Stability Analysis During Concrete Filling of Large-Span Steel Tube Concrete Arch Bridges

Literature Overview

The paper published in World Bridges (Vol. 47, Issue 5, 2019, pp. 49–53) by Wang Hongwei, Xie Kaizhong, Guo Xiao, and Zhou Jianxi from Guangxi University presents a stability analysis of the main arch rib during the concrete filling process of a large-span steel tube concrete (SRC) arch bridge. The study is based on the Pingnan No. 3 Bridge in Guangxi, China, which has a main span of 575 m and is a through-type SRC arch bridge. This research is of significant practical importance because the concrete filling process is a critical construction phase that can affect the structural integrity and long-term performance of the bridge.

Construction Background and Methodology

Bridge Description

The Pingnan No. 3 Bridge is a large-span through-type SRC arch bridge with the following key parameters:

Parameter	Value	Description
Main span	575 m	Length of the main arch span
Bridge type	Through-type SRC arch bridge	Main arch ribs are SRC composite sections
Arch rib configuration	Main truss arch structure	Two main arch ribs connected by cross bracing
Construction method	Steel arch erection followed by concrete filling	Sequential construction process

The construction sequence involves first erecting the steel arch ribs as a temporary structure, then filling the steel tubes with concrete to create the permanent composite arch ribs. The concrete filling process is typically performed from the arch feet towards the crown, and the stability of the structure during this process is critical to ensure construction safety.

Finite Element Modelling

The authors established a finite element model of the main truss arch structure based on the basic theory of stability. The model accounts for:

The geometric configuration of the steel arch ribs and cross bracing.
The material properties of the steel tubes and the concrete core.
The boundary conditions at the arch feet and the construction joints.
The progressive loading effect of the concrete filling from the arch feet to the crown.

The stability analysis is performed using the finite element method, which allows for the evaluation of the structural response at each stage of the concrete filling process. The key response parameters analysed include the line shape deviation, stress distribution, and stability coefficient.

Analysis Results

Line Shape Deviation

The analysis revealed that during the concrete filling process from the arch feet to the crown, the main arch rib experiences both lateral and vertical deviations:

Deviation Type	Trend During Filling	Maximum Location	Comparison
Lateral deviation	Present throughout the process	Mid-span section	Early stage > Late stage
Vertical deviation	Present throughout the process	Mid-span section	Early stage > Late stage

The key findings regarding line shape deviation are:

The deviation is larger in the early stages of filling compared to the later stages. This is because the initial concrete weight creates a significant additional load on the steel arch, which has not yet been reinforced by the composite action.
The mid-span section exhibits the largest deviation, which is consistent with the expected behaviour of a simply supported arch under symmetric loading.
Both lateral and vertical deviations are present, indicating that the steel arch is not perfectly aligned during the filling process.

Stress Distribution

The stress analysis during the concrete filling process showed the following patterns:

Stress Component	Maximum Location	Trend During Filling	Comparison
Steel tube stress	Arch foot section	Changes continuously	Larger variation than concrete
Concrete stress	Arch foot section	Changes continuously	Smaller variation than steel
Maximum stress	Arch foot section	Approaches but below allowable	Safe throughout the process

The critical findings regarding stress distribution are:

The maximum stress is concentrated at the arch foot section, which is the location of maximum axial force and bending moment in the arch.
The stress in both the steel tube and the concrete core changes continuously during the filling process, reflecting the progressive load application.
The steel tube stress variation is larger than the concrete stress variation, which is consistent with the findings of Peng Jianxin et al. (2007) on creep-induced stress redistribution.
The maximum stress at the arch foot remains below the allowable stress limit throughout the entire filling process, indicating that the structure is safe from a strength perspective.

Stability Coefficient

The stability coefficient analysis revealed the following trends:

Parameter	Trend During Filling	Early Stage	Late Stage
Stability coefficient	Gradually decreases	Larger decrease	Smaller decrease
Minimum stability coefficient	At the end of filling	—	—
Safety margin	Adequate throughout	Higher	Lower but still adequate

The key findings regarding stability are:

The stability coefficient gradually decreases as the concrete filling progresses from the arch feet to the crown.
The rate of decrease is larger in the early stages of filling compared to the later stages. This is because the initial concrete weight creates a significant additional load on the steel arch, which has a relatively low stability reserve.
The stability coefficient remains above the critical value throughout the entire filling process, indicating that the structure is stable from a buckling perspective.
The minimum stability coefficient occurs at the end of the filling process, when the full weight of the concrete has been applied.

Engineering Practice Implications

Construction Safety Measures

Based on the findings of this study, the following construction safety measures are recommended for large-span SRC arch bridges:

The steel arch should be erected with sufficient stability reserve to accommodate the additional load during concrete filling. The initial stability coefficient of the steel arch should be verified before starting the filling process.
The concrete filling rate should be controlled to avoid excessive loading rates that could lead to dynamic effects or instability. A recommended filling rate of 1–2 m³/h per arch rib is generally safe.
The line shape deviation should be monitored during the filling process using surveying instruments. If the deviation exceeds the specified tolerance, the filling should be paused and corrective measures should be taken.
The stress at the arch foot should be monitored using strain gauges or stress sensors. If the stress approaches the allowable limit, the filling rate should be reduced or the filling should be paused.
The cross bracing system should be fully installed and connected before starting the concrete filling process, as it provides lateral stability to the main arch ribs.

Design Considerations

The findings of this study also have implications for the design of SRC arch bridges:

The steel arch should be designed to have adequate stability during the construction phase, not just the service phase. The construction-phase stability check should be included in the design verification.
The arch foot section should be designed to accommodate the maximum stress during the concrete filling process, which may be higher than the service stress.
The cross bracing system should be designed to provide adequate lateral stability during the construction phase, particularly during the early stages of concrete filling.
The concrete filling sequence should be optimised to minimise the stability demand on the steel arch. Symmetric filling from both arch feet towards the crown is generally the preferred sequence.

Key Questions and Reflections

Several aspects of this study merit further consideration. First, the study focuses on a specific bridge (Pingnan No. 3 Bridge) and the findings may not be directly applicable to bridges with different geometric configurations or construction methods. However, the general trends and principles should be transferable to similar structures. Second, the study does not address the effect of temperature on the stability during the concrete filling process. Temperature variations during construction can affect the steel arch dimensions and the concrete properties, which may influence the stability. Third, the study assumes a uniform concrete filling rate, but in practice, the filling rate may vary due to construction logistics and weather conditions. The effect of non-uniform filling rates on the stability should be investigated. Fourth, the long-term stability of the composite arch rib after the concrete filling is complete is not addressed in this study. The creep-induced stress redistribution discussed by Peng Jianxin et al. (2007) could affect the long-term stability of the structure.

Study Insights and Implications

The most significant contribution of this paper is the systematic analysis of the stability during the concrete filling process of a large-span SRC arch bridge. The findings demonstrate that the concrete filling process is a critical construction phase that requires careful planning and monitoring to ensure construction safety. The study provides quantitative data on the line shape deviation, stress distribution, and stability coefficient during the filling process, which can be used to establish construction acceptance criteria and monitoring thresholds. For engineers involved in the design and construction of SRC arch bridges, this paper provides valuable guidance on the safety aspects of the concrete filling process. The collaboration between the research team and the construction of the Pingnan No. 3 Bridge demonstrates the importance of integrating research with practice to improve construction safety and quality. This paper should be referenced by any engineer involved in the construction of large-span SRC arch bridges, as it provides essential information on the stability behaviour during a critical construction phase that directly impacts the safety of workers and the integrity of the structure.