Ultimate Bearing Capacity Analysis of CFST Arch Bridges

Literature Overview and Analytical Framework

The paper by Zhang Jianmin, Zheng Jialian, and Xiao Rucheng (2004), published in the Central-South Highway Engineering journal, presents a comprehensive nonlinear analysis of the ultimate bearing capacity of a large-span concrete-filled steel tube (CFST) arch bridge. This research, supported by the China Postdoctoral Science Foundation (Grant No. 2003034279), focuses on the Nanning Yonghe Bridge and addresses the critical need for accurate assessment of the ultimate limit state of CFST arch structures. The study employs unlayered circular cross-section Timoshenko beam elements with a composite material constitutive model for the CFST section, capturing both material and geometric nonlinearities that govern the failure behavior of large-span arch bridges.

Computational Methodology and Constitutive Modeling

The analytical approach adopted in this paper is significant for its treatment of the complex nonlinear behavior exhibited by CFST arch bridges during the deformation and instability phases. The key methodological choices include:

Analytical Component	Approach	Justification
Element type	Unlayered circular Timoshenko beam	Captures shear deformation in short-to-medium span members; avoids layering complexity
Material model	CFST composite constitutive relation	Accounts for steel-concrete interaction and confinement effects
Nonlinearity type	Material + Geometric	Both are critical for large deformation and buckling analysis
Loading type	Static	Captures ultimate capacity under sustained loading
Verification	Model test comparison	Validates computational predictions against physical behavior

The Timoshenko beam formulation is particularly appropriate for CFST arch ribs because it accounts for transverse shear deformation, which becomes significant in shorter spans or when the depth-to-span ratio exceeds typical limits for Euler-Bernoulli beam theory. The unlayered approach simplifies the computational model while still capturing the essential composite behavior through an equivalent constitutive relationship.

The CFST composite constitutive model must account for several critical phenomena:

Confinement effect of the steel tube on the core concrete, which increases concrete compressive strength and ductility
Progressive yielding of the steel tube under combined axial and flexural loading
Interaction between steel and concrete under varying stress states (compression, tension, shear)
Potential local buckling of the steel tube under high compressive stresses

Analysis Results and Failure Mechanisms

The analysis of the Nanning Yonghe Bridge revealed important characteristics of the ultimate bearing capacity behavior:

Material Nonlinearity Effects:

Progressive yielding of the steel tube initiates at the arch crown and springing under symmetric loading
Concrete crushing occurs in the compressed zone of the arch rib cross-section
The confinement effect maintains load-carrying capacity even after concrete crushing initiates
Steel tube local buckling may occur in regions of high compressive stress if diameter-to-thickness ratio is excessive

Geometric Nonlinearity Effects:

Large lateral displacements develop as the structure approaches the ultimate limit state
P-Δ effects significantly amplify internal forces and accelerates the approach to failure
The arch action gradually transitions to beam action as deformations increase
Soil arch effects (mentioned in the keywords) may contribute additional lateral support at the springing

Instability Mechanisms:

The ultimate failure mode is characterized by a combination of material failure (steel yielding and concrete crushing) and structural instability (excessive lateral displacement)
The transition from stable to unstable equilibrium is gradual rather than sudden
Post-peak behavior shows significant ductility due to the composite nature of the CFST section

Verification Through Model Testing

The computational analysis was validated through physical model testing of the CFST arch. The model test results confirmed the accuracy of the numerical predictions, particularly regarding:

The magnitude of ultimate load
The location of initial yielding
The pattern of deformation development
The overall failure mode

The agreement between numerical and experimental results validates the chosen computational approach and provides confidence in its application to other CFST arch bridge designs.

Engineering Practice and Design Recommendations

For practical bridge engineering applications, this research provides several important insights:

Nonlinear analysis is essential for accurate assessment of CFST arch bridge ultimate capacity; linear elastic analysis significantly overestimates the load-bearing capacity.
The composite constitutive model must accurately represent steel-concrete interaction, particularly the confinement effect, to predict post-yield behavior.
Geometric nonlinearity cannot be neglected for large-span CFST arches, as P-Δ effects significantly reduce the ultimate capacity.
The unlayered Timoshenko beam approach provides a practical balance between computational efficiency and accuracy for routine design analysis.
Model testing remains essential for validating computational models before application to full-scale bridge design.

The research contributes to the growing body of knowledge on CFST arch bridges, which offer significant advantages over conventional RC arches in terms of construction speed, material efficiency, and structural performance. For engineers involved in the design of large-span CFST arch bridges, this paper provides a validated analytical framework that can be adapted to specific project requirements with appropriate constitutive model calibration.