Issue 72

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

Prediction model and strength reduction factor The peak axial load carrying capacity, ௨ , ௔௡௔ of each CFDST column is predicted using the modified EC-4 equation proposed by Pagoulatou et. al. [20] as presented with Eqn. 4, assuming that the outer steel tube served as the primary means of confinement to the sandwiched concrete. Confinement coefficients, ௔ and ௖ corresponding to steel and concrete, respectively as presented in Eqn. 4, were taken into account solely to predict the capacity of CS-CFDST column. They are eliminated for SS-CFDST columns following EC-4 recommendation for square-shaped CFST columns and also for CS CFDST with concrete imperfections. Peak axial load ratio, ௨ , ௔௡௔ ௨ , ௘௫௣ ⁄ are evaluated for all CFDST columns and are tabulated in Tab 4. It shows an underestimation of up to about 11%, when comparing the experimental and predicted peak loads of CS-CFDST and SS-CFDST composite column. Therefore, it is believed that these confinement coefficients are insufficient for the CS-CFDST column and there is an existence of confinement in SS-CFDST column as well. Hence SS CFDST column was re-assessed with set of equations proposed by Han et. al. [27] and the result showed a more accurate prediction of peak load with a variance of less than ~1%. Although it is evident that peak axial load estimated using Eqn. 4 for CS-CFDST and SS-CFDST columns with circumferential gap ratio of 1.1% and 2.2% and spherical or rectangular cap gap ratio of 4.4% and 8.8% yield close agreement with experimental results. ௨ , ௔௡௔ ൌ ௔ ሺ ௔௢ ൈ ௔௢ ൅ ௔௜ ൈ ௔௜ ሻ൅ ௖ ൈ ௖ ቀ 1 ൅ ௖ ௧ ೚ ஽ ೚ ௙ೌ ೚ ௙ ೎ ቁ (4) Here, ௔௢ is yield strength of outer steel tube, ௔௜ is yield strength of inner steel tube, ௖ is compressive strength of concrete (cylinder). Specimen ID (%) ௨ , ௘௫௣ (kN) ௨ , ௔௡௔ (kN) ௨ , ௔௡௔ ௨ , ௘௫௣ ⁄ CS-CFDST - 667 596 1.32 - 0.89 CS-CFDST-CCI1 1.1 609 589 1.30 0.91 0.97 CS-CFDST-CCI2 2.2 599 583 1.25 0.90 0.97 CS-CFDST-SCI4 4.4 621 594 1.21 0.93 0.96 CS-CFDST-SCI8 8.8 590 589 1.37 0.89 1.00 SS-CFDST - 664 591 3.19 - 0.89 SS-CFDST-CCI1 1.1 565 582 2.61 0.85 1.03 SS-CFDST-CCI2 2.2 541 574 1.29 0.82 1.06 SS-CFDST-SCI4 4.4 629 573 1.57 0.95 0.91 SS-CFDST-SCI8 8.8 557 556 1.45 0.84 1.00 C-CFST - 575 555 1.44 - 0.97 S-CFST - 520 481 1.55 - 0.93 Table 4: Experimental and analytical results for CFDST and CFST columns.

1,00

1,00

0,80

0,80

0,40 Strength Index, SI 0,60

0,40 Strength Index, SI 0,60

0,20

0,20

CS-CFDST SS-CFDST

CS-CFDST SS-CFDST

0,00

0,00

0,0

2,0

4,0

6,0

8,0

0,0

0,5

1,0

1,5

2,0

Gap ratio, χ (%)

Gap ratio, χ (%)

(a) CCI gap ratio

(b) SCI or RCI gap ratio

Figure 11: Strength Index versus gap ratio for CS-CFDST and SS-CFDST.

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