Calculation Method for Compression-Bending Bearing Capacity of Square CFST Columns Reinforced with Outer Square Steel Tube Sandwiched Concrete

Literature Overview

This study develops a calculation methodology for determining the compression-bending bearing capacity of square concrete-filled steel tube (CFST) columns that are externally reinforced with an additional square steel tube enclosing a sandwiched concrete layer. This innovative strengthening approach aims to enhance the load-carrying capacity and ductility of existing CFST columns, providing a viable retrofit solution for aging infrastructure and existing structures requiring capacity upgrades.

Core Technical Points

The research addresses the complex interaction between the original CFST column, the outer steel tube, and the sandwiched concrete layer under combined axial compression and bending moments. The key technical contributions include:

• Development of an interaction diagram (N-M curve) that accounts for the composite action of all three components
• Identification of the optimal reinforcement configuration that maximizes capacity enhancement while minimizing additional weight
• Consideration of the interface behavior between the original column and the reinforcement layer

Theoretical Framework

The calculation method is based on equilibrium equations combined with strain compatibility conditions. The fundamental assumptions include:

1. Plane sections remain plane after deformation (Bernoulli-Euler hypothesis)
2. The steel tubes and concrete layers deform together without slip (achieved through shear connectors)
3. Material stress-strain relationships follow the constitutive models for steel (bilinear with hardening) and confined concrete (Mander or similar model)

Interaction Diagram Development

The N-M interaction curve is generated by varying the neutral axis position and calculating the resultant axial force and bending moment for each position. The key steps include:

• Determining the strain distribution across the section based on the neutral axis location
• Computing stresses in each material layer using the appropriate constitutive model
• Integrating stresses over the cross-section to obtain the resultant forces and moments
• Plotting the N-M relationship for different axial load levels

The study demonstrates that the reinforced column can achieve a 30-60% increase in bending capacity and a 20-40% increase in axial capacity compared to the original CFST column, depending on the reinforcement configuration and loading eccentricity.

Failure Modes and Ductility

The research identifies several potential failure modes for the reinforced column:

• Local buckling of the outer steel tube under compressive stresses
• Concrete crushing in the sandwiched layer, particularly at the corners where stress concentrations occur
• Delamination between the original column and reinforcement if shear connectors are inadequate
• Overall column buckling under high axial loads

The ductility enhancement is quantified using the displacement ductility ratio (μ), which typically increases from 3-4 for the original CFST column to 5-7 for the reinforced column, indicating improved energy dissipation capacity under cyclic loading.

Engineering Practice Integration

For practical implementation of this reinforcement method, the following considerations are essential:

1. Surface preparation of the original CFST column to ensure proper bonding with the sandwiched concrete, including roughening and cleaning
2. Installation of shear connectors (studs or plates) with adequate embedment depth to ensure composite action between the original and reinforcement layers
3. Controlled concrete placement to avoid voids and ensure uniform confinement of the original column
4. Quality inspection procedures including ultrasonic testing of the sandwiched concrete and verification of shear connector welds

The method is particularly suitable for strengthening existing CFST columns in buildings, bridges, and industrial structures where internal reinforcement is impractical or impossible.

Study Insights and Reflections

This research represents a significant advancement in structural retrofit technology, offering a practical solution for enhancing the capacity of existing CFST columns. The development of a rigorous calculation method provides engineers with a reliable tool for design verification, while the experimental validation ensures that the theoretical predictions are grounded in reality.

I am particularly impressed by the study's attention to the interface behavior between the original column and reinforcement layer. In my experience, the long-term performance of externally reinforced columns often depends on the quality of this interface, which can degrade over time due to environmental exposure or differential thermal expansion. The recommended use of shear connectors to ensure composite action is a sound engineering approach that addresses this concern.

Conclusion

This literature provides a comprehensive calculation methodology for determining the compression-bending bearing capacity of square CFST columns reinforced with an outer steel tube and sandwiched concrete. The method is grounded in sound theoretical principles, validated by experimental evidence, and practical for engineering application. Engineers can use this approach to design effective retrofit solutions for existing CFST structures, ensuring enhanced load-carrying capacity and improved ductility. The emphasis on interface behavior and shear connector design is particularly important for ensuring long-term performance, and should be carefully considered in practical implementations. This work contributes significantly to the field of structural retrofit and strengthens our ability to extend the service life of existing CFST structures in a cost-effective and reliable manner.