Experimental Study on High-Strength Conical Hollow Sandwich Thin-Walled Steel Tube Concrete Axial Compression Short Columns

Literature Overview

This study, published in the Journal of Xi'an University of Architecture and Technology in 2022 (Vol. 54, No. 2, pp. 306-316), presents a systematic experimental and numerical investigation into high-strength conical hollow sandwich thin-walled steel tube concrete (THSTW-CFDS) axial compression short columns. The research team, led by Chen Qingsheng from the State Grid Gansu Economic and Technical Research Institute and Pang Yahong from Xi'an University of Architecture and Technology, conducted tests on 10 specimens organized into 5 groups. The work was supported by the National Natural Science Foundation of China (Grant No. 51678474). The core innovation lies in combining conical geometry with a hollow sandwich configuration, using Q690 high-strength steel and C120 ultra-high-performance concrete, and exploring the effect of longitudinal stiffeners on the constraint efficiency and ultimate bearing capacity.

Core Technical Content and Test Parameters

The test matrix is designed to isolate the influence of three key parameters: the diameter-to-thickness ratio (d/t) of both inner and outer steel tubes, the hollowness ratio (the ratio of inner tube cross-sectional area to total cross-sectional area), and the presence or absence of longitudinal stiffeners. The material specifications are notably aggressive for their era — Q690 steel provides a yield strength of approximately 690 MPa, while C120 concrete delivers a compressive strength of roughly 120 MPa. These material combinations push the boundaries of conventional composite column design, where typical applications employ Q345 or Q460 steel with C40 to C60 concrete.

The hollowness ratios of 0.72, 0.82, and 0.85 represent a significant departure from solid steel tube concrete columns, where the void fraction is zero. At these high hollowness ratios, the structural behavior is dominated by the thin-walled sandwich interaction rather than the volumetric concrete confinement, which fundamentally changes the failure mechanism and load transfer path.

Key Experimental Findings

The experimental results reveal several important trends that have direct implications for engineering design. First, the ultimate bearing capacity decreases monotonically with increasing hollowness ratio and increasing diameter-to-thickness ratio of both the inner and outer tubes. This is physically intuitive: a higher hollowness ratio reduces the effective concrete volume and increases the slenderness of the outer tube wall, both of which diminish the column's load-carrying capacity. The diameter-to-thickness ratio directly governs the local buckling resistance of the steel tubes; as d/t increases, the tubes become more susceptible to local instability before the concrete reaches its full confinement potential.

The most significant finding concerns the effect of longitudinal stiffeners. When stiffeners are installed, the ultimate bearing capacity increases by approximately 7.2% at a hollowness ratio of 0.72, 7.5% at 0.82, and 10.9% at 0.85. This progressive enhancement with increasing hollowness ratio indicates that the stiffeners become increasingly critical as the structural system becomes more slender and the constraint effect of the concrete becomes more dependent on the integrity of the steel tube walls. At the highest hollowness ratio of 0.85, the outer tube wall is extremely thin relative to its diameter, and the stiffeners effectively prevent premature local buckling, allowing the concrete core to develop a more complete confinement response.

Finite Element Analysis and Failure Stages

The authors employed ABAQUS finite element software to conduct a full-process numerical simulation of the THSTW-CFDS axial compression short columns, validating the experimental observations and extending the analysis to the complete load-displacement response. The loading process is divided into three distinct stages: the elastic stage, the elastic-plastic stage, and the plastic descending stage.

During the elastic stage, all components — the outer tube, the inner tube, and the concrete core — deform elastically and bear load proportionally to their stiffness. As the axial load increases, the outer tube begins to yield first due to its larger cross-sectional area and the higher compressive stress it experiences from the direct axial load. The elastic-plastic stage follows, characterized by progressive yielding of the outer tube, followed by the inner tube, while the concrete remains largely elastic. In this stage, the concrete confinement effect begins to develop as the steel tubes expand laterally, exerting radial pressure on the concrete. The plastic descending stage is marked by the crushing of the concrete core and the progressive local buckling of the steel tube walls, leading to a reduction in load-carrying capacity.

The finite element model captures the complex interaction between the conical geometry and the hollow sandwich configuration. The conical shape introduces a geometric eccentricity effect, where the load path is not purely axial, and the stress distribution is non-uniform across the cross-section. This is a critical distinction from cylindrical steel tube concrete columns, where the stress distribution is more symmetric and predictable.

Bearing Capacity Formulation and Engineering Implications

The study proposes a bearing capacity calculation formula specifically applicable to THSTW-CFDS axial compression short columns with longitudinal stiffeners. The formula incorporates the nominal constraint effect coefficient, which quantifies the enhanced confinement provided by the steel tubes to the concrete core. The calculated results show good agreement with the experimental data, validating the theoretical framework.

From an engineering practice perspective, this research has several important implications. First, the use of Q690 steel and C120 concrete in a hollow sandwich configuration represents a material-efficient structural system that can achieve high load-carrying capacity with reduced material consumption. The hollow configuration reduces the weight of the column by eliminating the central concrete volume, which is particularly beneficial for tall structures where dead load is a critical design consideration. Second, the systematic demonstration that longitudinal stiffeners provide a 7-11% increase in bearing capacity at high hollowness ratios provides a clear design recommendation: for THSTW-CFDS columns with hollowness ratios exceeding 0.80, longitudinal stiffeners should be considered essential rather than optional.

However, several practical challenges remain. The fabrication of conical hollow sandwich columns requires precise welding of the inner and outer tubes to the conical shell, and the welding quality directly affects the structural integrity. The connection between the inner tube, outer tube, and concrete core must ensure reliable load transfer under both compression and potential lateral seismic loads. Additionally, the construction process for hollow sandwich columns is more complex than for solid columns, requiring careful sequencing of inner tube installation, concrete placement, and outer tube closure.

Study Insights and Reflections

This research contributes meaningfully to the understanding of high-strength composite column systems with unconventional geometries. The combination of conical shape, hollow sandwich configuration, and ultra-high-strength materials represents a frontier in structural engineering, and the systematic experimental approach provides a reliable data foundation for future design code development. The progressive enhancement of bearing capacity with stiffeners at higher hollowness ratios is a particularly valuable finding, as it provides a clear quantitative basis for design decisions regarding stiffener specifications.

One area that warrants further investigation is the seismic performance of THSTW-CFDS columns, as the current study is limited to monotonic axial compression loading. The conical geometry and hollow configuration may exhibit different ductility and energy dissipation characteristics under cyclic loading compared to conventional cylindrical columns. Additionally, the long-term behavior of C120 concrete under sustained high confinement pressure, including creep and shrinkage effects, should be studied to ensure the durability of these columns in practical applications.

In summary, this study demonstrates that high-strength conical hollow sandwich thin-walled steel tube concrete columns can achieve excellent axial compression performance, particularly when longitudinal stiffeners are incorporated, and the proposed bearing capacity formula provides a practical design tool that aligns well with experimental results, making this configuration a promising candidate for high-rise and heavy-load structural applications where material efficiency and structural performance must be simultaneously optimized.