Sensitivity Analysis of Mechanical Properties of Circular Steel Tube Recycled Concrete Columns via Orthogonal Design and Finite Element Simulation

Literature Overview

This paper by Dong Suqin, Wei Xianzhong, and Gao Bing (2018), published in Progress in Steel Building Structures, investigates the influence of slenderness ratio, steel ratio, and recycled aggregate replacement rate on the mechanical behavior of circular steel tube recycled concrete columns. The study employs both orthogonal experimental design and finite element (FE) simulation, combining nine full-scale axial compression tests with numerical analysis to identify the most sensitive parameters governing strain distribution and deflection behavior. The work is directly relevant to engineers working on composite steel-concrete structures where recycled aggregates are substituted for natural aggregates to meet sustainability targets.

Core Technical Content

The experimental matrix is constructed using an L9 orthogonal array, with three factors each at three levels:

The nine specimens were subjected to monotonic axial loading until failure. Strain gauges were placed on both the steel tube and the concrete core at multiple heights to capture the composite action. The FE model was validated against the experimental load-displacement curves, showing good agreement in both the elastic and post-peak stages.

The key finding is the hierarchy of sensitivity: slenderness ratio exerts the greatest influence on both strain and deflection, followed by steel ratio, while the recycled aggregate replacement rate has the smallest effect. This is somewhat counterintuitive for practitioners who might expect the replacement rate to dominate, but the explanation lies in the fact that the steel tube provides significant confinement that partially compensates for the lower strength and stiffness of recycled aggregate concrete.

Process and Standards Analysis

The study derives a specific formula for the axial compressive bearing capacity of circular steel tube recycled concrete columns. The formula accounts for the composite action between the steel tube and the recycled aggregate concrete core, incorporating a confinement coefficient that reflects the effective lateral restraint provided by the thin-walled steel tube.

From a standards perspective, this work bridges the gap between GB 50017 (Standard for Design of Steel Structures), which provides design methods for conventional concrete-filled steel tube columns, and the emerging need to incorporate recycled aggregate concrete into structural design. The Chinese standard GB/T 25177 (Technical Specification for Recycled Concrete) defines the properties of recycled aggregate concrete but does not provide specific design provisions for composite columns. This paper's formula offers a practical tool for designers working on sustainable structural projects.

The FE simulation approach is particularly valuable because it allows parametric studies beyond the nine physical specimens. Engineers can extend the analysis to intermediate levels of each factor, explore different steel tube geometries (e.g., elliptical or square cross-sections), and evaluate the effect of loading eccentricity.

Integration with Engineering Practice

In practice, the finding that slenderness ratio is the most critical factor reinforces the well-established principle that slender columns are more susceptible to buckling, and the composite action between steel and concrete becomes less effective as the slenderness increases. For recycled concrete columns with slenderness ratios exceeding 10, additional lateral bracing or increased wall thickness of the steel tube should be considered.

The relatively minor effect of replacement rate suggests that recycled aggregate concrete can be used in composite columns with moderate confidence, provided the slenderness ratio is controlled. However, this conclusion is limited to the replacement rates studied (up to 100%). For applications requiring very high recycled content, additional testing at higher replacement rates and with different concrete strengths would be prudent.

Key Questions and Reflections

A critical question arises: how does the sensitivity hierarchy change when the steel tube is subjected to local buckling? The study focuses on global behavior (strain and deflection), but in practice, local buckling of the steel tube can govern the failure mode, especially for thick-walled tubes or under high confinement pressures. Future work should incorporate local buckling checks into the FE model.

Another concern is the long-term durability of recycled aggregate concrete within steel tubes. The permeability and porosity of recycled aggregate concrete are typically higher than those of natural aggregate concrete, which could accelerate corrosion of the steel tube in the event of a breach in the protective coating. The paper does not address this issue, and engineers should exercise caution when extrapolating the results to environments with aggressive exposure conditions.

Study Insights and Implications

The orthogonal experimental design approach is efficient and well-suited for identifying the dominant factors among multiple variables with limited test specimens. This methodology can be readily applied to other composite structural members, such as steel-reinforced concrete beams or hybrid steel-concrete slabs. The FE simulation complementing the experimental work is essential for validating the model and extending the parametric study beyond the tested matrix.

Overall, this paper provides a valuable foundation for the rational design of recycled concrete-filled steel tube columns, offering both empirical data and a practical design formula. Engineers should note that the conclusions are specific to circular cross-sections and axial loading; the behavior under combined loading (axial force plus bending) and for non-circular cross-sections requires further investigation. The work contributes meaningfully to the sustainable construction agenda by demonstrating that recycled aggregate concrete can perform adequately in composite columns when the slenderness ratio is properly controlled.