Stability Analysis of Large-Span Upper-Deck CFST Arch Bridges

Literature Overview

This paper, published in Highway and Transport Research (2006, Vol. 23, No. 11, pp. 82–85), addresses the lateral stability analysis of large-span upper-deck composite steel tube concrete (CFST) arch bridges. Using a specific project in Gansu Province with a main span of 180 meters and a width-to-span ratio of 1/25.7 as a case study, the authors established a full three-dimensional finite element model to analyze the lateral stability of the bridge in both the completed state and during construction. The research was supported by the National Natural Science Foundation of China.

Structural Stability Challenges

Upper-deck CFST arch bridges with large spans and small width-to-span ratios face unique stability challenges. The combination of a slender arch structure and a relatively narrow deck creates a configuration that is susceptible to lateral buckling. Unlike strength failure, which typically provides visible warning signs such as cracking or yielding, lateral instability can occur suddenly with minimal precursors, making it more dangerous and difficult to mitigate.

Case Study Parameters

The width-to-span ratio of 1/25.7 is notably small, indicating a relatively narrow deck compared to the span length. This geometric proportion is common in arch bridges where the deck width is constrained by traffic requirements, but it significantly reduces the lateral stiffness of the deck system. The arch ribs, being slender CFST members, also have limited lateral bending stiffness, creating a system where lateral stability is governed by the interaction between the arch ribs and the deck structure.

Finite Element Analysis Approach

The authors developed a full three-dimensional finite element model to capture the complex behavior of the bridge during construction and in the completed state. The model incorporated the geometric nonlinearity and material nonlinearity inherent in CFST arch bridges, providing a realistic representation of the structural behavior.

Lateral Stability Analysis Methodology

The lateral stability analysis involved several key considerations:

1. The boundary conditions of the deck structure and arch ribs were carefully modeled, particularly the connection between the arch ribs and the deck through hangers and spandrel columns.
2. The plate rubber bearings at the supports were modeled with appropriate lateral stiffness, as these bearings provide limited lateral restraint and significantly influence the overall lateral stability.
3. The construction sequence was simulated step by step, as the lateral stability during construction may be more critical than in the completed state due to incomplete structural systems and asymmetric loading.

The analysis revealed that the lateral stability of the bridge is sensitive to the boundary condition treatment of the deck structure and the arch rib connections. Different modeling assumptions for these connections can lead to significantly different stability results, highlighting the importance of accurate boundary condition representation in finite element analysis.

Construction Phase Stability

During construction, the bridge structure is in a partially completed state, with some components not yet installed. This incomplete configuration can result in lower lateral stability compared to the completed bridge. The analysis showed that certain construction stages are more critical than others, and appropriate temporary support measures may be necessary to ensure lateral stability during these stages.

Engineering Practice Recommendations

Based on the analysis results, several practical recommendations emerge for the design and construction of large-span upper-deck CFST arch bridges:

• The lateral stability should be analyzed at every construction stage, not just the completed state.
• The boundary conditions of the deck structure and arch rib connections should be modeled with high fidelity, as these significantly influence the stability results.
• Temporary lateral bracing may be required during construction to ensure stability at critical stages.
• The plate rubber bearings should be designed with sufficient lateral stiffness to provide adequate lateral restraint to the arch ribs.
• The width-to-span ratio should be considered carefully during preliminary design, as narrower decks require more robust lateral stability measures.

The CFST material used for the arch ribs provides excellent combination of strength and ductility, but the lateral stability of the overall bridge system depends on the interaction between all structural components. The steel tube in the CFST rib provides both structural strength and a formwork for the concrete, but the composite action between the steel tube and concrete must be properly accounted for in the stability analysis.

Summary

This paper demonstrates the critical importance of lateral stability analysis for large-span upper-deck CFST arch bridges, particularly those with small width-to-span ratios. The case study of the 180-meter span bridge in Gansu Province illustrates how the lateral stability behavior is sensitive to boundary condition modeling and construction sequencing. The findings emphasize that lateral instability is a more dangerous failure mode than strength failure due to its sudden nature and lack of visible precursors. Engineers designing similar bridges should conduct comprehensive three-dimensional stability analyses at every construction stage, pay careful attention to boundary condition representation, and implement appropriate temporary lateral support measures during construction to ensure structural safety throughout the entire construction and service life of the bridge.