Axial Compression Mechanical Performance Analysis of Hollow Sandwich Steel Tube Concrete Columns

Literature Overview

This paper, published in the Journal of Huazhong University of Science and Technology in 2011 by Xu Hanyong and Yu Zhiwu from Central South University, presents a theoretical and experimental study on the axial compression mechanical performance of hollow sandwich steel tube concrete columns with octagonal cross-sections. The research proposes a novel octagonal hollow sandwich steel tube concrete column configuration and develops theoretical models for predicting the load-bearing capacity, typical failure modes, and load-strain curves of both short and long columns. The study combines theoretical analysis, experimental testing, and numerical simulation to derive a rational load-bearing capacity formula that accounts for the influence of various design parameters.

Core Technical Points

The hollow sandwich steel tube concrete column represents an advanced composite structural member that combines the advantages of steel tube concrete with the weight reduction benefits of a hollow core. The octagonal cross-section is selected for its favorable combination of structural efficiency, geometric simplicity, and ease of fabrication. The sandwich configuration consists of an outer steel tube, an inner steel tube, and a concrete layer between the two tubes, with a hollow core in the center. This configuration provides enhanced confinement of the concrete, improved load-bearing capacity, and increased ductility compared to conventional steel tube concrete columns.

The theoretical analysis of the hollow sandwich steel tube concrete column begins with the equilibrium and compatibility conditions for the composite cross-section. The load-bearing capacity is determined by the sum of the contributions from the outer steel tube, the inner steel tube, and the confined concrete. The confinement effect of the steel tubes on the concrete increases the compressive strength and ductility of the concrete, and this effect is quantified using confinement models that relate the confining pressure to the concrete strength enhancement.

The failure modes of the hollow sandwich steel tube concrete columns depend on the slenderness ratio and the confinement ratio. Short columns with low slenderness ratios fail by crushing of the concrete and local buckling of the steel tubes, with the failure mode governed by the confinement ratio. Long columns with high slenderness ratios fail by global buckling, with the failure mode governed by the slenderness ratio. The transition between short and long column behavior occurs at a critical slenderness ratio that depends on the confinement ratio and the material properties.

Interpretation of Technical Points

The load-strain curve of the hollow sandwich steel tube concrete column exhibits a characteristic behavior that reflects the interaction between the steel tubes and the concrete. In the elastic stage, the load-strain relationship is linear, with the stiffness determined by the combined axial stiffness of the steel tubes and the concrete. In the plastic stage, the concrete begins to crush, but the steel tubes continue to provide confinement, resulting in a plateau or gradual increase in load. In the post-peak stage, the load decreases as the concrete degrades and the steel tubes buckle, with the rate of load decrease depending on the confinement ratio and the ductility of the steel tubes.

The theoretical model developed in this study accounts for the nonlinear behavior of the concrete under confinement and the elastic-plastic behavior of the steel tubes. The model is validated against experimental data, and the agreement between theoretical predictions and experimental results is good, indicating that the model captures the essential mechanics of the hollow sandwich steel tube concrete column. The model is then extended to account for the influence of various design parameters, including the dimensions of the steel tubes, the concrete strength, and the slenderness ratio, through a parametric study and regression analysis.

The numerical simulation of the hollow sandwich steel tube concrete column provides additional insights into the failure mechanism and the influence of design parameters. The finite element model captures the nonlinear material behavior of the steel and concrete, the interaction between the steel tubes and the concrete, and the geometric nonlinearities due to buckling. The simulation results are used to validate the theoretical model and to extend the parametric study to a wider range of design parameters than can be covered experimentally.

Process and Standards Analysis

The fabrication of hollow sandwich steel tube concrete columns with octagonal cross-sections involves several manufacturing processes that must be carefully controlled to ensure the quality and performance of the final product. The octagonal steel tubes can be fabricated by rolling, welding, or extrusion, depending on the required dimensions and tolerances. The welding of the steel tubes requires qualified welding procedures that ensure the integrity of the welds and the dimensional accuracy of the tubes. The concrete infilling of the sandwich space must be performed with appropriate concrete mixes that have the required flowability and strength, and the infilling process must be controlled to avoid voids and ensure complete filling of the sandwich space.

The relevant standards for the design and fabrication of steel tube concrete columns include GB 50017-2017 for steel structure design, GB 50010-2010 for concrete structure design, and CECS 230-2004 for steel tube concrete structures. These standards provide guidelines for the design, fabrication, and testing of steel tube concrete members, including the material specifications, design methods, and acceptance criteria. The hollow sandwich steel tube concrete column is a novel configuration that is not explicitly covered by existing standards, so the design and fabrication must be based on engineering judgment and the results of experimental and analytical studies.

The testing of hollow sandwich steel tube concrete columns involves axial compression tests that measure the load-strain response, the failure mode, and the ultimate load-bearing capacity. The test specimens are prepared according to the design specifications, with careful attention to the dimensional accuracy, the surface finish, and the concrete infilling quality. The test setup includes load cells, displacement transducers, and strain gauges to measure the load, the deformation, and the strain in the steel tubes and the concrete. The test results are analyzed to determine the load-bearing capacity, the failure mode, and the influence of design parameters on the mechanical performance.

Integration with Engineering Practice

In engineering practice, the hollow sandwich steel tube concrete column offers several advantages over conventional steel tube concrete columns, including reduced weight, improved load-bearing capacity per unit weight, and enhanced ductility. These advantages make the hollow sandwich configuration attractive for applications where weight is a critical design constraint, such as high-rise buildings, long-span bridges, and offshore platforms. The octagonal cross-section provides additional benefits in terms of structural efficiency and ease of fabrication, as the flat faces of the octagon simplify the connection design and the formwork for concrete infilling.

The design of hollow sandwich steel tube concrete columns requires a thorough understanding of the interaction between the steel tubes and the concrete, the confinement effect of the steel tubes on the concrete, and the failure mechanisms under axial compression. The theoretical model developed in this study provides a rational basis for the design of these columns, and the load-bearing capacity formula derived from the model can be used for preliminary design and verification. However, the design must also consider the fabrication tolerances, the construction quality, and the long-term performance under service conditions, which may require additional safety factors and quality control measures.

The fabrication of hollow sandwich steel tube concrete columns requires specialized equipment and expertise, particularly for the welding of the octagonal steel tubes and the concrete infilling of the sandwich space. The welding of the steel tubes must be performed with qualified welding procedures that ensure the integrity of the welds and the dimensional accuracy of the tubes. The concrete infilling must be performed with appropriate concrete mixes that have the required flowability and strength, and the infilling process must be controlled to avoid voids and ensure complete filling of the sandwich space. The quality control of the fabrication process includes dimensional inspection, weld testing, concrete strength testing, and non-destructive testing of the concrete infilling.

Key Questions and Reflections

Several questions arise from the study of hollow sandwich steel tube concrete columns that warrant further investigation. First, the behavior of these columns under eccentric loading, bending, and combined loading is not addressed in this axial compression-focused study. The interaction between axial compression and bending is an important design consideration for practical applications, and the development of interaction diagrams for hollow sandwich steel tube concrete columns is a valuable extension of this study. Second, the long-term performance of these columns under sustained loading, including creep and shrinkage of the concrete, is not fully characterized. The long-term behavior may affect the load-bearing capacity and the serviceability of the columns, and it deserves further study. Third, the fatigue behavior of these columns under cyclic loading is not addressed, which is relevant for applications in seismic regions or under dynamic loading.

The paper demonstrates the value of combining theoretical analysis, experimental testing, and numerical simulation in the study of novel structural configurations. The theoretical model provides a rational understanding of the mechanics of the hollow sandwich steel tube concrete column, the experimental tests validate the model and provide data for parameter identification, and the numerical simulation extends the analysis to a wider range of design parameters. This integrated approach is essential for the development of new structural systems and for the improvement of existing design methods.

Study Insights and Implications

The study of hollow sandwich steel tube concrete columns with octagonal cross-sections provides valuable insights for the design and fabrication of advanced composite structural members. The theoretical model and the load-bearing capacity formula derived from the study offer a rational basis for the design of these columns, and the experimental and numerical results provide confidence in the applicability of the model. For engineers involved in the design and fabrication of steel tube concrete structures, this paper serves as a reference for the development of novel configurations that combine structural efficiency with practical fabrication considerations. The insights gained from this study can be applied to improve the design of steel tube concrete columns and to develop new structural systems that meet the demands of modern construction.