Axial Compression Performance of Composite Concrete-Filled Steel Tube Columns with Internal Square Steel Tubes

Literature Overview

This paper by Yao Guohuang, Chen Yiyan, Guo Ming, and Zhao Qunchang from Shenzhen Municipal Design and Research Institute investigates the axial compression behavior of composite CFST columns that incorporate internal square steel tubes. The study employs finite element analysis using appropriate material constitutive models for both steel and concrete to simulate the load-deformation response of these hybrid columns. Published in the Journal of Building Steel Structure Progress in 2013, the research provides numerical insights into the structural performance of a composite column system that combines the benefits of circular CFST columns with the additional strength and confinement provided by internal square steel tubes.

Structural Configuration and Material Modeling

The composite column system studied consists of an outer circular steel tube filled with concrete, with one or more internal square steel tubes also filled with concrete. This hybrid configuration aims to combine the excellent confinement properties of circular CFST columns with the additional load-bearing capacity and confinement provided by the internal square tubes. The square tubes also offer advantages in terms of section efficiency and ease of connection to other structural members.

The finite element analysis employed appropriate constitutive models for both materials. For the steel tubes, a bilinear or multilinear elastic-plastic model was used, capturing the elastic response, yield plateau, and strain hardening behavior. For the concrete, a confinement model that accounts for the lateral restraint provided by the steel tubes was adopted. The concrete model typically includes an unconfined concrete branch and a confined concrete branch, with the transition between the two branches governed by the level of lateral confinement pressure.

Numerical Analysis Results and Key Findings

The finite element analysis revealed several important aspects of the composite column behavior under axial compression. The load-deformation relationship exhibits a characteristic curve with an initial linear elastic branch, a nonlinear transition region as yielding begins in the steel tubes and micro-cracking initiates in the concrete, and a post-peak branch that may show either gradual degradation or, in some configurations, a secondary strength increase due to the progressive engagement of confined concrete.

The failure mode analysis indicates that the composite columns fail through a combination of steel tube local buckling and concrete crushing. The internal square tubes provide additional confinement to the concrete, delaying the onset of concrete crushing and enhancing the post-peak ductility. The interaction between the outer circular tube and the internal square tubes is a critical factor in determining the overall column performance. The interface between the steel tubes and concrete involves frictional contact, which affects the load transfer mechanism and the confinement effectiveness.

The stress development patterns show that the concrete in the regions between the internal square tubes and the outer circular tube experiences a complex stress state with varying levels of confinement. The confinement pressure is not uniform throughout the concrete core, with higher confinement near the steel tube walls and lower confinement in the interior regions. This non-uniform confinement has implications for the overall load-bearing capacity and ductility of the composite column.

Engineering Design Considerations

From a design perspective, the composite CFST column system offers several advantages over conventional CFST columns. The additional internal square tubes increase the load-bearing capacity without significantly increasing the column diameter, which is beneficial for space-constrained applications. The square tubes also provide more favorable connection geometries for beam-column joints, as square sections align more naturally with the rectangular cross-sections of typical steel beams.

However, several design challenges must be addressed. The fabrication of the composite column requires precise alignment and welding of the internal square tubes within the outer circular tube, which adds complexity to the manufacturing process. The concrete pouring and compaction must be carefully controlled to ensure complete filling of the space between the internal and external tubes without voids or honeycombing. The weld connections between the internal square tubes and any end plates or connection elements must be designed to prevent premature failure under seismic loading.

The interaction between the outer and inner tubes also raises questions about the potential for internal buckling of the square tubes under high axial loads. If the internal square tubes buckle before the outer circular tube reaches its full capacity, the composite action may be compromised. Design guidelines should specify minimum wall thickness ratios and maximum unsupported lengths for the internal tubes to prevent this failure mode.

Key Technical Questions and Reflections

The paper's reliance on finite element analysis raises questions about the validation of the numerical models against experimental data. While the constitutive models used are well-established in the literature, the specific interaction between the internal square tubes and the surrounding concrete may not be fully captured by standard contact algorithms. Experimental validation with physical specimens would strengthen the confidence in the numerical predictions.

Another important consideration is the effect of the composite column on seismic performance. While the paper focuses on axial compression behavior, the ductility and energy dissipation capacity under cyclic loading are equally important for structures in seismic regions. The internal square tubes may influence the hysteretic behavior of the column, potentially providing additional confinement that enhances post-yield ductility. Future research should investigate the cyclic loading response of these composite columns.

The practical feasibility of the composite column system also deserves consideration. The additional fabrication complexity, concrete placement challenges, and quality control requirements may increase the overall cost of the column system. A comparative cost-benefit analysis between the composite CFST column and alternative systems such as double-skin CFST columns or conventional CFST columns with increased wall thickness would be valuable for practical engineering decision-making.

Reference Value and Outlook

This research provides valuable numerical insights into the axial compression behavior of a novel composite column system that combines circular and square steel tubes with concrete infill. The finite element analysis reveals the complex interaction mechanisms between the different components and highlights the potential benefits of the composite configuration in terms of load-bearing capacity and ductility. For structural engineers designing high-rise buildings or industrial structures that require large column capacities, this composite system represents a promising alternative to conventional CFST columns.

The study also underscores the importance of developing comprehensive design guidelines that address fabrication, construction, and quality control aspects of the composite column system. As the structural engineering community continues to explore innovative column configurations to meet increasing demands for taller and more slender structures, research like this contributes to the expanding toolkit of available structural solutions.