Zhuojin Pipe Fitting Co., LtdPreliminary Selection of Circular Steel Tube Concrete Section Parameters Based on Combined Effect
Literature Overview
This 2023 paper published in Progress in Steel Building Structures presents a systematic methodology for the preliminary selection of section parameters for circular steel tube concrete (CFST) columns based on the combined effect between the steel tube and the concrete core. Funded by the National Natural Science Foundation of China (52278158) and the Fujian Provincial Natural Science Foundation (2022J05188), the research addresses a critical practical challenge in composite structural design: how to efficiently select the optimal combination of steel grade, concrete strength, column diameter, and wall thickness to achieve a target load-bearing capacity while maximizing material efficiency. The authors from Fujian University of Technology and Fuzhou University developed analytical methods for ultimate bearing capacity and residual bearing capacity coefficients under combined effect conditions.
Combined Effect Mechanism and Material Strength Matching
The core concept of this study is the combined effect coefficient, which quantifies the enhancement in load-bearing capacity of a CFST column beyond the simple sum of the individual contributions of the steel tube and the concrete core. This enhancement arises from the confinement effect, where the steel tube restrains the lateral expansion of the concrete core under axial compression, thereby increasing the triaxial compressive strength of the concrete.
A key finding from this research is that the timing of material yielding is critical for optimizing the combined effect. The authors demonstrated that the concrete should enter its plastic state slightly after the steel tube enters its plastic state, but this delay should not be excessive. If the concrete yields too early relative to the steel tube, the confinement effect is not fully utilized because the concrete has already lost significant stiffness. If the concrete yields too late, the steel tube may undergo excessive plastic deformation before the concrete reaches its full load-bearing potential, leading to premature local buckling of the steel tube.
The following table summarizes the material strength matching conditions identified in this study:
| Matching Condition | Description | Design Implication |
|---|---|---|
| Concrete yields after steel tube | Concrete f_c / Steel f_y ratio is relatively low | Good combined effect, moderate confinement |
| Concrete yields simultaneously with steel tube | Specific f_c / f_y ratio | Maximum combined effect coefficient |
| Concrete yields before steel tube | High f_c / f_y ratio | Reduced combined effect, concrete underutilized |
| Ultra-thin wall steel tube | D/t ratio exceeds critical value | Local buckling dominates, combined effect degraded |
The study also established that, under the premise of proper material strength matching and excluding ultra-thin wall steel tube configurations, the combined effect coefficient can be simplified to consider only the influence of concrete strength. This simplification is practically significant because it reduces the complexity of the preliminary design process and allows engineers to use a single-parameter approach for initial section sizing.
Ultimate and Residual Bearing Capacity Analysis
The research developed analytical expressions for both the ultimate bearing capacity and the residual bearing capacity coefficient of CFST short columns. The ultimate bearing capacity represents the maximum axial load that the column can sustain, while the residual bearing capacity coefficient characterizes the load-carrying capacity remaining after the ultimate state has been reached and some level of deformation has occurred.
The residual bearing capacity coefficient is particularly important for seismic design because it determines the post-peak behavior of the column, which directly affects the energy dissipation capacity and the overall ductility of the structural system. The authors proposed a recommended range for the residual bearing capacity coefficient based on two criteria: the strength loss due to inelastic deformation should not be too large, and the material strength utilization should have reached its maximum value.
For a given ultimate bearing capacity and residual bearing capacity coefficient, the study analyzed the characteristics of different section parameter combinations. A significant practical finding is that increasing the concrete strength is more effective in reducing the column outer diameter and section self-weight, while increasing the steel strength is more effective in reducing the steel consumption. This insight is directly applicable to cost optimization in CFST column design, where the relative cost of steel versus concrete can guide the selection of the optimal material combination.
Engineering Application and Design Workflow
The methodology presented in this paper can be integrated into a practical design workflow for CFST column preliminary sizing. The workflow proceeds as follows:
- Determine the design axial load demand and the required residual bearing capacity coefficient based on the seismic performance objective.
- Select the steel grade and concrete strength combination that satisfies the material strength matching condition, ensuring that the concrete yields slightly after the steel tube.
- Use the simplified combined effect coefficient formula to estimate the required section area.
- Select the column outer diameter and wall thickness based on the estimated section area, ensuring that the diameter-to-thickness ratio does not exceed the local buckling limit.
- Verify the ultimate bearing capacity and residual bearing capacity against the design requirements using the developed analytical expressions.
- If the verification fails, adjust the section parameters and repeat steps 3 through 5 until convergence is achieved.
This workflow provides a structured approach to CFST column design that is both efficient and technically rigorous. The emphasis on the combined effect ensures that the design achieves optimal material utilization, which is particularly important for large-scale infrastructure projects where the quantity of steel and concrete used is substantial.
This literature represents a significant advancement in the practical design methodology for circular steel tube concrete columns, providing engineers with a clear and systematic approach to section parameter selection that accounts for the complex interaction between steel and concrete materials. The methodology bridges the gap between theoretical research on composite action and practical engineering design, making it directly applicable to current and future CFST structural projects.