Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Stiffened Thin-Walled High-Strength Square Steel Tube Concrete Short Columns
Literature Overview and Research Significance
This paper, authored by Yang Youfu and Guo Hongxin from the State Key Laboratory of Coastal and Offshore Engineering at Dalian University of Technology, was published in the Journal of South China University of Technology (Natural Science Edition) in 2021 (Vol. 49, No. 8, pp. 43-52). The research was supported by the National Natural Science Foundation of China (Grant 51678105). The study investigates the axial compression behavior of stiffened and unstiffened thin-walled high-strength square steel tube concrete (CFST) short columns, evaluating the effectiveness of different stiffening measures through experimental testing and finite element analysis.
Experimental Program and Specimen Design
The experimental program included both stiffened and unstiffened specimens to provide direct comparison of the effects of stiffening measures. The specimens were square-section CFST short columns with thin walls, using high-strength steel tubes and concrete cores. Multiple types of stiffening measures were designed and applied, including steel plate ribs with circular holes and diagonal tension steel plate ribs, as well as double-row steel plate rib configurations. The unstiffened specimens served as control specimens for comparison.
| Specimen Parameter | Stiffened Specimens | Unstiffened Specimens |
|---|---|---|
| Steel grade | High-strength structural steel | Same as stiffened |
| Steel yield strength | Elevated levels | Same as stiffened |
| Wall thickness | Thin-walled | Same as stiffened |
| Concrete strength | High-strength concrete | Same as stiffened |
| Stiffening measure | Multiple types | None |
| Section shape | Square | Square |
| Column length | Short column (low slenderness) | Short column (low slenderness) |
The experimental setup included axial loading with displacement control, and the specimens were instrumented with strain gauges on the steel tube surface and core concrete, as well as displacement transducers to measure axial deformation. The loading was applied at a constant displacement rate to simulate quasi-static axial compression, and the load-displacement and load-strain relationships were recorded throughout the test.
Key Experimental Findings
The experimental results revealed several important findings regarding the effects of stiffening measures on the axial compression performance of thin-walled high-strength square CFST short columns. First, the stiffening measures were found to alter the failure progression and the steel tube failure mode compared to unstiffened specimens. In unstiffened specimens, local buckling of the thin steel tube walls typically initiates at relatively low load levels, leading to a sudden loss of confinement effectiveness and premature concrete crushing. The stiffening measures delay the onset of local buckling and maintain the confinement effectiveness of the steel tube over a wider range of deformation.
Second, while stiffening measures generally improve the axial compression performance, no single stiffening measure was found to comprehensively improve all mechanical indicators. The stiffening measure with circular-hole diagonal tension steel plate ribs was found to be more effective in improving load-carrying capacity and ductility, while the double-row steel plate rib configuration was more effective in improving the combined elastic modulus. This finding is significant because it demonstrates that the optimization of stiffening measures must be targeted based on the specific performance requirements of the application.
The steel yield strength was found to have a significant influence on the failure mode and load-carrying capacity of the specimens. Higher steel yield strength generally increased the load-carrying capacity but may have reduced the ductility due to the reduced strain capacity of the high-strength steel. The interaction between the steel yield strength and the stiffening measure type was also investigated, revealing that the optimal stiffening measure may depend on the steel grade used.
Finite Element Modeling and Parametric Analysis
A finite element (FE) model was developed using appropriate constitutive models for both the steel and concrete materials. For the steel, a von Mises yield criterion with isotropic hardening was employed, calibrated to the high-strength steel material properties. For the concrete, a confined concrete model accounting for the lateral confinement provided by the steel tube was used, with the confinement pressure calculated based on the steel tube deformation. The FE model was validated against the experimental results, showing good agreement in terms of load-displacement curves, failure modes, and strain distributions.
| FE Model Parameter | Specification | Purpose |
|---|---|---|
| Steel constitutive model | Von Mises with isotropic hardening | Capture plastic behavior |
| Concrete constitutive model | Confined concrete model | Account for lateral confinement |
| Contact formulation | Penalty or augmented Lagrange | Model steel-concrete interaction |
| Element type | Shell (steel), solid (concrete) | Efficient mesh discretization |
| Boundary conditions | Axial compression with displacement control | Match experimental setup |
The validated FE model was then used for a parametric study to investigate the effects of key parameters on the axial compression performance of stiffened thin-walled high-strength square CFST short columns. The parameters studied included the steel yield strength, wall thickness, concrete strength, stiffening measure type, stiffening measure spacing, and the geometric dimensions of the stiffening elements. The parametric study provided a comprehensive understanding of the parameter sensitivities and identified the most influential factors for design optimization.
Engineering Practice and Quality Control Implications
The fabrication of stiffened thin-walled high-strength square CFST short columns involves several critical quality control steps. The high-strength steel tubes require careful selection and certification to ensure that the material properties meet the specified requirements, with particular attention to the yield strength, tensile strength, elongation, and impact toughness. The steel tubes should be inspected using UT or MT to detect any manufacturing defects, and the surface quality should be assessed to ensure that there are no dents, scratches, or other imperfections that could initiate buckling under compression.
The stiffening measures, whether steel plate ribs or other configurations, must be welded to the steel tube with high-quality welds. The welds should be designed and executed in accordance with applicable welding standards such as GB 50661 or AWS D1.1, with appropriate pre-heat and interpass temperature control to prevent cracking in the high-strength steel. Post-weld inspection using UT or PT is essential to ensure weld integrity, and any defects must be repaired before the concrete is placed.
The concrete filling of the square section must be carefully controlled to ensure complete filling without voids, particularly in the corners where the stiffening measures create complex geometries. Self-compacting concrete or vibratory compaction with internal vibrators should be used, and the filling process should be monitored to ensure that the concrete reaches all regions of the section. The concrete strength should be verified through cube testing in accordance with GB/T 50081, and the actual strength should be within the specified tolerance range.
Critical Reflections and Study Insights
The study provides valuable insights into the design of stiffened thin-walled high-strength square CFST short columns, but several limitations and areas for further investigation are apparent. The experimental program, while comprehensive, was limited to short columns under pure axial compression, and the findings may not directly apply to columns subjected to combined axial compression and bending, which is more representative of actual structural applications. The slenderness ratio and eccentricity of loading can significantly affect the behavior of CFST columns, and future research should extend the investigation to these more realistic loading conditions.
The stiffening measures studied were all based on external steel plate ribs, and the study did not consider internal stiffening measures such as internal ribs or ring stiffeners that could be incorporated during the concrete placement process. The comparison between external and internal stiffening measures, and the optimal combination of both, would provide a more complete design framework. Additionally, the study did not address the fatigue performance of the stiffened columns under cyclic loading, which is relevant for applications in offshore structures or bridges where fatigue is a design consideration.
The FE model, while validated against experimental data, relies on constitutive models that may not fully capture the complex behavior of confined concrete under high strain levels. The confinement effect in thin-walled high-strength steel tubes may differ from that in thicker-walled or lower-strength tubes due to the different buckling behavior of the steel tube, and the constitutive model should be calibrated specifically for the material combination studied.
Summary
This study provides a comprehensive investigation of the axial compression performance of stiffened and unstiffened thin-walled high-strength square CFST short columns, demonstrating through both experimental testing and finite element analysis that stiffening measures can significantly improve load-carrying capacity, ductility, and combined elastic modulus. The key finding that no single stiffening measure comprehensively improves all mechanical indicators is particularly important for practical design, as it highlights the need for targeted optimization based on specific performance requirements. For practicing engineers, the work offers valuable guidance on the selection of stiffening measures for thin-walled high-strength CFST columns, emphasizing the importance of weld quality, concrete filling integrity, and material certification in achieving the predicted performance. The study contributes to the advancement of CFST technology for high-strength applications and identifies opportunities for further research into combined loading, fatigue behavior, and alternative stiffening configurations.