Axial Compression Behavior of Square Hollow Sandwich Steel-Concrete Columns

Overview of the Literature

This paper by Huang Hong and Zhang Anguo from the School of Civil Engineering and Architecture at East China Jiaotong University, published in Railway Architecture in 2008, investigates the axial compression behavior of square hollow sandwich steel-concrete columns using finite element analysis with ABAQUS software. The sandwich column configuration consists of an outer square steel tube, an inner square steel tube, and a concrete core sandwiched between them. The study analyzes the full load-deformation response, examines the load-sharing mechanism between the outer tube, inner tube, and concrete, and investigates the influence of hollow ratio, nominal steel ratio, and material strength on the interaction forces.

Structural Configuration and Design Rationale

The sandwich steel-concrete column concept is an innovative structural system that combines the advantages of steel tubes and concrete in a composite configuration. Unlike conventional concrete-filled steel tube (CFST) columns, where concrete fills the entire interior of a single steel tube, the sandwich configuration introduces an inner steel tube that creates a hollow space within the concrete core. This hollow space can serve multiple purposes, including reduction of self-weight, provision of conduits for utilities, and creation of a thermal break in fire-exposed conditions.

The design rationale for this configuration is rooted in the complementary mechanical behaviors of steel and concrete under axial compression. Steel provides high tensile strength and ductility, while concrete provides compressive strength and confinement to the steel tubes. In the sandwich configuration, both the outer and inner tubes contribute to the confinement of the concrete core, creating a multi-layer confinement effect that can enhance the compressive strength and ductility of the concrete beyond what is achievable with a single tube.

Finite Element Modeling and Load-Deformation Analysis

The researchers developed a three-dimensional finite element model of the sandwich column using ABAQUS software. The model incorporated realistic material constitutive laws for both steel and concrete, including the confinement effect on concrete strength due to the lateral restraint provided by the steel tubes. The concrete was modeled using the Concrete Damage Plasticity (CDP) model, which captures both tensile cracking and compressive crushing behavior. The steel tubes were modeled using the von Mises yield criterion with isotropic hardening.

The load-deformation curves obtained from the finite element analysis showed good agreement with experimental results, validating the modeling approach. The full load-deformation response typically exhibited three stages:

1. Elastic stage: Linear load-deformation behavior where all components (outer tube, inner tube, concrete) deform elastically and share the load proportionally to their stiffness.
2. Plastic stage: Progressive yielding of steel tubes and microcracking of concrete, leading to a nonlinear load-deformation response. The concrete confinement effect becomes more pronounced as the steel tubes yield and begin to provide lateral restraint.
3. Post-peak stage: After the peak load, the concrete crushes progressively while the steel tubes continue to carry load through membrane action and arching effects. The rate of load degradation depends on the ductility of the steel tubes and the residual strength of the crushed concrete.

Load-Sharing Mechanism and Interaction Forces

One of the most valuable contributions of this study is the detailed analysis of how the load is distributed among the outer tube, inner tube, and concrete throughout the loading process. The researchers decomposed the total load into contributions from each component and analyzed the interaction forces between them.

At low load levels, the load distribution is primarily governed by the relative stiffness of each component. The steel tubes, having a much higher elastic modulus than concrete, carry a disproportionate share of the load. As the load increases and the steel tubes begin to yield, the concrete progressively takes on a larger share of the load. The interaction forces between the steel tubes and concrete arise from the Poisson effect: as the concrete is compressed axially, it tends to expand laterally, pressing against the steel tubes. The steel tubes, in turn, provide lateral confinement that increases the concrete's compressive strength.

The hollow ratio was found to have a significant effect on the interaction forces. A higher hollow ratio reduces the concrete cross-sectional area, which increases the stress in the remaining concrete and intensifies the confinement effect. However, this also reduces the total load-carrying capacity of the column. The nominal steel ratio, on the other hand, directly affects the confinement capacity: higher steel ratios provide greater lateral restraint and thus higher concrete confinement strength.

Influence of Design Parameters

The parametric study revealed several important trends:

• Hollow ratio: Increasing the hollow ratio from 0.2 to 0.5 reduced the peak load by approximately 15 to 25 percent but increased the ductility by 10 to 20 percent due to the enhanced confinement effect on the remaining concrete.
• Nominal steel ratio: Increasing the steel ratio from 0.1 to 0.3 increased the peak load by 20 to 35 percent and significantly improved the post-peak ductility.
• Concrete strength: Higher concrete strength increased the peak load but had a more modest effect on ductility, as the confinement effect became less dominant at higher strength levels.
• Steel yield strength: Higher steel yield strength improved both the peak load and the confinement capacity, with diminishing returns at higher strength levels.

Engineering Applications and Practical Considerations

The sandwich steel-concrete column concept is particularly attractive for applications where weight reduction is important, such as long-span bridges, high-rise buildings, and offshore structures. The hollow space can accommodate structural steel reinforcement, utility conduits, or be used for post-tensioning tendons. In railway bridge applications, which are the context of this paper's publication venue, the reduced self-weight of sandwich columns can lead to significant savings in foundation costs and improved seismic performance.

For welding engineers involved in fabricating sandwich steel-concrete columns, the inner tube-to-concrete interface presents unique challenges. The inner tube must be precisely positioned within the concrete pour to ensure uniform concrete coverage and avoid voids. Welding connections between the inner and outer tubes, if any, must be designed to accommodate differential thermal expansion during the concrete curing process. The welding sequence and procedure must also consider the residual stress state introduced by the welding operations, which can affect the column's buckling resistance.

From a quality control perspective, the sandwich column configuration introduces additional inspection requirements. The integrity of the concrete-concrete interface, the position of the inner tube, and the quality of any welding connections must all be verified. Non-destructive testing methods such as ultrasonic testing and radiographic testing may be required to confirm the absence of defects at critical interfaces.

In conclusion, this paper provides valuable insights into the mechanical behavior of sandwich steel-concrete columns under axial compression. The finite element analysis approach, validated against experimental data, offers a powerful tool for predicting the performance of these innovative structural elements. The parametric study results provide practical design guidance for optimizing the hollow ratio, steel ratio, and material strengths to achieve the desired balance between load capacity, ductility, and weight efficiency.