Ultimate Bearing Capacity Analysis of Large-Span Half-Through Steel Tube Concrete Arch Bridges

Literature Overview

This paper by Yun Di and Zhang Sumei from the School of Civil Engineering, Harbin Institute of Technology (published in Journal of Jilin University (Engineering and Technology), Vol. 37, No. 6, 2007, pp. 1308-1312, funded by NSFC Project 50608023) investigates the ultimate bearing capacity of large-span half-through steel tube concrete (STC) arch bridges. The authors analyze the complete process of static instability under design load conditions, explore the load-bearing characteristics and failure mechanisms, and discuss the contribution of non-conservative forces from the deck system to structural stability. Using ANSYS software, they perform ultimate capacity analysis on a 360 m span half-through STC arch bridge under combined loading of dead load, full-span bidirectional vehicle load, full-span bidirectional crowd load, and wind load.

Core Technical Findings

The study reveals several important aspects of the structural behavior of large-span half-through STC arch bridges:

• The out-of-plane instability failure at ultimate load is a typical extremum point instability problem, meaning the structure reaches a limit point where the tangent stiffness matrix becomes singular.
• The non-conservative forces from the deck system contribute to both in-plane and out-of-plane stiffness of the arch ribs, with greater contribution during the elastic loading stage.
• The complete instability process can be described through internal force and deformation indicators, providing a clear picture of the progressive structural degradation.

The load combination analyzed (dead load + full-span bidirectional vehicle load + full-span bidirectional crowd load + wind load) represents a severe but realistic design scenario for large-span arch bridges. The inclusion of wind load is particularly important for large-span structures where aerodynamic effects can significantly influence stability.

Stability Analysis and Failure Mechanism

The static instability analysis of the STC arch bridge reveals a complex interaction between in-plane and out-of-plane behavior:

The extremum point instability is characterized by the structure reaching a maximum load capacity beyond which any additional loading causes rapid, unstable deformation. This is distinct from bifurcation buckling, where the structure can theoretically follow a new equilibrium path after buckling.

The non-conservative forces from the deck system are particularly interesting because they represent a feedback mechanism: as the arch ribs deform, the deck system exerts forces that can either stabilize or destabilize the structure depending on the deformation pattern. During the elastic stage, these forces generally have a stabilizing effect by adding stiffness to the system. However, as the structure approaches the limit state, the non-conservative forces may become destabilizing, accelerating the collapse process.

Engineering Practice Implications

For engineers designing or assessing large-span STC arch bridges, this study provides several important insights:

• Stability analysis must consider both in-plane and out-of-plane behavior simultaneously, as the coupling between these modes can significantly affect the ultimate capacity.
• The deck system is not merely a load-carrying component but also contributes to the overall structural stability through non-conservative forces.
• The ultimate capacity is governed by extremum point instability rather than material failure, which has implications for safety factor determination and code compliance.
• Wind load should be included in stability analysis even though it may not govern the strength design, as it can significantly reduce the stability margin.

From a steel tube perspective, the arch ribs of STC arch bridges are typically fabricated from large-diameter steel tubes filled with concrete. The steel tube serves multiple functions:

1. Formwork during construction — The steel tube acts as permanent formwork for the concrete fill, eliminating the need for temporary formwork.
2. Confinement of concrete — The steel tube provides lateral confinement that enhances the compressive strength and ductility of the concrete core.
3. Structural member — The steel tube itself contributes to the flexural and compressive capacity of the arch rib.

The quality of the steel tube fabrication is therefore critical. Engineers should pay attention to:

• Weld quality at the tube joints, as weld defects can initiate buckling under compressive loading.
• Ovality and out-of-roundness of the tube, as geometric imperfections reduce the buckling resistance.
• The interface bond between steel tube and concrete, as poor bonding can lead to local buckling of the tube under compression.

Study Insights and Reflections

The concept of non-conservative forces from the deck system is a sophisticated structural mechanics concept that is often overlooked in practical bridge design. Most engineers treat the deck system as a load-carrying component that transfers loads to the arch ribs, without considering the feedback effect of the deck forces on arch rib stability. This study demonstrates that such feedback can be significant, particularly during the elastic loading stage.

The extremum point instability mechanism has important implications for safety assessment. Unlike bifurcation buckling, which can be detected by a sudden lateral deflection, extremum point instability is preceded by a gradual increase in deformation under increasing load. This means that visual inspection may not detect the approach to the limit state, making it essential to include stability analysis in the design and assessment process.

One area for future research is the effect of long-term degradation on the ultimate bearing capacity. Steel tube concrete arch bridges are exposed to environmental conditions that can cause corrosion of the steel tubes, degradation of the concrete, and fatigue damage from traffic loading. These degradation mechanisms can reduce the effective stiffness and strength of the arch ribs, potentially shifting the failure mode from extremum point instability to material failure.

In conclusion, this paper provides a rigorous analysis of the ultimate bearing capacity of large-span half-through STC arch bridges, highlighting the importance of considering non-conservative forces and the extremum point instability mechanism. The findings have direct implications for the design and safety assessment of such structures, particularly in terms of stability analysis and load combination selection.