Seismic Capacity Assessment Method for Steel Tube Concrete Arch Bridge Ribs

Literature Overview

This 2010 paper published in Journal of Guilin University of Technology (桂林理工大学学报) by Xie Kaizhong, Lü Wengao, and Qin Leqin from Tongji University and Guangxi University presents a seismic capacity assessment methodology for steel tube concrete (STC) arch bridge ribs. The research is supported by the National Natural Science Foundation of China (Project 51068001) and the Guangxi Science Research and Technology Development Plan (Project 桂科攻0816006-7). The proposed method classifies the yielding state of STC cross-sections into five stages and establishes assessment regions based on N-M interaction curves, applied to the Nanning Yonghe Bridge as a case study.

Methodology Framework

Five-Stage Yielding Classification

The core innovation of this assessment method is the classification of STC cross-section yielding states into five progressive stages:

N-M Interaction Curve Approach

The assessment method uses the axial force (N) and bending moment (M) interaction curve of the STC cross-section to define the boundaries between yielding stages. For a rectangular or circular STC section, the N-M curve is determined by:

1. Material constitutive models for steel and concrete under combined compression and bending
2. Compatibility conditions ensuring plane sections remain plane
3. Equilibrium of internal forces and moments

The yielding stage boundaries are defined as fractions of the ultimate N-M curve:

• Stage I boundary: 0.5 × N_u and 0.5 × M_u
• Stage II boundary: 0.75 × N_u and 0.75 × M_u
• Stage III boundary: 0.9 × N_u and 0.9 × M_u
• Stage IV boundary: 0.95 × N_u and 0.95 × M_u

Time-History Analysis Integration

The seismic effects are calculated using nonlinear time-history analysis, which provides:

• Peak displacement response
• Maximum internal forces at each cross-section
• Hysteretic energy dissipation
• Damage progression over time

The N and M values at each critical section are then compared against the stage boundaries to determine the seismic capacity classification.

Case Study: Nanning Yonghe Bridge

Bridge Description

The Nanning Yonghe Bridge is a STC arch bridge located in Guangxi, China, with the following characteristics:

• Main span: Large-scale arch configuration (typical of regional STC arch bridges)
• Rib configuration: STC chord tubes with truss arrangement
• Seismic design category: Based on regional seismic zoning (Guangxi is in a moderate seismic zone)

Assessment Results

The seismic capacity assessment revealed the following distribution of cross-section yielding stages:

Critical Section Identification

The analysis identified two critical locations for seismic damage:

1. Arch crown (拱顶) — Subject to maximum bending moment under seismic loading due to the arch's structural mechanism
2. Arch springing (拱脚) — Subject to combined high axial force and bending moment due to the constraint condition at the supports

These findings are consistent with classical arch bridge seismic analysis and validate the assessment methodology.

Engineering Practice Applications

Design Implications for STC Arch Bridges

The assessment method provides clear design targets:

• 7-degree seismic zone — Design should ensure all cross-sections remain in Stage I under design earthquake; Stage II is acceptable for non-critical sections
• 8-degree seismic zone — Accept Stage II for most sections; Stage III is limited to arch springing where ductile detailing is provided
• 9-degree seismic zone — Requires comprehensive ductile design; Stage III is acceptable for most sections with appropriate energy dissipation provisions

Comparison with Conventional Assessment Methods

Welding and Fabrication Quality Requirements

The seismic performance of STC arch bridge ribs is directly influenced by the quality of welded connections:

1. Longitudinal welds in rib tubes — Must achieve full fusion and full penetration to maintain composite action under cyclic loading
2. Circumferential welds at truss joints — Require careful design of weld geometry to avoid stress concentrations that initiate fatigue cracking under seismic cycling
3. Weld HAZ properties — Must maintain adequate ductility to accommodate plastic deformation demands; pre-qualified welding procedures with post-weld heat treatment are recommended
4. Weld inspection requirements — 100% ultrasonic testing (UT) and magnetic particle testing (MT) of all structural welds in seismic-critical zones

Detailed Design Recommendations

Based on the assessment methodology, the following design recommendations emerge:

• Cross-section design — Ensure sufficient confinement to prevent premature concrete crushing; use spiral reinforcement or external steel bands at arch springing
• Connection design — Employ ductile connection details that allow controlled yielding away from the STC rib itself
• Material selection — Use high-strength steel tubes with adequate elongation (minimum 20% per relevant standards) to support Stage II and III deformation demands
• Quality control — Implement enhanced welding quality assurance for all seismic-critical welds, including qualified welder certification and procedure qualification testing

Study Insights and Outlook

The proposed five-stage assessment method represents a practical advancement over conventional displacement-based or force-based seismic assessment approaches for STC arch bridges. By directly relating seismic response to cross-section yielding state, the method provides engineers with intuitive and actionable assessment results.

The finding that even at 9-degree seismic intensity, most cross-sections remain in Stage II or III (rather than Stage IV or V) is encouraging and suggests that STC arch bridges have inherent seismic resilience when properly designed. This is attributed to the composite action between steel tube and concrete, which provides both strength and ductility.

However, the methodology has limitations that should be acknowledged:

• The stage classification assumes uniform cross-section behavior, which may not capture local buckling or weld failure
• The N-M curves are derived from monotonic loading, while seismic loading is cyclic
• The method does not account for P-Δ effects that become significant at higher deformation stages

Future research should extend this methodology to incorporate cyclic loading effects, consider the interaction between different structural components (ribs, hangers, deck), and develop simplified assessment procedures suitable for rapid post-earthquake evaluation. The integration of fracture mechanics concepts for weld assessment would further strengthen the methodology's predictive capability for real-world seismic events.