Cross-Section Selection for Concrete-Filled Steel Tube Bridge Piers

Literature Overview

This study by Zhang Guojing, Liu Yongjian, and Jiang Lei from Chang'an University, published in the Journal of Architecture, Civil and Engineering (2020, Vol. 37, No. 3, pp. 94-101), presents a systematic methodology for selecting optimal cross-sectional configurations for concrete-filled steel tube (CFST) bridge pier columns. Supported by the National Natural Science Foundation of China (Grants 51778058, 51378068), the research compares three common cross-sectional forms—circular, rectangular, and rectangular hollow with interlayer—using the concept of critical eccentricity as an economic efficiency indicator.

Core Technical Content and Methodology

Cross-Section Forms Compared

Section Type	Geometric Characteristic	Typical Application
Circular	Uniform D/t ratio	Standard pier columns
Rectangular	Variable h/b ratio	Architecturally constrained piers
Rectangular hollow with interlayer	Variable void ratio	Material-efficient design

Critical Eccentricity as Economic Indicator

The concept of critical eccentricity (界限偏心率) represents the eccentricity at which the cross-section transitions from compression-dominated to bending-dominated behavior. This parameter serves as an economic efficiency indicator because:

It characterizes the section's ability to efficiently utilize both axial compression and flexural capacity
Higher critical eccentricity indicates better material utilization for combined loading
It enables direct comparison between different cross-sectional geometries under equivalent material conditions

Parametric Study Variables

Parameter	Range Studied	Effect on Critical Eccentricity
Confinement coefficient (套箍系数)	Variable	Higher = higher critical eccentricity
Rectangular section h/b ratio	Variable	Higher = higher critical eccentricity
Hollow section void ratio (挖空率)	Variable	Higher = higher critical eccentricity

Key Technical Findings

Economic Performance Relationships

The study establishes clear trends in section economic performance:

Confinement coefficient effect: As the confinement coefficient increases (indicating greater steel tube contribution relative to concrete), the economic performance of rectangular and rectangular hollow sections improves relative to circular sections. When the confinement coefficient exceeds 0.8, the influence on critical eccentricity diminishes, suggesting an optimum design range.
Rectangular section aspect ratio: For rectangular sections, increasing the height-to-width ratio (h/b) improves economic performance. When h/b exceeds 2.0, the marginal improvement in critical eccentricity becomes negligible, indicating a practical design limit.
Hollow section void ratio: For rectangular hollow interlayer sections, increasing the void ratio improves economic performance by reducing material usage while maintaining adequate confinement. When the void ratio exceeds 0.3, further increases have minimal effect on critical eccentricity.

Decision Framework for Section Selection

The study provides a practical decision framework:

When confinement coefficient > 0.8, rectangular section h/b > 2.0, and hollow section void ratio > 0.3, the influence of these parameters on critical eccentricity is small, and section selection should be guided by other considerations (construction feasibility, architectural requirements, cost).
Below these thresholds, the critical eccentricity table enables rapid comparison and selection of the most economical section form for given internal forces and material properties.

Engineering Practice Implications

Steel Tube Fabrication and Supply Considerations

For engineers involved in steel tube manufacturing and supply for bridge pier applications:

Section geometry requirements: Rectangular steel tubes require more complex forming processes compared to circular tubes, involving additional rolling or bending operations. The quality of corner radii in rectangular tubes affects the concrete placement and confinement effectiveness.
Wall thickness uniformity: For rectangular sections, wall thickness variations between the longer and shorter sides can significantly affect confinement effectiveness and critical eccentricity. Tight dimensional tolerances are essential.
Hollow section fabrication: The rectangular hollow interlayer section requires internal tube fabrication or casting with internal formwork, presenting additional manufacturing challenges. Welding quality at internal tube connections becomes critical.

Design Optimization Approach

Design Step	Action	Consideration
1. Determine loads	Calculate axial force and bending moment	Service and ultimate limit states
2. Select material	Choose steel grade and concrete strength	Cost-performance balance
3. Calculate confinement coefficient	Evaluate steel tube contribution	Target > 0.8 for optimal economics
4. Consult critical eccentricity table	Compare section forms	Select most economical option
5. Verify detailed design	Check buckling, connections, detailing	Code compliance

Quality Control for CFST Bridge Piers

From a welding and fabrication quality perspective:

Longitudinal welded tubes (LSAW/UOE) require rigorous weld inspection (RT, UT, MT) as any weld defect reduces the effective confinement capacity
Spiral welded tubes may introduce anisotropic behavior in rectangular sections that affects critical eccentricity predictions
Surface coating or corrosion protection systems must not interfere with the tube-concrete interface bonding

Key Questions and Reflections

Several important questions arise from this study:

How does the critical eccentricity concept account for the different buckling behavior of circular versus rectangular steel tubes under combined compression and bending?
What are the seismic performance implications of section selection based on critical eccentricity, particularly regarding ductility demands?
How do construction tolerances (tube straightness, concrete placement quality) affect the predicted critical eccentricity values?
Can the critical eccentricity methodology be extended to include fatigue considerations for bridge piers subject to traffic-induced dynamic loading?

Study Insights and Reference Value

This research provides a practical, systematic approach to cross-section selection for CFST bridge piers that is particularly valuable for design engineers working within project timelines and budget constraints. The critical eccentricity concept elegantly bridges the gap between theoretical section analysis and practical economic optimization.

From a steel tube engineering perspective, the study highlights the importance of understanding how geometric parameters (D/t ratio, aspect ratio, void ratio) interact with material properties to determine structural efficiency. The finding that rectangular sections can outperform circular sections in economic terms when properly designed challenges the conventional preference for circular CFST columns and opens new design possibilities for bridge engineering applications.

The methodology's strength lies in its simplicity: given the internal forces and material properties, engineers can rapidly consult the critical eccentricity table to identify the optimal section form without performing extensive parametric analysis. This practical utility makes the research directly applicable to real-world bridge design projects, particularly in highway infrastructure where pier column optimization significantly impacts project cost and constructability.