Abstract
This paper systematically investigated the axial compression behavior of circular concrete-filled high-strength thin-walled steel tubular (CFHTST) columns with out-of-code diameter-to-thickness (D/t) ratios. The axial compression test was first conducted to examine the failure mode, load-displacement curves, and composite mechanism effect. The finite element (FE) model was thereafter established to perform full-range analysis on the load versus displacement curve as well as the interaction behavior, where the parametric study was performed to investigate the influences of the material strengths and geometric sizes. Subsequently, the applicability of typical design methods was evaluated, and a revised equation for determining strain εscy corresponding to ultimate strength was established to assess the plastic deformation capacity of CFHTST columns. Finally, a theoretical model for calculating axial bearing capacity was derived based on unified twin-shear strength theory by considering the influence of intermediate principal stress. The research results indicate that a relatively high confine effect can be guaranteed for CFHTST columns under out-of-code D/t ratios, given that the ratio Nu/Nnom between the measured capacity (Nu) and nominal cross-sectional capacity (Nnom) mainly distributes within 1.179∼1.292; the full-range analysis reflects that the axial load-deformation curve can be distinguished by four various loading stages; the scope b = 0.3∼0.55 of intermediate stress coefficient is generally suggested for predicting axial strength of circular CFST columns within an error of ±5%. The abovementioned study can provide the meaningful design reference for the analysis and application of CFHTST columns.
Highlights
Circular concrete-filled steel tubular (CFST) columns have been gradually applied in many building structures, e.g., the ultrahigh-rise buildings, long-span bridges, urban viaducts, and heavy-load pylons due to their efficient fabrication procedure and favorable mechanical behavior in which the core concrete can prevent outer steel tube out of local buckling; the steel tube can confine the concrete infill into a triaxial compression state for improving the strength and ductility [1]. e steel tube acts as the framework for bearing the self-weight of upper structure before casting core concrete, making a reduced labor employed and construction cost [2]
The above studies mainly focus on concretefilled thick-walled steel tubular columns with NS steel; for the CFSTcolumns, using HS steel may bring about larger D/t ratios that were beyond the specific requirements of design codes, namely, the CFST columns with out-of-code D/t ratios
Compared with the thick-walled CFST columns, one urgent issue for CFHTSTcolumns is that many design guidelines restrict the application of HS steel and impose strict limitation on the diameter-to-thickness (D/t) ratio, i.e., American AISC 360-16 [30] applies to the structural steel of nominal yield strength not more than 525 MPa; European EC4 [31] and Chinese GB 50936-2014
Summary
Circular concrete-filled steel tubular (CFST) columns have been gradually applied in many building structures, e.g., the ultrahigh-rise buildings, long-span bridges, urban viaducts, and heavy-load pylons due to their efficient fabrication procedure and favorable mechanical behavior in which the core concrete can prevent outer steel tube out of local buckling; the steel tube can confine the concrete infill into a triaxial compression state for improving the strength and ductility [1]. e steel tube acts as the framework for bearing the self-weight of upper structure before casting core concrete, making a reduced labor employed and construction cost [2]. [32] specify the steel yield strength limitation of 460 MPa. Considering the local buckling induced by the thin-walled steel tube, the aforementioned codes [30,31,32], respectively, stress the limitations of D/t ≤ 0.31Es/fy, D/t ≤ 90·235/fy, and D/t ≤ 135·235/fy, which obstructs the popularization and application of CFHTST columns. Considering the local buckling induced by the thin-walled steel tube, the aforementioned codes [30,31,32], respectively, stress the limitations of D/t ≤ 0.31Es/fy, D/t ≤ 90·235/fy, and D/t ≤ 135·235/fy, which obstructs the popularization and application of CFHTST columns For those CFHTST columns with out-of-code D/t ratios, there is still an uncertainty on the applicability of current design methods, in which the composite behavior between the core concrete and thinwalled steel tube needs to be examined to ensure the confine effect for enhancing the ductility and bearing capacity of CFST columns. The study mentioned in this paper can provide the meaningful design reference for the analysis and application of CFHTST columns
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