Local Compression Mechanical Properties of Round-End Steel Tubular Concrete Columns

Literature Overview

The paper by Liu Huadong and Tang Yongqiang (2025), published in Journal of Shenyang Ligong University (Vol. 44, No. 5, pp. 74-81), investigates the local compression mechanical behavior of round-end steel tubular concrete (STC) columns using finite element analysis (FEA) with ABAQUS software. This research, supported by the Liaoning Provincial Department of Education Basic Research Project (JYTMS20231591), addresses a specific but important structural engineering problem: the behavior of STC columns subjected to concentrated local loads, which is a common loading condition in multi-story buildings, industrial facilities, and water supply infrastructure.

Core Technical Concept

The round-end STC column configuration refers to a steel tubular concrete member where the cross-section transitions from a circular steel tube to a rectangular or square cross-section through rounded (filleted) corners. This geometry is advantageous because it:

• Reduces stress concentration at the transition zone compared to sharp-cornered rectangular sections.
• Facilitates fabrication through rolling and welding processes.
• Provides a more uniform confinement effect on the concrete core compared to fully rectangular sections.

The local compression scenario involves a concentrated load applied over a portion of the column cross-section, which is typical in structural connections where beam-column joints transfer loads to the column through a limited contact area. This loading condition creates a complex stress distribution that includes:

• Direct compressive stress under the loaded area.
• Stress diffusion (Boussinesq-type) into the un-loaded portions of the cross-section.
• Interaction between the steel tube and the concrete core through interface contact and bond.
• Potential local buckling of the steel tube under concentrated load.

Finite Element Modeling Approach

The ABAQUS-based FEA model captures several critical aspects of the local compression behavior:

• Contact mechanics: The model incorporates contact elements between the steel tube and the concrete core to simulate the interface interaction, including friction and potential separation under tensile stresses.
• Material nonlinearity: Both the steel tube and the concrete core are modeled with nonlinear material constitutive laws. The concrete model accounts for confinement effects through a modified stress-strain relationship that depends on the lateral confinement pressure from the steel tube.
• Geometric nonlinearity: Large deformation analysis is employed to capture the geometric changes that occur under significant local compression, including the bulging of the steel tube and the crushing of the concrete core.
• Parameter variation: The model is used to systematically vary key parameters to quantify their influence on the local compression behavior.

Parameter Study Results

The parametric analysis reveals several important trends:

Effect of Cross-Section Geometry

The round-end cross-section exhibits a non-uniform constraint distribution. The curved (arc) segments of the cross-section provide stronger confinement on the concrete core compared to the flat segments, resulting in higher contact forces at the curved portions. Furthermore, the contact force distribution varies along the column height, with the highest contact forces occurring near the loaded end and decreasing progressively toward the free end.

Effect of Local Compression Area Ratio

The local compression area ratio (ratio of loaded area to total cross-section area) has a significant and nonlinear effect on the local compression bearing capacity:

The substantial reduction in bearing capacity with increasing area ratio indicates that the stress diffusion mechanism in round-end STC columns is limited. The concrete core cannot effectively redistribute concentrated loads over the full cross-section, particularly when the loaded area is large relative to the total cross-section. This finding has direct implications for the design of column-beam connections in STC structures.

Effect of Steel Yield Strength

The steel yield strength has a significant effect on the bearing capacity but minimal effect on the initial stiffness. This is because the initial stiffness is primarily governed by the elastic modulus of the steel (which is approximately constant across different grades) and the elastic modulus of the concrete, while the bearing capacity is directly related to the yield strength of the steel tube.

Effect of Concrete Compressive Strength

The concrete compressive strength has a relatively modest effect on the bearing capacity compared to the steel yield strength, but it produces a more pronounced effect on the stiffness. This is because the concrete core contributes significantly to the initial stiffness through its elastic modulus, while the steel tube dominates the post-yield bearing capacity.

Effect of Steel Ratio

The steel ratio (ratio of steel cross-sectional area to total cross-sectional area) has the most dramatic effect on both the bearing capacity and the stiffness. This is because increasing the steel ratio simultaneously increases the direct load-carrying capacity of the steel tube and the confinement effect on the concrete core, creating a synergistic strengthening mechanism.

Engineering Practice Implications

The findings of this study have several important implications for the structural design of round-end STC columns:

1. Local compression design provisions: The significant reduction in bearing capacity with increasing local compression area ratio indicates that existing design codes for local compression in STC members may need revision. Engineers should apply appropriate reduction factors for concentrated loads, particularly when the loaded area exceeds 1.44 times the effective bearing area.
2. Steel ratio optimization: The dramatic improvement in bearing capacity with increasing steel ratio suggests that for applications requiring high local compression resistance (such as column-beam connections in industrial buildings), a higher steel ratio should be considered despite the increased material cost.
3. Material selection strategy: For local compression applications, the steel yield strength should be prioritized over the concrete compressive strength, as the former has a more significant effect on the bearing capacity. However, for stiffness-sensitive applications, higher concrete strength should be selected.
4. Cross-section geometry considerations: The non-uniform confinement distribution in round-end cross-sections should be accounted for in the design of connections and local reinforcement. The curved segments provide superior confinement and should be oriented to receive the primary load transfer.

Key Reflections

This study provides valuable quantitative data on the local compression behavior of round-end STC columns, which is a topic that has received relatively limited attention in the structural engineering literature. The parametric analysis reveals that the steel ratio and local compression area ratio are the most critical design parameters, while the concrete strength has a relatively modest influence on the bearing capacity. The finding that the steel yield strength has negligible effect on the initial stiffness but a significant effect on the bearing capacity is particularly important for the design of connections that must satisfy both stiffness and strength requirements. Engineers involved in the design of STC structures should incorporate these findings into their local compression design procedures, particularly for applications involving concentrated loads from beam-column joints, crane rails, or equipment foundations.