. Y. R. (2004). Static strength of collar plate reinforced CHS X-joints loaded by in-plane bending. Journal of Constructional Steel Research, 60(12), 1745–1760. doi:10.1016/j.jcsr.2004.05.005.

Nassiraei, H., Mojtahedi, A., & Lotfollahi-Yaghin, M. A. (2018). Static strength of X-joints reinforced with collar plates subjected to brace tensile loading. Ocean Engineering, 161, 227–241. doi:10.1016/j.oceaneng.2018.05.017.

Shao, Y. B. (2016). Static strength of collar-plate reinforced tubular T-joints under axial loading. Steel and Composite Structures, 21(2), 323–342. doi:10.12989/scs.2016.21.2.323.

Nassiraei, H., Zhu, L., Lotfollahi-Yaghin, M. A., & Ahmadi, H. (2017). Static capacity of tubular X-joints reinforced with collar plate subjected to brace compression. Thin-Walled Structures, 119(June), 256–265. doi:10.1016/j.tws.2017.06.012.

Feng, R., Chen, Y., & Chen, D. (2017). Experimental and numerical investigations on collar plate and doubler plate reinforced SHS T-joints under axial compression. Thin-Walled Structures, 110, 75–87. doi:10.1016/j.tws.2016.10.017.

Rajić, A., Lukačević, I., Skejić, D., & Ungureanu, V. (2023). Cold-formed Steel-Concrete Composite Beams with Back-to-Back Channel Sections in Bending. Civil Engineering Journal, 9(10), 2345-2369. doi:10.28991/CEJ-2023-09-10-01.

Choo, Y. S., Liang, J. X., Van Der Vegte, G. J., & Liew, J. Y. R. (2004). Static strength of doubler plate reinforced CHS X-joints loaded by in-plane bending. Journal of Constructional Steel Research, 60(12), 1725–1744. doi:10.1016/j.jcsr.2004.05.004.

Gjukaj, A., Salihu, F., Muriqi, A., & Cvetanovski, P. (2023). Numerical Behavior of Extended End-Plate Bolted Connection under Monotonic Loading. HighTech and Innovation Journal, 4(2), 294-308. doi:10.28991/HIJ-2023-04-02-04.

Gao, F., Tang, Z., Guan, X., Zhu, H., & Chen, Z. (2018). Ultimate strength of tubular T-joints reinforced with doubler plates after fire exposure. Thin-Walled Structures, 132(April), 616–628. doi:10.1016/j.tws.2018.09.021.

Zavvar, E., Sadat Hosseini, A., & Lotfollahi-Yaghin, M. A. (2021). Stress concentration factors in steel tubular KT-connections with FRP-Wrapping under bending moments. Structures, 33(July), 4743–4765. doi:10.1016/j.istruc.2021.06.100.

Lesani, M., Bahaari, M. R., & Shokrieh, M. M. (2014). Experimental investigation of FRP-strengthened tubular T-joints under axial compressive loads. Construction and Building Materials, 53, 243–252. doi:10.1016/j.conbuildmat.2013.11.097.

Nassiraei, H., & Rezadoost, P. (2021). Stress concentration factors in tubular X-connections retrofitted with FRP under compressive load. Ocean Engineering, 229(April), 108562. doi:10.1016/j.oceaneng.2020.108562.

Hosseini, A. S., Bahaari, M. R., & Lesani, M. (2020). SCF distribution in FRP-strengthened tubular T-joints under brace axial loading. Scientia Iranica, 27(3 A), 1113–1129. doi:10.24200/SCI.2018.5471.1293.

Masilamani, R., & Nallayarasu, S. (2021). Experimental and numerical investigation of ultimate strength of ring-stiffened tubular T-joints under axial compression. Applied Ocean Research, 109, 102576. doi:10.1016/j.apor.2021.102576.

Lee, M. M. K., & Llewelyn-Parry, A. (2005). Strength prediction for ring-stiffened DT-joints in offshore jacket structures. Engineering Structures, 27(3), 421–430. doi:10.1016/j.engstruct.2004.11.004.

Lee, M. M. K., & Llewelyn-Parry, A. (1999). Strength of ring-stiffened tubular T-joints in offshore structures: A numerical parametric study. Journal of Constructional Steel Research, 51(3), 239–264. doi:10.1016/S0143-974X(99)00027-9.

Lan, X., Wang, F., Ning, C., Xu, X., Pan, X., & Luo, Z. (2016). Strength of internally ring-stiffened tubular DT-joints subjected to brace axial loading. Journal of Constructional Steel Research, 125, 88–94. doi:10.1016/j.jcsr.2016.06.012.

Li, Q., Zhou, X., Wang, Y., Lim, J. B. P., Wang, B., & Gao, S. (2024). Static performance of multi-planar CFST chord-CHS brace KK joints. Journal of Constructional Steel Research, 213, 108428. doi:10.1016/j.jcsr.2023.108428.

Boretzki, J., Albiez, M., Myslicki, S., Vallée, T., & Ummenhofer, T. (2024). Hybrid grouted joints: Load bearing and failure behaviour under static, axial loading. Construction and Building Materials, 413, 134691. doi:10.1016/j.conbuildmat.2023.134691.

Lin, G., Zeng, J., Li, J., & Chen, G. M. (2024). Chord axial compressive behavior of hybrid FRP-concrete-steel double-skin tubular member T-joints. Thin-Walled Structures, 196, 111535. doi:10.1016/j.tws.2023.111535.

Ahmadi, H., & Lotfollahi-Yaghin, M. A. (2013). Experimental and numerical investigation of geometric SCFs in internally ring-stiffened tubular KT-joints of offshore structures. Journal of the Persian Gulf, 4(12), 1-12.

Wimpey Offshore. (1991). In-service database for ring-stiffened tubular joints. Report WOL, 35, 91, London, United Kingdom.

Azari Dodaran, N., Ahmadi, H., & Lotfollahi-Yaghin, M. A. (2018). Static strength of axially loaded tubular KT-joints at elevated temperatures: Study of geometrical effects and parametric formulation. Marine Structures, 61(June), 282–308. doi:10.1016/j.marstruc.2018.06.009.

Sadat Hosseini, A., Zavvar, E., & Ahmadi, H. (2021). Stress concentration factors in FRP-strengthened steel tubular KT-joints. Applied Ocean Research, 108(September), 102525. doi:10.1016/j.apor.2021.102525.

Iqbal, M., Karuppanan, S., Perumal, V., Ovinis, M., & Rasul, A. (2023). Numerical investigation of crack mitigation in tubular KT-joints using composite reinforcement. Engineering Proceedings, 56(1), 255. doi:10.3390/ASEC2023-16290.

Ahmadi, H., Lotfollahi-Yaghin, M. A., & Yong-Bo, S. (2013). Chord-side SCF distribution of central brace in internally ring-stiffened tubular KT-joints: A geometrically parametric study. Thin-Walled Structures, 70, 93–105. doi:10.1016/j.tws.2013.04.011.

Ahmadi, H., Ali Lotfollahi-Yaghin, M., Yong-Bo, S., & Aminfar, M. H. (2012). Parametric study and formulation of outer-brace geometric stress concentration factors in internally ring-stiffened tubular KT-joints of offshore structures. Applied Ocean Research, 38, 74–91. doi:10.1016/j.apor.2012.07.004.

Ahmadi, H., & Lotfollahi-Yaghin, M. A. (2015). Stress concentration due to in-plane bending (IPB) loads in ring-stiffened tubular KT-joints of offshore structures: Parametric study and design formulation. Applied Ocean Research, 51, 54–66. doi:10.1016/j.apor.2015.02.009.

Ahmadi, H., & Zavvar, E. (2015). Stress concentration factors induced by out-of-plane bending loads in ring-stiffened tubular KT-joints of jacket structures. Thin-Walled Structures, 91, 82–95. doi:10.1016/j.tws.2015.02.011.

Kohnke, P. (2009). Theory reference for the mechanical APDL and mechanical applications. Ansys Inc, Pennsylvania, United States.

Marshall, P. W. (2013). Design of welded tubular connections: Basis and use of AWS code provisions. Elsevier, Amsterdam, Netherlands.

Zavvar, E., Hectors, K., & De Waele, W. (2021). Stress concentration factors of multi-planar tubular KT-joints subjected to in-plane bending moments. Marine Structures, 78. doi:10.1016/j.marstruc.2021.103000.

Iskander, M. S., Shaat, A. A., Sayed-Ahmed, E. Y., & Soliman, E. A. (2017). Strengthening CHS T-joints subjected to brace axial compression using through-bolts. Journal of Constructional Steel Research, 128, 555–566. doi:10.1016/j.jcsr.2016.09.019.

Wardenier, J., Kurobane, Y., Packer, J. A., Van der Vegte, G. J., & Zhao, X. L. (2008). Design guide for circular hollow section (CHS) joints under predominantly static loading. Cidect, Geneva, Switzerland.

Swensson, K. D., & Yura, J. A. (1986). Stress Concentration Factors in Double-Tee Tubular Joints. PMFSEL Report No.

Li, X., Zhang, L., Xue, X., Wang, X., & Wang, H. (2018). Prediction on ultimate strength of tube-gusset KT-joints stiffened by 1/4 ring plates through experimental and numerical study. Thin-Walled Structures, 123, 409–419. doi:10.1016/j.tws.2017.11.029.

Qu, H., Li, A., Huo, J., & Liu, Y. (2017). Dynamic performance of collar plate reinforced tubular T-joint with precompression chord. Engineering Structures, 141, 555-570. doi:10.1016/j.engstruct.2017.03.037.

Ahmadi, H., Yeganeh, A., Mohammadi, A. H., & Zavvar, E. (2016). Probabilistic analysis of stress concentration factors in tubular KT-joints reinforced with internal ring stiffeners under in-plane bending loads. Thin-Walled Structures, 99, 58–75. doi:10.1016/j.tws.2015.11.010.

Efthymiou, M. (1988). Development of SCF formulae and generalized influence functions for use in fatigue analysis. OTJ 88. Recent Developments in Tubular Joints Technology, Surrey, United Kingdom.

Khan, M. B., Iqbal Khan, M., Shafiq, N., Abbas, Y. M., Imran, M., Fares, G., & Khatib, J. M. (2023). Enhancing the mechanical and environmental performance of engineered cementitious composite with metakaolin, silica fume, and graphene nanoplatelets. Construction and Building Materials, 404(March), 133187. doi:10.1016/j.conbuildmat.2023.133187.

Nassiraei, H. (2020). Local joint flexibility of CHS T/Y-connections strengthened with collar plate under in-plane bending load: parametric study of geometrical effects and design formulation. Ocean Engineering, 202, 107054. doi:10.1016/j.oceaneng.2020.107054.

Karim, M. A., Abdullah, M. Z., Waqar, A., Deifalla, A. F., Ragab, A. E., & Khan, M. (2023). Analysis of the mechanical properties of the single layered braid reinforced thermoplastic pipe (BRTP) for oil & gas industries. Results in Engineering, 20, 101483. doi:10.1016/j.rineng.2023.101483.

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DOI: 10.28991/CEJ-2024-010-05-012


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