Experimental Study on Axial Compressive Bearing Capacity of Double Steel Tube High-Strength Concrete Columns

Literature Overview

The paper authored by Zhang Chunmei, Yin Yi, and Zhou Yun from Guangzhou University published in 2004 in the Journal of Guangzhou University (Natural Science Edition) presents an experimental investigation into the axial compressive behavior of double steel tube high-strength concrete (DST-HSC) columns. The study examined specimens with two different steel ratios and varying steel tube wall thicknesses, subjecting them to static axial compression loading until failure. The primary objectives were to observe the failure process and characteristics, analyze the load-bearing mechanism, and propose an approximate calculation formula for axial compressive bearing capacity.

Core Technical Content and Experimental Setup

The experimental program involved DST-HSC columns where two concentric steel tubes were filled with high-strength concrete, creating a composite structural system that differs fundamentally from conventional single steel tube concrete (CFST) columns. The steel ratio and tube wall thickness were the two key variables studied, as these parameters directly govern the load-sharing mechanism between the steel and concrete components.

The specimens were designed to represent realistic engineering configurations where high bearing capacity is required in space-constrained conditions, such as high-rise building cores and heavy industrial equipment supports.

Failure Process and Mechanical Behavior

The failure process of DST-HSC columns under axial compression typically proceeds through several distinct stages. In the elastic stage, both the steel tubes and the concrete core deform uniformly, with the load distributed according to the relative stiffness of each component. As the load increases and the concrete enters the plastic range, the outer and inner steel tubes begin to provide progressive confinement to the concrete core, delaying cracking and spalling.

At the ultimate stage, the failure mode is characterized by progressive local buckling of the outer steel tube, followed by loss of confinement and eventual crushing of the concrete core. The inner steel tube continues to bear load even after the outer tube has yielded, providing a secondary load-bearing mechanism that contributes to the overall ductility of the column. This sequential failure mechanism is one of the key advantages of the double tube configuration over single tube systems.

The load-bearing mechanism can be understood through the interaction between the steel tubes and the concrete. The concrete core, confined by both the inner and outer tubes, experiences a triaxial compressive stress state that significantly enhances its compressive strength compared to unconfined concrete. The outer tube also provides lateral restraint against the inner tube, preventing premature inward buckling of the inner tube under high axial loads.

Bearing Capacity Calculation Method

The authors proposed an approximate calculation formula for the axial compressive bearing capacity of DST-HSC columns. The formula generally follows the superposition principle with confinement enhancement factors, accounting for:

1. The direct bearing contribution of the outer steel tube.
2. The direct bearing contribution of the inner steel tube.
3. The enhanced concrete bearing capacity due to the combined confinement effect of both tubes.

The confinement enhancement is typically expressed through a modified concrete strength term that incorporates the steel ratio, tube thickness, and geometric parameters. The formula provides a practical tool for preliminary design, though it should be validated against experimental data for specific applications.

Engineering Practice Insights

From a manufacturing and engineering perspective, the double steel tube configuration introduces several practical considerations. The fabrication of DST-HSC columns requires precise concentricity between the inner and outer tubes to ensure uniform concrete filling and avoid voids that could compromise structural integrity. The welding of end plates or connection details to both tubes must be carefully controlled to prevent distortion that could affect the column's verticality and load path.

The use of high-strength concrete in these columns demands attention to concrete placement and compaction within the confined space between the two tubes. Vibration during placement must be sufficient to achieve full compaction but controlled to avoid damaging the inner tube. The shrinkage and creep behavior of high-strength concrete within the confined geometry also warrants consideration in long-term performance assessments.

The study provides valuable data for designers seeking to maximize bearing capacity in situations where the structural cross-section is limited. The double tube approach effectively increases the steel ratio without proportionally increasing the overall column dimensions, making it suitable for applications such as transfer columns in high-rise buildings, equipment foundations in industrial plants, and bridge piers where space is at a premium.

Study Reflections and Implications

This research contributes meaningfully to the understanding of composite column behavior by systematically isolating the effects of steel ratio and tube wall thickness on bearing capacity. The proposed calculation formula, while approximate, offers a practical design tool that bridges the gap between theoretical analysis and engineering application. Future work should extend these findings to include cyclic loading conditions, fire resistance, and long-term durability, as these factors are critical for real-world structural applications. The experimental data also serves as a benchmark for validating finite element models of DST-HSC columns, which can subsequently be used for parametric studies covering a wider range of design variables.