Ultimate Bearing Capacity of Prefabricated Circular Semi-Continuous Steel Tube Concrete Columns

Literature Overview

Published in the Journal of Anhui University of Engineering, Vol. 40, Issue 4, 2025 (pages 66-73), this paper by Qian Huashan, Xie Lei, Song Yanwei, Zhu Longfei, and Cao Bing presents the results of 10 eccentric compression tests on prefabricated circular semi-continuous steel tube concrete (CFST) columns. The research is supported by the Anhui Provincial Natural Science Foundation (2408085ME122) and the Anhui Provincial Key Research Project of Natural Science in Higher Education Institutions (KJ2021A0504). The study introduces a multi-component coupled system analysis method to characterize failure modes and predict ultimate bearing capacity.

Core Technical Innovation: Multi-Component Coupled System Analysis

The multi-component coupled system analysis method represents a significant methodological contribution to the field of CFST structural analysis. Rather than treating the column as a monolithic element, this approach decomposes the structure into identifiable components and analyzes their individual behaviors and interactions:

Component Classification

Load Transfer Path

The study identifies the load transfer sequence as: upper steel tube → bolts → outer sleeve steel tube → lower steel tube. This sequential transfer mechanism is critical for understanding failure initiation and propagation in the prefabricated column.

Test Results and Failure Mode Analysis

Three Primary Failure Modes

The experimental results reveal three distinct failure modes:

1. Local buckling: Outward buckling of the steel tube wall in the confined concrete region, typically occurring at low eccentricity ratios where axial compression dominates
2. Overall buckling accompanied by local buckling: A combined mode where global column instability initiates with concurrent local wall buckling, occurring at intermediate eccentricity ratios
3. Instability failure: Sudden loss of load-carrying capacity due to connection failure or progressive local buckling, occurring at high eccentricity ratios

Bearing Capacity Prediction Performance

The proposed prediction method demonstrates excellent accuracy:

Steel Tube Manufacturing and Connection Quality Considerations

From a steel pipe manufacturing and welding engineering perspective, this prefabricated CFST column system raises several critical technical considerations:

Steel Tube Requirements

The circular steel tubes used in this system must meet stringent manufacturing requirements:

Bolted Connection Design and Quality

The bolt-outer sleeve connection is the critical weak link in the prefabricated system. Key considerations include:

• Bolt grade: High-strength bolts (Grade 8.8 minimum, preferably 10.9) are required for the connection to ensure adequate shear and bearing capacity
• Bolt hole alignment: The concentricity of bolt holes in the upper tube, outer sleeve, and lower tube must be maintained within ±1 mm to prevent eccentric loading
• Contact surface preparation: Machined or flame-cut contact surfaces must be flat within 0.5 mm over any 100 mm length to ensure uniform bearing pressure distribution
• Pre-tension control: Bolt pre-tension must be controlled to 70-80% of proof load to ensure slip-critical connection behavior

Welding Considerations (if applicable)

In some prefabricated CFST column designs, field welding may be used for the outer sleeve connection. If welding is employed:

• Procedure qualification: Welding procedures must be qualified per GB/T 9948 or AWS D1.1
• Preheating: Required for thick-walled tubes (t > 25 mm) or high-carbon equivalent materials (CEV > 0.45)
• Post-weld inspection: UT or MT inspection of all field welds is mandatory to detect lack of fusion, cracks, and porosity
• Residual stress management: Post-weld stress relief or controlled cooling to minimize through-thickness residual stresses

Multi-Component Coupled Analysis: Technical Methodology

The multi-component coupled system analysis method can be understood through the following framework:

Analysis Steps

1. Component identification: Decompose the column into upper tube, lower tube, outer sleeve, bolts, and core concrete
2. Individual component analysis: Determine the load-deformation behavior of each component independently
3. Coupling relationship establishment: Define the interaction conditions between adjacent components (contact, friction, constraint)
4. System-level assembly: Combine component behaviors through equilibrium and compatibility conditions
5. Failure mode identification: Determine which component reaches its limit first and how failure propagates

Engineering Significance

This methodology is particularly valuable for prefabricated structures because:

• It identifies the weakest component (typically the bolt connection or the outer sleeve)
• It quantifies the load redistribution after component failure
• It enables targeted strengthening of critical components without over-designing the entire system

Engineering Practice Integration

Quality Control Checklist for Prefabricated CFST Columns

FMEA Analysis for Prefabricated Connection

Key Questions and Reflections

Several technical questions emerge from this research:

1. Seismic performance: The study focuses on eccentric compression (quasi-static). Under seismic cyclic loading, the bolted connection may exhibit fatigue degradation and slip, fundamentally changing the load transfer mechanism. The connection's energy dissipation capacity and ductility need separate investigation.
2. Concrete filling quality in field conditions: The laboratory tests assume full concrete filling with high compaction. In field conditions, achieving uniform concrete filling in a vertical prefabricated column is challenging, particularly for tall columns. Void formation would significantly reduce the confinement effect.
3. Long-term behavior: Creep and shrinkage of concrete, combined with bolt relaxation, may alter the load distribution in the prefabricated connection over time. The long-term performance of the bolted connection under sustained loads warrants investigation.
4. Scale effect: The test specimens likely represent medium-scale columns. Full-scale columns (height > 3 m) may exhibit different buckling behavior due to amplified geometric imperfections and second-order effects.

Study Insights and Outlook

This research makes a significant contribution to the understanding of prefabricated CFST column behavior through the innovative application of multi-component coupled system analysis. The method provides a systematic framework for identifying critical components and predicting failure sequences, which is directly applicable to design and quality assurance in prefabricated structural systems.

For steel pipe manufacturers and structural engineers, the key insight is that the prefabricated connection (bolt-outer sleeve assembly) is the governing weak link. Design and quality control efforts should be concentrated on ensuring the reliability of this connection, including bolt material quality, hole alignment, surface preparation, and pre-tension control. The conservative prediction method (mean ratio 0.970) provides adequate safety margins for design application, validating the engineering feasibility of this prefabricated system.

The broader implication is that modular, prefabricated structural systems can achieve performance comparable to monolithic construction when the connection design is properly engineered and the quality control system is rigorously implemented. This aligns with the global trend toward prefabricated construction that reduces construction time, improves quality consistency, and enables factory-controlled manufacturing of structural components.