Local Compression Behavior of Rectangular Hexagonal Steel Tube Concrete Columns

Literature Overview

This 2023 paper published in the Journal of Hunan University (Natural Science) by Wang Zhibin, Dai Hongwei, Wang Bingkun, and Li Dong from Fuzhou University investigates the local compression (bearing plate) behavior of composite columns with a novel rectangular hexagonal cross-section steel tube filled with concrete. The research was supported by the National Natural Science Foundation of China (No. 52178122) and provincial-level funding sources. Twenty experimental specimens were tested under local compression loading, and finite element analyses were conducted to elucidate the load-bearing mechanisms. The study concludes with the proposal of a simplified predictive model for local compression bearing capacity.

Background and Motivation

In structural engineering, steel tube concrete (STC) columns are widely used in high-rise buildings, bridges, and industrial structures because of their high strength-to-weight ratio and excellent ductility. Traditional STC columns employ circular or square/rectangular cross-sections. The rectangular hexagonal cross-section studied here offers a hybrid geometry that combines the corner confinement advantages of hexagonal shapes with the flat face characteristics of rectangular sections. This geometry is particularly interesting for applications requiring connection plates at specific angles, such as truss joints or moment-resisting frames.

The local compression behavior of STC columns is a critical design consideration because the actual load transfer at beam-column connections rarely involves full-section bearing. Instead, loads are transmitted through bearing plates or connection plates that cover only a fraction of the column cross-section. Understanding how the steel tube, concrete core, and connection plate interact under local compression is essential for rational connection design and for avoiding premature local buckling or concrete crushing.

Experimental Program and Key Parameters

Specimen Configuration

The twenty specimens varied in three primary parameters:

The local compression area ratio is defined as the ratio of the loaded area to the total cross-sectional area of the column. A lower area ratio means a more concentrated load, which is more critical from a design perspective.

Key Experimental Findings

1. End plate thickness effect: The local compression bearing capacity is directly proportional to the end plate thickness. Thicker end plates distribute the load more effectively over the cross-section, reducing stress concentrations at the loaded region.
2. Local compression area ratio effect: The bearing capacity is inversely proportional to the area ratio. A smaller loaded area concentrates stress and leads to lower overall capacity due to the reduced load-spreading zone.
3. End plate presence: The presence of an end plate significantly improves the bearing capacity compared to specimens without end plates. This is because the end plate acts as a load-spreading mechanism, transferring the concentrated load to a larger portion of the concrete-steel composite cross-section.

Finite Element Analysis and Mechanism Interpretation

The numerical analysis provides deeper insight into the load-bearing mechanisms:

Load Transfer Path

The analysis reveals that the majority of the applied load is carried by the concrete core rather than the steel tube. This is consistent with the high compressive strength of concrete relative to the relatively thin steel tube walls. However, the steel tube plays a crucial role in confining the concrete, particularly at the corners.

Confinement Distribution

The steel tube confinement force is concentrated primarily at the corners of the hexagonal section. This is a geometric consequence of the cross-section shape: at corners, the steel tube walls meet at angles that create a natural stress concentration and enhanced confinement effect on the adjacent concrete. In contrast, the flat faces provide less effective confinement because the tube walls are parallel to the loading direction and cannot develop lateral restraint forces as efficiently.

Load-Spreading Zone

The analysis identifies a critical load-spreading zone extending from 0 to 1.2B (where B is the side length of the steel tube) from the loaded end. Within this zone, the steel tube exerts strong confinement on the core concrete. Beyond this depth, the confinement effect diminishes significantly. This finding has direct implications for connection design: the load-spreading plate thickness should be designed to cover at least this critical zone to ensure effective load distribution.

Effect of Loading Pad Shape

For specimens without end plates, the shape of the loading pad has a substantial influence on the local compression bearing capacity. A pad with a geometry that matches the cross-section contour provides better load distribution than a simple rectangular pad. This finding underscores the importance of connection detail design in local compression-critical applications.

Simplified Predictive Model

The authors propose a simplified predictive model for the local compression bearing capacity of rectangular hexagonal STC columns. The model incorporates the key parameters identified through testing and analysis:

• Concrete compressive strength
• Steel tube material properties
• End plate thickness
• Local compression area ratio
• Steel ratio

The model captures the observed trends: capacity increases with concrete strength and end plate thickness, and decreases with increasing area ratio. The influence of steel ratio, steel grade, and eccentricity on local compression capacity is found to be relatively minor, which simplifies the design calculations.

Engineering Practice Integration

From a practical design perspective, this study has several important implications:

1. Connection plate design: The 1.2B load-spreading zone provides a quantitative basis for determining the minimum bearing plate dimensions. Engineers can use this criterion to design connection plates that ensure adequate load distribution without excessive material usage.
2. End plate recommendation: The significant improvement in capacity provided by end plates justifies their inclusion in most local compression-critical connections, even when the added cost is considered.
3. Cross-section selection: The rectangular hexagonal cross-section offers a viable alternative to conventional shapes, particularly where connection geometry constraints favor hexagonal alignment. The corner concentration of confinement forces means that corner regions of the column cross-section are the most effective for load transfer.
4. Code compliance: The simplified predictive model can be used as a basis for developing design provisions in relevant standards such as GB 50017 (Steel Structure Design Code) or JGJ 138 (Technical Specification for Concrete-Filled Steel Tubular Structures).

Key Questions and Reflections

The study raises several important questions for future research. First, the interaction between local compression and global buckling has not been addressed. In real structures, columns are subject to both local and global loads, and the interaction between local compression-induced damage and global stability needs to be understood. Second, the cyclic loading behavior under local compression is not investigated, which is a concern for seismic applications where columns experience repeated local compression from beam-column connections. Third, the effect of concrete strength grades beyond typical structural concrete (e.g., ultra-high performance concrete) on the confinement mechanism is not explored. Finally, the study focuses on static loading, and the dynamic response of these columns under impact or blast loading remains unknown.

Study Insights and Implications

This paper makes a meaningful contribution to the understanding of local compression behavior in STC columns with non-conventional cross-sections. The experimental data, supported by finite element analysis, provide a comprehensive picture of the load-bearing mechanisms and the influence of key design parameters. The proposed simplified predictive model offers a practical tool for engineers to estimate local compression capacity without resorting to complex numerical analyses. The finding that the load-spreading zone extends to 1.2B is particularly valuable for connection design, as it provides a clear geometric criterion for bearing plate sizing. For engineers working on STC structure design, this study reinforces the importance of connection detail design and the potential benefits of non-conventional cross-section geometries. The rectangular hexagonal cross-section represents an innovative approach that merits further investigation and potential standardization.