PSI - Issue 70

Oberoi Kabrambam et al. / Procedia Structural Integrity 70 (2025) 74–81

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7. Conclusion

• Increase in ultimate moment with increase in strength of concrete is observed in this study. Increase in strength of concrete from 20 to 25, 25 to 30, 30 to 35 MPa increases its ultimate moment by 6.33%, 6.84% and 5.78% respectively in the case of 4-point bend test of thin-walled regular hexagonal CFST beam by keeping all the other parameters same. • Steel yields more in the bottom part of the beam in the case of low strength of concrete, this is because lower strength of concrete has low compressive strength and stiffness causing it to crush early during loading. This leads to the concrete core not being able to resist the internal bending moment effectively and more load is shifted to the steel tube particularly in the tension zone, this leads to more yielding of steel in the bottom part of the beam. Another reason is that the neutral axis of the beam also moves downward when weaker concrete is used, this increases lever arm between the tension and compressive forces which increases the tensile strain in the bottom steel. The top concrete of lower grade of concrete of CFST beam crushes early whereas the steel in the bottom which is free from brittle failure continues to elongate and absorb more strain and force the steel to enter plastic stage well before the steel in beam with higher strength of concrete. • Local buckling could be observed in bending test of thin-walled hexagonal CFST beam. This is because when the beam is subjected to bending, the upper part of the beam undergoes compression so this pushes the steel tube outward but in the case of thin wall, they have low stiffness so they are prone to the out-of-plane deformation which causes local buckling. Each thin plate has its own critical buckling stress determined by its width, thickness etc. when the compressive stress exceeds this point, buckling starts. References American Concrete Institute, 1999. Building code requirements for structural concrete (ACI 318-99) and commentary (ACI 318R-99). Farmington Hills, MI: American Concrete Institute. Ami, M. and Patel, V.I., 2023. Numerical investigation and design of PM interaction capacity of CFRP-jacketed hexagonal CFST short beam columns. Structures 52, 983-1008. Ellobody, E., Young, B. and Lam, D., 2006. Behaviour of normal and high strength concrete-filled compact steel tube circular stub columns. Journal of Constructional Steel Research, 62(7), 706-715. European Committee for Standardization, 2006. Eurocode 3: Design of steel structures – Part 1-4: General rules – Supplementary rules for stainless steels. EN 1993-1-4. Brussels: CEN. Giakoumelis, G. and Lam, D., 2004. Axial capacity of circular concrete-filled tube columns. Journal of constructional steel research 60(7), 1049 1068. Han, L.H., Li, W. and Bjorhovde, R., 2014. Developments and advanced applications of concrete-filled steel tubular (CFST) structures: Members. Journal of constructional steel research 100, 211-228. Hu, H.T., Huang, C.S., Wu, M.H. and Wu, Y.M., 2003. Nonlinear analysis of axially loaded concrete-filled tube columns with confinement effect. Journal of structural engineering, 129(10), 1322-1329. Hu, H.T. and Schnobrich, W.C., 1989. Constitutive modeling of concrete by using nonassociated plasticity. Journal of Materials in Civil Engineering 1(4), 199-216. İlgün, A. and Sancioğlu, S., 2023. Flexural behaviour of different CFSTs cross -section shapes with the same steel cross- sectional area. Sādhanā, 48(2), 53. Ma, D.Y., Han, L.H., Ji, X. and Yang, W.B., 2018. Behaviour of hexagonal concrete-encased CFST columns subjected to cyclic bending. Journal of Constructional Steel Research 144, 283-294. Mander, J.B., Priestley, M.J. and Park, R., 1988. Theoretical stress-strain model for confined concrete. Journal of structural engineering, 114(8), 1804-1826. Mazlan, Z.H. and Al Zand, A., 2022. Flexural Behavior of Concrete-Filled Double-Skin Hexagonal Tubular Beams Using Finite Element Analysis. Knowledge-Based Engineering and Sciences 3(2), 18-35. Mursi, M. and Uy, B., 2003. Strength of concrete filled steel box columns incorporating interaction buckling. Journal of Structural Engineering 129(5), 626-639. Saenz, L.P., 1964. Discussion of ‘Equation for the stress – strain curve of concrete’ by P. Desayi, and S. Krishnan. Journal of the American Concrete Institute 61, 1229 – 1235. TOMII, M., 1991. Ductile and strong columns composed of steel tube, infilled concrete and longitudinal steel bars. Proceedings of the third international conference on steel-concrete composite structures, 39-66. Xu, W., Han, L.H. and Li, W., 2016. Performance of hexagonal CFST members under axial compression and bending. Journal of constructional steel research 123, 162-175. Xu, W., Han, L.H. and Li, W., 2016. Seismic performance of concrete-encased column base for hexagonal concrete-filled steel tube: experimental study. Journal of Constructional Steel Research 121, 352-369.