Axial Compression Performance of Double Square Steel Tube Confinement Concrete Columns

Literature Overview

This study by Chen Zongping and Ning Fan from Guangxi University investigates the axial compression behavior of a novel composite column system that combines an internally inclined square steel tube with an externally upright square steel tube, both confining concrete. Published in 2022 in the journal Industrial Construction, the paper reports on nine test specimens subjected to concentric compression loading. The research is funded by the National Natural Science Foundation of China and several Guangxi provincial programs, indicating its significance in the context of advanced structural systems for seismic and high-load applications.

Core Technical Parameters and Test Design

The experimental program varied several key parameters to systematically evaluate their influence on structural performance. The following table summarizes the principal design variables and their ranges:

A reference specimen (SC1) with a conventional single square steel tube was included for baseline comparison. The steel ratio was kept approximately constant between the composite columns and the reference to ensure a fair comparison of confinement effectiveness.

Key Experimental Findings

The failure modes of the composite columns exhibited two distinct characteristics during the plastic stage: overall bending instability and local buckling. The inner inclined tube developed continuous, progressive wrinkles under the combined action of core concrete and interstitial concrete, while the outer upright tube bulged outward due to lateral concrete pressure. De-bonding zones between the inner and outer tubes showed varying degrees of outward bulging, and larger de-bonding regions had a pronounced influence on the ultimate failure pattern.

The most striking result was a 35% increase in load-bearing capacity for the composite columns compared to the reference SC1 specimen at comparable steel ratios. Furthermore, the load-displacement curves of the composite columns exhibited no descending branch, demonstrating excellent strain-hardening behavior. This absence of a descending branch is a critical indicator of superior ductility and energy dissipation capacity, which is of paramount importance in seismic design.

Influence of Individual Parameters

Increasing the inner tube side length led to improvements in both initial stiffness and peak load. However, increasing the wall thickness of either the inner or outer tube did not proportionally increase the peak load, likely due to non-uniform concrete mixing effects that reduced the effective confinement efficiency. Despite this, thicker walls significantly improved post-peak mechanical behavior, confirming that wall thickness primarily governs ductility rather than strength.

When the slenderness ratio increased to 6, second-order effects became dominant, altering the failure mode and causing degradation in several mechanical indicators. The study also found that, at comparable steel ratios, increasing the core column area ratio better exploits the complementary strengths of steel and concrete materials.

Analytical Model and Engineering Relevance

The authors proposed a calculation method that accounts for the confinement effects of both inner and outer tubes on the enclosed concrete. The analytical predictions showed good agreement with experimental results, validating the proposed approach. For engineering practice, this dual-confinement system offers a viable solution for columns requiring high axial capacity and ductility, particularly in high-seismicity regions where post-yield performance is critical.

Study Insights and Implications

The inclined configuration of the inner tube is an innovative design feature that creates a wedge-like interlocking mechanism between the two tube layers, enhancing the transfer of lateral confining pressure. From a manufacturing and welding perspective, the fabrication of such a system would require precise control of the inner tube geometry, careful joint design between inner and outer tubes, and rigorous quality control of the concrete placement process to avoid the non-uniformity issues noted in the study. The finding that wall thickness primarily improves ductility rather than peak strength has direct implications for economic design optimization. This research provides valuable data for the development of design codes for hybrid confined concrete columns and opens new possibilities for lightweight, high-capacity structural systems.