Elastic-Plastic Bearing Capacity Analysis of Concrete-Filled Steel Tube Compression Members

Literature Overview

The paper by Yan Quansheng, Wang Weiye, and Zou Qiaohong, published in the Journal of Zhengzhou University (Engineering Science) (2003, Vol. 24, No. 2, pp. 29-32), presents a rigorous analytical framework for determining the elastic-plastic bearing capacity of concrete-filled steel tube (CFT) compression-bending members. The work employs a fiber-model-based segmented finite element method to derive the elastic-plastic stiffness matrix for CFT members and proposes a computational model for calculating the elastic-plastic stability bearing capacity. This is a foundational contribution to the structural analysis of CFT columns, which are widely used in high-rise buildings, bridges, and industrial structures.

Core Technical Methodology

The authors adopted a fiber model approach, dividing the cross-section into discrete fiber elements, each assigned a specific stress-strain relationship. This approach is particularly well-suited to CFT members because it can capture the complex interaction between the steel tube and the confined concrete core. The steel tube undergoes elastic-plastic deformation, while the concrete core experiences confinement-induced strength enhancement, both of which are nonlinear phenomena that require a sophisticated analytical framework.

The key innovation is the derivation of the elastic-plastic stiffness matrix for CFT compression-bending members using the fiber model. This matrix accounts for the geometric nonlinearity arising from P-Δ effects and the material nonlinearity of both steel and concrete. The authors developed a corresponding computational program and validated the results against experimental and theoretical data reported in the literature, demonstrating good agreement.

Technical Points and Analytical Framework

Technical Element	Description	Significance
Fiber model	Cross-section divided into discrete fiber elements	Captures nonlinear stress-strain behavior of steel and concrete
Elastic-plastic stiffness matrix	Derived from fiber model with geometric and material nonlinearity	Enables accurate load-displacement analysis
Incremental equilibrium equation	Governs the iterative solution of the nonlinear problem	Ensures convergence to the true equilibrium state
Segmented finite element method	Divides member into segments with distinct stiffness properties	Accounts for varying cross-sectional properties along member length
Circular CFT section	Primary geometry analyzed in the study	Most common CFT column cross-section in practice

The stress-strain relationships used in the fiber model are critical to the accuracy of the analysis. For the steel tube, a bilinear or multilinear elastic-plastic model is typically employed, with yield strength values ranging from 235 MPa to 460 MPa depending on the steel grade (Q235 to Q345 per GB/T 1591). For the confined concrete, a model that accounts for the lateral confinement pressure exerted by the steel tube is essential. The confinement effect increases the compressive strength of the concrete and extends its ductile range, which is a key advantage of the CFT composite action.

Engineering Practice Integration

From a steel pipe fabrication and welding perspective, CFT columns present several important considerations. The steel tubes used for CFT columns are typically seamless steel pipes or welded steel pipes conforming to standards such as GB/T 8162 (seamless) or GB/T 3091 (welded). Common dimensions for structural applications range from 219 mm to 630 mm in outer diameter, with wall thicknesses of 8 mm to 25 mm.

The fabrication of CFT columns involves several critical welding operations:

Tube-to-tube splicing: Full-penetration butt welds joining pipe segments, typically using SAW or GTAW processes with prequalified WPS per GB/T 985.
Tube-to-base plate connections: Fillet welds or full-penetration welds connecting the tube to the base plate, requiring careful control of distortion.
Tube-to-end plate connections: Similar to base plate connections, these welds must accommodate the high axial and bending stresses at column ends.
Lateral brace connections: Fillet welds attaching lateral bracing to the tube, which must maintain the structural integrity of the tube wall.

The quality of these welds directly affects the bearing capacity of the CFT column. Defects such as incomplete fusion, lack of penetration, or porosity in the splice welds can create stress concentrations that initiate premature buckling or fracture. Therefore, rigorous non-destructive testing (NDT) is essential, including ultrasonic testing (UT) per GB/T 11345 for butt welds and radiographic testing (RT) per GB/T 3323 for critical connections.

The elastic-plastic analysis framework presented in this paper has direct implications for weld quality control. The analysis identifies the critical cross-sections where plastic deformation initiates and propagates, which correspond to the locations where weld defects would have the most severe consequences. This information can be used to prioritize NDT resources and to develop risk-based inspection procedures.

Key Questions and Reflections

A significant question raised by this study is the applicability of the fiber model approach to other cross-sectional geometries, such as rectangular, oval, or multi-cell CFT columns. The paper focuses on circular sections, which are the most common, but rectangular and oval sections are increasingly used in certain applications due to their architectural advantages and efficient use of space. The confinement effect varies significantly with cross-sectional shape, and the fiber model parameters would need to be adjusted accordingly.

Another reflection concerns the effect of weld quality on the actual bearing capacity of CFT columns. The theoretical analysis assumes perfect material properties and ideal geometric conditions, but in practice, weld defects, residual stresses, and geometric imperfections reduce the actual capacity. The gap between theoretical and actual capacity should be quantified and incorporated into design safety factors.

Study Insights and Implications

The fiber model-based elastic-plastic analysis framework presented in this paper provides a powerful tool for the design and assessment of CFT columns. The approach is particularly valuable for evaluating existing structures, where the actual material properties and geometric conditions may differ from the original design assumptions. It also provides a basis for developing design codes and standards for CFT structures.

For the welding engineering community, the study underscores the importance of understanding the structural behavior of CFT columns in order to develop appropriate welding procedures and quality control measures. The elastic-plastic analysis identifies the critical regions of the column where weld quality is most important, enabling a risk-based approach to inspection and testing.

The study also highlights the value of computational methods in structural engineering. The development of a dedicated computational program allows for the efficient analysis of complex CFT structures, which would be impractical using hand calculation methods. This is particularly important for the design of large-scale structures such as long-span bridges and tall buildings, where the number of columns and the complexity of loading conditions make manual analysis infeasible.

In conclusion, this paper makes a significant contribution to the understanding of the elastic-plastic behavior of CFT compression members. The fiber model approach, combined with the derived elastic-plastic stiffness matrix, provides a rigorous analytical framework that can be applied to a wide range of CFT structural applications. Engineers working on CFT structures should familiarize themselves with this methodology and consider its implications for design, fabrication, welding, and quality control.