Local Bearing Performance of Rectangular-Right-Angle Hexagonal Steel Tube Concrete Columns

Literature Overview

Published in the Journal of Hunan University (Natural Science Edition) in 2023, this study investigates the local bearing performance of rectangular-right-angle hexagonal steel tube concrete (CHST) columns. Conducted at the College of Civil Engineering, Fuzhou University, under the support of the National Natural Science Foundation of China (No. 52178122), Fujian Provincial Natural Science Foundation (No. 2021J01603), and Quanzhou Science and Technology Plan Projects (No. 2021C015R, 2020N010s), the research addresses a relatively novel cross-sectional shape that combines the geometric advantages of hexagonal sections with the structural efficiency of composite construction.

Research Methodology and Test Program

The study involved 20 local bearing tests on CHST columns, systematically varying key geometric and material parameters. The test matrix was designed to investigate the influence of end plate thickness, local bearing area ratio, and steel ratio on the local bearing capacity and failure behavior. Finite element models were developed to provide mechanistic understanding of the load transfer mechanisms and constraint effects, followed by a parametric analysis to validate and extend the experimental findings.

The test parameters and their effects are summarized below:

Parameter	Range of Variation	Effect on Bearing Capacity
End plate thickness	Multiple levels	Positive correlation (higher thickness, higher capacity)
Local bearing area ratio	Multiple levels	Inverse correlation (larger ratio, lower capacity)
Steel ratio (steel-to-concrete area ratio)	Multiple levels	Minor effect
Concrete strength	Multiple levels	Positive correlation
Steel material strength	Multiple levels	Minor effect
Eccentricity	Multiple levels	Minor effect

Key Findings and Mechanism Analysis

The experimental results reveal several important relationships governing the local bearing behavior of CHST columns:

End plate thickness: The local bearing capacity is directly proportional to end plate thickness. The end plate serves as a load-spreading element that distributes the concentrated load over a larger area of the steel tube wall, thereby reducing stress concentrations and improving the overall load-carrying capacity.
Local bearing area ratio: The capacity is inversely proportional to the local bearing area ratio. A larger bearing area ratio implies a more concentrated load application relative to the column cross-section, leading to higher stress levels and earlier failure.
Load transfer mechanism: The finite element analysis demonstrates that the majority of the local bearing load is carried by the core concrete rather than the steel tube. This finding is consistent with the general principle in composite column design where concrete is the primary load-bearing material.
Constraint distribution: The steel tube constraint force is concentrated at the corners of the hexagonal section. This is attributed to the geometric configuration where corner regions experience more pronounced lateral confinement due to the angular geometry.
End plate effect on constraint: Specimens without end plates exhibit a greater steel tube constraint force on the core concrete. This counterintuitive result can be explained by the fact that without an end plate, the steel tube is free to deform laterally more significantly, thereby providing more effective confinement to the concrete.
Loading block shape: For specimens without end plates, the shape of the loading pad has a significant influence on the local bearing capacity, as it directly affects the stress distribution on the steel tube surface.
Constraint zone extent: The steel tube provides strong constraint to the core concrete within a range of 0 to 1.2B from the loading end, where B is the side length of the steel tube. Beyond this distance, the constraint effect diminishes rapidly.

Simplified Prediction Model

Based on the combined experimental and numerical findings, the authors proposed a simplified prediction model for the local bearing capacity of CHST columns. The model incorporates the key influencing parameters identified through the study: end plate thickness, local bearing area ratio, concrete strength, and steel ratio. While the specific formulation is detailed in the original paper, the model's development represents a practical contribution to the design of this novel composite column type.

Engineering Practice Integration

The research on hexagonal steel tube concrete columns addresses an emerging structural application where non-circular cross-sections are preferred for architectural or functional reasons. Hexagonal sections offer several advantages over circular sections, including better packing efficiency in certain structural arrangements, improved resistance to lateral forces in multiple directions, and potentially more efficient use of material. However, the non-circular geometry introduces complex stress states at the corners and flat faces, which necessitate thorough investigation of local bearing behavior.

From a fabrication standpoint, hexagonal steel tubes can be manufactured through cold rolling or hot forming processes. The corner radius and flat face dimensions are critical geometric parameters that influence both the structural performance and the manufacturing feasibility. Engineers should pay particular attention to the corner geometry when specifying hexagonal steel tubes for composite column applications, as the corner radius affects the stress concentration factor and the constraint effectiveness.

Key Questions and Reflections

The study raises several important considerations for practical engineering applications:

How does the hexagonal geometry affect the seismic performance of these columns? Local bearing behavior is closely related to the ductility and energy dissipation capacity under cyclic loading, which is critical for seismic design.
What are the recommended design limits for the local bearing area ratio and end plate thickness? The study provides empirical relationships, but practical design codes require clear limit states and safety factors.
How does the manufacturing tolerance of the hexagonal cross-section affect the local bearing performance? Deviations in corner radius or flat face length could influence the stress distribution and constraint effectiveness.
What is the optimal balance between steel ratio and concrete strength for cost-effective design? The study indicates that steel ratio and steel strength have minor effects on local bearing capacity, suggesting that concrete strength optimization may be more beneficial.

Study Insights and Implications

This research provides valuable foundational data for the structural design of hexagonal steel tube concrete columns, a cross-section that is gaining attention in modern construction. The finding that the core concrete carries the majority of the local bearing load reinforces the importance of concrete quality and strength in composite column design. The identification of the constraint zone extent (0 to 1.2B) offers practical guidance for determining the effective load distribution length in design calculations. The proposed simplified prediction model serves as a useful tool for preliminary design and code development. For engineers working on composite structures, this study highlights the need for shape-specific investigation rather than relying solely on design provisions developed for conventional circular or rectangular sections.