Issue 72

A. J. Patel et alii, Fracture and Structural Integrity, 72 (2025) 1-14 DOI: 10.3221/IGF-ESIS.72.01

0 100 200 300 400 500 600 700 800

CS

SS

667

664

629

621

609

599

590

565

557

541

Ulimate axial load ( N u,exp ) in kN

CFDST CFDST-CCI1 CFDST-CCI2 CFDST-SCI4 CFDST-SCI8

Specimen Designation

Figure 5: Experimental ultimate load-carrying capacity of CFDST columns. Load-Displacement behaviour ( െ∆ ) and ductility Axial load-displacement ( െ∆ )curves were plotted for CS-CFDST and SS-CFDST columns in Fig. 6(a) and Fig. 6(b), respectively. CS-CFDST columns exhibit steep stiffness degradation of the axial load-displacement curve in the post-peak region with an increase in circumferential gap ratio. This is attributed to the global buckling behaviour of CS-CFDST column test specimens. It can further be realized that stiffness degradation is higher for CS-CFDST column test specimens with concrete imperfection since imperfection affects the composite action. Axial load-displacement curves of square-shaped CFDST columns shown in Fig. 6(b) depict many humps indicating the presence of multiple local buckling and strain hardening following each local buckling until the testing stopped. It can be realized that in case of concrete imperfection, the inner steel tube stabilizes post-peak ultimate axial load behaviour and prevents premature buckling up to a certain extent. Inelastic axial displacement behaviour further gets strengthened because of the inner steel tube since sandwiched concrete may yield uniform confinement in addition to the delayed local buckling.

100 150 200 250 300 350 400 450 500 550 600 650 700

100 150 200 250 300 350 400 450 500 550 600 650 700

Axial load ( N ) in kN

Axial load ( N ) in kN

SS-CFDST

SS-CFDST-CCI1 SS-CFDST-RCI4

CS-CFDST CS-CFDST-CCI2 CS-CFDST-SCI8

CS-CFDST-CCI1 CS-CFDST-SCI4

SS-CFDST-CCI2 SS-CFDST-RCI8

0 50

0 50

C-CFST

S-CFST

0

5

10

15

20

0

10 20 30 40 50 60 70

Axial deformation ( Δ ) in mm

Axial deformation ( Δ ) in mm

(a) (b) Figure 6: Axial load versus axial displacement behaviour of columns (a) CS-CFDST; (b) SS-CFDST.

Ductility of each CS-CFDST and SS-CFDST column was evaluated through the Ductility Index ( DI ) as listed in Tab. 4. DI is defined as a ratio of axial shortening, ∆ ௨ ,଼ ହ % corresponding to the axial load once the peak load falls by 15% and axial shortening, ∆ ௨ corresponding to peak axial load as represented by Eqn. 3 [26]. ൌ ∆ ௨ ,଼ ହ % ∆ ௨ (3)

7