Zhuojin Pipe Fitting Co., LtdFull-Process In-Plane Loading Behavior of CFST Rib Arch Members
Literature Overview
This paper by Chen Baichun and Chen Youjie, published in Engineering Mechanics (2000, Vol. 17, No. 2, pp. 44-50), presents experimental research on the full-process in-plane force behavior of concrete-filled steel tube (CFST) single circular tube rib arch members. The research was conducted at the College of Civil Engineering, Fuzhou University. This early study contributed to the understanding of CFST arch behavior, which is particularly relevant for bridge engineering applications where arch structures are commonly used.
Core Technical Content
Two full-scale CFST single circular tube rib arch specimens were tested under in-plane loading to investigate their complete loading process from initial elastic behavior through yielding to ultimate failure. The test results demonstrated that CFST rib arches possess good elastoplastic performance and high load-bearing capacity. The authors analyzed the in-plane force behavior throughout the entire loading process and proposed a preliminary simplified calculation method for the in-plane ultimate load-bearing capacity.
The full-process testing approach is significant because it captures not only the ultimate strength but also the progressive degradation of stiffness, the development of plastic hinges, and the redistribution of internal forces as the arch deforms. This comprehensive understanding is essential for both structural design and failure prediction.
Arch Performance Characteristics
| Performance Aspect | Observed Behavior |
|---|---|
| Elastic behavior | Linear load-displacement relationship in initial stage |
| Elastoplastic transition | Gradual transition from elastic to plastic behavior |
| Load-bearing capacity | High ultimate capacity compared to conventional RC arches |
| Ductility | Good deformation capacity before failure |
| Failure mode | In-plane instability with plastic hinge formation |
Structural Analysis and Design Implications
The in-plane behavior of arch structures is fundamentally different from that of beam-like members. In an arch, the primary load-carrying mechanism is axial compression, with bending moments arising from geometric imperfections, eccentric loading, and asymmetric deformation. For CFST rib arches, the composite action between the steel tube and concrete core provides enhanced axial capacity and confinement, which delays the onset of local buckling and increases the overall stability of the arch.
The simplified calculation method for in-plane ultimate load-bearing capacity proposed by the authors is of practical value for design engineers. In the absence of detailed nonlinear analysis, a simplified method that accounts for the composite behavior of the CFST cross-section can provide a reasonable estimate of the arch's capacity while being computationally efficient.
From a steel pipe manufacturing perspective, the circular steel tubes used as arch ribs must meet specific requirements. The tubes should have uniform wall thickness to ensure consistent cross-sectional properties along the arch length. Any variation in wall thickness can lead to localized weakness that may initiate failure at unexpected locations. The steel tubes should be manufactured to meet standards such as GB/T 8163 for structural steel tubes or API 5L for line pipe, with appropriate mechanical property verification.
Welding and Fabrication Considerations
The fabrication of CFST rib arches typically involves welding of steel plates into circular tubes (for fabricated tubes) or the use of seamless tubes. For arch structures, the steel tubes may need to be curved to match the arch geometry. The cold bending of steel tubes introduces residual stresses and potential ovalization of the cross-section, both of which can affect the structural performance.
If the steel tube is bent from a flat plate, the longitudinal weld seam must be carefully designed and executed. The weld should be placed at the crown of the tube (the point farthest from the neutral axis in bending) or at the bottom, depending on the loading conditions. The weld metal should have mechanical properties at least equal to those of the base steel, and the weld should be inspected using non-destructive testing methods such as radiographic testing (RT) or ultrasonic testing (UT).
For arch segments that are fabricated in the shop and erected in the field, the field welds between segments are critical. These welds must be designed for full-strength connection and should be inspected in accordance with the applicable welding code. The welding procedure should be qualified through welding procedure qualification testing (WPQT) to ensure that the welds achieve the required mechanical properties and geometric tolerances.
Study Insights and Reflections
This study, published in 2000, represents early research into CFST arch behavior and contributed to the growing body of knowledge in this area. The full-process testing approach is commendable because it provides data on the entire structural response, not just the ultimate failure point. This comprehensive data set is essential for calibrating numerical models and developing design methods.
The finding that CFST rib arches exhibit good elastoplastic performance is consistent with the general behavior of CFST members. The steel tube provides confinement to the concrete, which enhances the concrete's compressive strength and ductility. In an arch structure, this enhanced ductility allows the arch to redistribute internal forces as plastic hinges form, providing a more gradual failure process compared to conventional RC arches.
The simplified calculation method for in-plane ultimate capacity should be interpreted with caution, as it represents a preliminary approach. Modern design practice would typically employ nonlinear finite element analysis to capture the complex behavior of CFST arches, including the interaction between axial force, bending moment, and shear force. However, the simplified method remains valuable for preliminary design and for checking the results of detailed analysis.
One area that this study does not address is the out-of-plane stability of CFST rib arches. In practice, arch structures are susceptible to lateral-torsional buckling, which can be a governing failure mode. Future research should investigate the combined in-plane and out-of-plane behavior of CFST rib arches to provide a more comprehensive understanding of their structural performance.
The application of CFST technology in arch bridges has grown significantly since this study was published. Modern CFST arch bridges, such as those in China and Europe, demonstrate the practical viability of this structural system. The fundamental understanding of CFST arch behavior established in this early study continues to inform current design and construction practices.