PSI - Issue 70

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

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Fig. 8. Moment vs displacement graph of hexagonal CFST beam.

Fig. 9. M u vs concrete strength of hexagonal CFST beam.

Fig. 10. Yielding of steel in B-20 specimen.

Fig. 11. Yielding of steel in B-35 specimen.

Fig. 12. Local buckling observed in hexagonal CFST beam specimen. The grey colour in Fig. 10. and Fig.11. represents the part where the steel has been yielded, when the CFST beam has been bend up to its ultimate point, here the yield stress of the steel is 236.3 MPa. As we can see 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. 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 as shown in Fig. 12. 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.