Axial Compression Bearing Capacity Analysis of FRP-Concrete-Steel Tube Composite Solid Square Short Columns

Literature Overview

This paper, authored by Hou Yulin, Zhao Junhai, and Cao Xueye from Chang'an University, was published in Industrial Construction in 2016. Supported by multiple national and provincial research funds, the study develops a theoretical model for calculating the axial compression bearing capacity of FRP-concrete-steel tube composite solid square short columns. The research addresses an emerging structural system that combines fiber-reinforced polymer (FRP) wrapping with concrete-filled steel tube (CFST) construction to achieve enhanced confinement and load-bearing capacity.

Core Technical Content

The paper proposes a novel analytical approach that divides the sandwich concrete into two zones based on the effectiveness of confinement: the effective confinement zone (near the steel tube and FRP interfaces) and the ineffective confinement zone (the core region where lateral restraint is insufficient). The key innovation is the conversion of the square FRP jacket into an equivalent circular FRP jacket, which simplifies the analytical treatment while accounting for the double confinement effect from both the steel tube and the FRP layer.

Analytical Framework

The bearing capacity calculation is based on the unified strength theory, which considers:

The intermediate principal stress effect through the intermediate principal stress coefficient (alpha)
The material tensile-to-compressive strength ratio (beta)
The fillet radius effect at the column corners
The combined confinement pressure from both steel tube and FRP

Key Parameters and Their Influence

Parameter	Symbol	Effect on Capacity	Physical Mechanism
Intermediate principal stress coefficient	alpha	Positive (increases capacity)	Better utilization of intermediate stress
Lateral pressure coefficient	-	Positive (increases capacity)	Enhanced concrete confinement
Fillet radius	r	Non-monotonic (increases then decreases)	Initial stress distribution improvement, then reduced confinement area
Steel tube diameter-to-thickness ratio	D/t	Negative (decreases capacity)	Reduced local buckling resistance
FRP thickness	t_FRP	Positive (increases capacity)	Increased lateral restraint pressure
Concrete strength	f_c	Positive (increases capacity)	Higher base compressive strength

Theoretical Model Development

The paper derives the axial compression bearing capacity formula through the following logical sequence:

Establish the stress state of confined concrete considering both steel tube and FRP confinement pressures
Apply the unified strength theory criterion to determine the failure condition
Account for the non-uniform stress distribution in square sections through the effective confinement zone concept
Convert the square geometry to an equivalent circular geometry for analytical tractability
Validate against experimental data from published literature

The validation results show good agreement between theoretical predictions and experimental measurements, confirming the correctness and rationality of the proposed calculation formula.

Engineering Practice Integration

From a structural engineering perspective, this composite system offers several advantages for practical applications:

Enhanced seismic performance through the ductile confinement provided by FRP and steel tube
Improved fire resistance compared to all-FRP systems due to the steel tube protection
Potential for replacing steel reinforcement in high-compression members
Applicability to retrofitting of existing concrete structures

However, several practical concerns must be addressed:

The long-term durability of FRP under cyclic loading and environmental exposure
The interface bond performance between FRP and concrete under high confinement pressures
The constructability of the composite system, particularly the sequence of steel tube installation, concrete placement, and FRP wrapping
Cost-effectiveness compared to conventional reinforced concrete or pure CFST solutions

Key Questions and Reflections

The paper's approach of converting a square section to an equivalent circular section is mathematically elegant but raises questions about the accuracy of this simplification for highly confined conditions. In practice, the stress distribution in square sections is inherently non-uniform, with higher confinement effectiveness near the corners and lower effectiveness at the mid-span of each face. The equivalent circle approach may overestimate or underestimate the actual bearing capacity depending on the relative proportions of the steel tube and FRP layers.

Another important consideration is the failure mode transition. As the confinement strength increases (through thicker FRP or steel tube), the failure mode transitions from concrete crushing to steel tube local buckling or FRP rupture. The paper's model should be validated against cases where different failure modes govern, as the unified strength theory may not accurately predict the post-buckling behavior of the steel tube component.

Study Insights and Implications

This paper contributes a valuable analytical tool for the design of FRP-concrete-steel tube composite columns. The systematic consideration of multiple parameters through the unified strength theory framework provides engineers with a clear understanding of how each design variable influences the structural performance. For practical design applications, the proposed formula can serve as a preliminary sizing tool, with detailed finite element analysis recommended for critical structures. The research direction of combining FRP confinement with steel tube protection represents a promising approach to achieving high-strength, ductile compression members, particularly for applications requiring enhanced seismic performance in corrosive environments where steel reinforcement would be impractical.