Axial Compression Performance of Rectangular Steel Tube Concrete Columns with Built-in Irregular Stirrups

Literature Overview

This paper, published in the Journal of South China University of Technology (Natural Science Edition) (Vol. 52, Issue 5, 204, pp. 101-113) by Kang Lan, Chen Xuan, and Hong Shutao from South China University of Technology, investigates the axial compression mechanical properties of a novel rectangular steel tube concrete column incorporating built-in irregular stirrups. The research is supported by the National Natural Science Foundation of China (Grant 52178286) and the Guangdong Provincial Key Laboratory of Modern Civil Engineering Technology (Grant 2021B1212040003). The study addresses two key problems in rectangular steel tube concrete columns: inconsistent confinement between long and short sides, and insufficient confinement of the core concrete. Through experimental testing of 15 specimens and parametric analysis, the research demonstrates that redistributing steel from the tube wall to internal stirrups can effectively improve both axial compression capacity and ductility.

Core Technical Content and Methodology

Motivation and Innovation

The research identifies two fundamental problems with conventional rectangular steel tube concrete (SRC) columns:

Inconsistent confinement: In a rectangular steel tube, the confinement effect on the core concrete is stronger at the corners and weaker at the mid-span of the long sides, creating non-uniform concrete behavior.
Insufficient core confinement: The steel tube provides confinement primarily at the perimeter, leaving the core region of the concrete inadequately confined, particularly for large-section columns.

The proposed solution is to reduce the wall thickness of the rectangular steel tube and use the saved steel to fabricate irregular-shaped stirrups that are embedded within the core concrete. This redistribution of steel from the perimeter to the interior creates a more uniform confinement distribution throughout the concrete cross-section.

Experimental Program

The experimental program included 15 specimens divided into three groups:

Specimen Group	Quantity	Configuration
Built-in irregular stirrup SRC columns	11	Various parameters
Built-in track-shaped stirrup SRC columns	2	Comparison group
Conventional SRC columns	2	Baseline reference

The key parameters investigated include:

Bundle spacing (spacing between stirrup bundles)
Steel tube wall thickness
Concrete strength grade
Stirrup spacing
Stirrup diameter
Built-in steel ratio

Test Results and Failure Behavior

The axial compression process of the built-in irregular stirrup SRC columns was identified as having four distinct stages:

Stage	Description	Characteristics
Elastic stage	Linear elastic behavior	Proportional load-deformation relationship
Elastic-plastic stage	Initial yielding of steel components	Non-linear load-deformation, initial cracking
Plastic hardening stage	Strain hardening of confined concrete	Full plastic hardening, significant ductility
Descending stage	Post-peak strength degradation	Progressive failure, residual strength

A key finding is that the built-in irregular stirrup SRC columns exhibit a more complete plastic hardening stage compared to conventional SRC columns, indicating superior ductility and energy absorption capacity.

Interpretation of Technical Points

Confinement Mechanism Enhancement

The innovation of this research lies in the redistribution of steel from the tube wall to internal stirrups. In a conventional SRC column, the steel tube provides confinement primarily at the perimeter, with the confinement effect diminishing toward the core. By embedding irregular stirrups within the core concrete, the confinement effect is extended throughout the entire cross-section. The irregular shape of the stirrups is designed to conform to the geometry of the core region, providing more effective confinement than conventional rectangular stirrups.

Steel Ratio Optimization

The research demonstrates that with the total steel ratio held constant, reducing the tube wall thickness and increasing the internal stirrup steel content improves both capacity and ductility. This finding challenges the conventional design approach that maximizes tube wall thickness for confinement. The optimal steel distribution appears to be a balance between perimeter confinement (provided by the tube) and core confinement (provided by the internal stirrups).

Confinement Concrete Constitutive Model

The researchers employed an existing confined concrete constitutive model to derive the axial compression capacity formula. The model accounts for the increased strength and ductility of concrete under multi-axial compression, with the confining pressure determined by the combined action of the steel tube and the internal stirrups. The derived formula provides a practical design tool for predicting the axial compression capacity of the proposed column type.

Comparison with Conventional SRC Columns

The comparison specimens (conventional SRC columns and track-shaped stirrup columns) provide valuable benchmarks. The results show that:

Built-in irregular stirrup columns outperform conventional SRC columns in both capacity and ductility when the total steel ratio is the same.
The irregular stirrup configuration is more effective than the track-shaped stirrup configuration, likely due to better conformity with the core geometry and more uniform confinement distribution.
The plastic hardening stage is significantly more developed in the irregular stirrup columns, indicating superior energy absorption capacity.

Engineering Practice Implications

Design Guidelines

The research provides the following practical design guidelines for engineers:

Steel redistribution: When designing rectangular SRC columns, consider redistributing steel from the tube wall to internal stirrups to improve core confinement and overall ductility.
Stirrup geometry: The irregular stirrup shape should be designed to conform to the core concrete geometry for maximum confinement effectiveness.
Parameter optimization: The key parameters (stirrup spacing, stirrup diameter, bundle spacing) should be optimized based on the specific column dimensions and loading conditions.
Capacity calculation: The derived axial compression capacity formula can be used for preliminary design, with appropriate safety factors applied for final design.

Manufacturing and Construction Considerations

From a steel pipe and structural fabrication perspective, several practical considerations arise:

Steel tube fabrication: The reduced wall thickness requires precise control of the steel tube manufacturing process to ensure dimensional accuracy and surface quality.
Stirrup fabrication: The irregular stirrup shapes require specialized fabrication techniques, potentially involving bending and welding operations.
Assembly process: The internal placement of stirrups within the steel tube requires careful construction sequencing, with the stirrups positioned before concrete placement.
Quality control: Inspection of the internal stirrup placement and welding connections is essential to ensure proper composite action.
Concrete placement: The presence of internal stirrups may affect concrete flow and consolidation, requiring appropriate concrete mix design and placement techniques.

Key Questions and Reflections

The research raises several important questions for further investigation:

How does the proposed column type perform under combined axial compression and bending, which is more representative of actual structural loading?
What is the effect of the welding connections between the internal stirrups and the steel tube on the overall performance?
How does the column behave under cyclic loading conditions relevant to seismic design?
What are the long-term effects of concrete creep and shrinkage on the confinement effectiveness of the internal stirrups?
Can the findings be extended to other cross-section shapes, such as square or circular steel tube concrete columns?
What is the cost-effectiveness of the proposed approach compared to conventional SRC columns, considering both material costs and construction labor?

The experimental database of 15 specimens provides a solid foundation for the proposed design formula, but further parametric studies through finite element analysis could expand the applicable range and provide additional insights into the failure mechanisms.

Study Insights and Implications

This research presents a novel and practical approach to improving the axial compression performance of rectangular steel tube concrete columns. The key insight is that the distribution of steel within the column cross-section is as important as the total amount of steel. By redistributing steel from the tube wall to internal stirrups, the confinement effect is enhanced throughout the entire cross-section, leading to improved capacity and ductility. The four-stage failure behavior identified in the tests provides a clear understanding of the structural response and can inform design for different performance levels. For steel pipe manufacturers and structural engineers, the practical implication is that thinner-walled steel tubes with internal stirrup reinforcement can achieve superior performance compared to conventional thick-walled tubes, potentially offering cost savings and improved structural performance. The derived capacity formula provides a practical design tool that can be integrated into existing design codes with appropriate validation and safety factors.