Numerical simulation of size effect in concrete-encased high-strength steel columns under axial compression

Concrete-encased steel (CES) columns are extensively utilized in practical engineering due to their outstanding performance. However, there have been limited studies on the size effect of CES columns. To investigate the influences of stirrup ratio, steel ratio, and steel strength grade on the axial capacity and the mechanism of the size effect of members under axial compression, a finite element (FE) analysis model that considers the interaction between the size effect and the confinement effect is established and verified. The study results demonstrate that the nominal stress of CES columns can be remarkably increased by employing high-strength steel and enhancing the steel ratio. However, the nominal stress decreases as the cross-sectional size of specimen increases, indicating the presence of the size effect. Moreover, increasing the steel strength grade, stirrup ratio, and steel ratio can decrease the degree of reduction of the nominal stress. Furthermore, comparison of the FE analysis findings and the calculation results of EN1994-1-1:2010(Europe), AISC360-16 (U.S.), and JGJ138-2016(China) revealed that the three codes fail to accurately assess the axial capacity of CES columns. The accuracy and safety reserve of the codes calculation results are even more inadequate for the larger cross-sectional specimens. Finally, a theoretical formula that considers the size effect and confinement effect is developed to calculate the bearing capacity of CES columns, and its applicability and reliability are verified.