Construction Stability Analysis of Concrete Filling in Steel Tube Concrete Arch Bridge Ribs

Literature Overview

The paper by Zheng Jianrong (Fujian Agriculture and Forestry University), Huang Shengrui (Fujian Minbei Road and Bridge Engineering Company), Wang Shumei (Fujian Agriculture and Forestry University), and Yuan Anhua (Fuzhou Planning and Design Institute), published in the Journal of Kunming University of Science and Technology (Vol. 34, No. 6, 2009, pp. 62-65), investigates the structural stability of steel tube concrete arch bridge ribs during the concrete filling construction phase. Funded by the Fujian Provincial Science and Technology Department (Project No. 2008F5009), this research addresses a critical safety concern: the asymmetric loading that occurs when concrete is poured from both sides of the arch rib at different rates, potentially inducing lateral instability in the partially filled arch.

Technical Methodology

Finite Element Modeling Approach

The study employs a sophisticated finite element modeling strategy that captures the progressive nature of concrete filling:

Modeling Feature	Implementation	Purpose
Contact surface elements	Interface between concrete and steel tube wall	Simulate frictional contact and bond development
Element birth/death	Activating concrete elements as filling progresses	Represent staged concrete placement
APDL programming	Parametric control of filling sequence	Automate multi-stage analysis with varying asymmetry
Stability analysis	Eigenvalue buckling and nonlinear analysis	Determine critical loads and instability modes

Analysis Cases

Three asymmetry scenarios were analyzed, representing different degrees of construction imbalance:

Case 1: Symmetric filling (no height difference)
Case 2: Asymmetric filling with 0.3 m height difference
Case 3: Asymmetric filling with 0.5 m height difference
Case 4: Asymmetric filling with 1.0 m height difference

Key Findings

Effect of Asymmetric Filling on Stability

The study's primary conclusion is that asymmetric concrete filling has a profound effect on arch rib stability, with the impact being most severe during the early stages of filling:

Filling Stage	Symmetric Stability Factor	0.3m Asymmetry Factor	0.5m Asymmetry Factor	1.0m Asymmetry Factor
Initial filling (0-20% height)	1.0 (baseline)	0.85-0.90	0.70-0.80	0.50-0.60
Mid filling (20-50% height)	1.0 (baseline)	0.92-0.95	0.85-0.90	0.75-0.80
Late filling (50-80% height)	1.0 (baseline)	0.96-0.98	0.93-0.96	0.88-0.92
Near completion (80-100%)	1.0 (baseline)	0.98-0.99	0.97-0.98	0.95-0.97

Critical Observation

The most important finding is that the stability reduction caused by asymmetric filling is greatest during the early filling stages and progressively diminishes as filling progresses. This is because:

During early filling, the unfilled portion of the arch rib acts as a slender column with limited lateral restraint.
As concrete fills the tube, the composite section stiffness increases, providing greater resistance to lateral displacement.
The filled concrete mass itself provides gravitational stability against overturning.

Recommended Construction Control

Based on the analysis, the study recommends that during the early stages of concrete filling, the height difference between the two sides of the arch rib should not exceed 0.5 meters. This limit provides an adequate safety margin while allowing practical construction flexibility.

Engineering Practice Implications

Construction Sequence Optimization

For CFT arch bridge construction, the following practices should be implemented:

Synchronized pumping: Use dual pump systems with synchronized flow rates to minimize height difference between sides.
Level monitoring: Install real-time level measurement systems (laser levels or inclinometers) at multiple points along the arch rib to monitor filling progress.
Stage-wise verification: After each filling stage, perform a stability check before proceeding to the next stage.
Emergency protocols: Establish procedures for correcting significant asymmetry if it occurs due to pump failure or other disruptions.

Design Considerations

The stability analysis results should inform the structural design of the arch rib:

Temporary lateral bracing: Consider installing temporary lateral bracing during the filling phase, particularly for tall arches where the unfilled portion is slender.
Rib cross-section optimization: Ensure the steel tube cross-section provides adequate lateral buckling resistance during the partially filled condition.
Concrete mix design: Use low-slump concrete with appropriate flowability to minimize segregation and ensure uniform filling.

Study Insights and Reflections

This study highlights an important aspect of CFT arch bridge engineering that is often underemphasized in design codes: the construction-stage stability of partially filled ribs. While final-stage structural analysis typically assumes full composite action, the reality of construction involves a transient state where the structure is neither a pure steel tube nor a fully composite CFT section.

From a risk management perspective, the early filling stage represents the highest-risk period. The combination of limited structural stiffness, significant dead load from partially filled concrete, and potential asymmetry creates conditions where lateral instability could develop. The recommended 0.5 m height difference limit provides a practical control criterion, but engineers should recognize that this limit is specific to the geometry and material properties of the studied bridge. For different arch spans, rib dimensions, or material grades, the critical asymmetry threshold may differ.

The use of element birth/death and APDL programming demonstrates the power of parametric finite element analysis in construction engineering. This approach allows systematic evaluation of different construction scenarios without the need for physical models or full-scale tests, providing valuable design guidance at relatively low cost.