Dynamic Analysis of Hanger Cable Fracture in Underpass Concrete-Filled Steel Tube Rigid Frame Tied Arch Bridge

Literature Overview

This paper by Wu Qingxiong, Yu Yingen, and Chen Baochun (Fuzhou University, 2014), published in Journal of Vibration and Shock (Vol. 33, No. 15, pp. 144–149), investigates the dynamic response of an underpass concrete-filled steel tube (CFST) rigid frame tied arch bridge following hanger cable fracture. The study was supported by the Fujian Provincial Outstanding Youth Science Foundation (2012J06013). Using ANSYS/LS-DYNA finite element software with contact-collision modeling, the authors simulate the sudden rupture of hanger cables and analyze the post-fracture force redistribution among the arch ribs, deck system, and remaining hangers.

Core Technical Findings

The study establishes that hanger cable fracture is not a static event but a highly dynamic process that induces significant impact loads on adjacent structural members. The key findings can be summarized as follows:

Dynamic Load Amplification

During the fracture process, the hanger cables, CFST arch ribs, and deck system all experience impact loading from the snapped cable. The hanger adjacent to the fractured cable experiences the largest impact effect, with its axial force change exceeding three times the initial static axial force. This dynamic amplification factor is critical for design considerations, as it means that the remaining hangers must be designed to withstand forces substantially greater than their static design values.

Failure Mode Analysis

The study identifies a critical vulnerability: the weld connections between transverse beams and longitudinal beams on either side of the fractured hanger are susceptible to failure due to insufficient weld strength. If these welds fail, the deck system may experience local collapse. This finding has direct implications for welding quality control in bridge construction, particularly for the fillet welds and butt welds connecting the deck system members.

Welding Quality Implications

From a welding engineering perspective, this study highlights a critical issue that is often overlooked in bridge construction: the weld connections in the deck system must be designed and inspected with consideration for dynamic overload scenarios, not just static service loads. The welds connecting transverse beams to longitudinal beams are typically designed for gravity and traffic loads, but the dynamic impact from hanger fracture can impose forces well beyond these design values.

Recommended Welding Practices

Based on the findings of this study, the following welding practices are recommended for CFST rigid frame tied arch bridges:

1. Weld detail design: The beam-to-beam connections in the deck system should be designed with adequate weld size and weld metal toughness to resist dynamic impact loading.
2. Weld metal selection: High-toughness weld metals (such as E7010 or E8010 equivalents) should be specified for critical connections to ensure adequate fracture resistance under dynamic loading.
3. Weld inspection: Ultrasonic testing (UT) and magnetic particle testing (MT) should be performed on all critical welds in the deck system, with particular attention to the beam-to-beam connections adjacent to hanger attachment points.
4. Heat-affected zone (HAZ) control: Preheating and interpass temperature control should be implemented to minimize HAZ hardness and ensure adequate toughness in the weldment.

Engineering Practice Integration

The study's use of ANSYS/LS-DYNA with contact-collision modeling represents a practical approach to dynamic fracture analysis. The contact algorithm used to simulate the snapping cable interaction is critical for accurately capturing the dynamic load transfer mechanism. For engineers involved in bridge design and construction, this study underscores the importance of considering cable fracture scenarios in the design of CFST arch bridges, particularly for the deck system weld connections.

The finding that the arch ribs experience relatively small internal force changes upon hanger fracture is reassuring from a structural redundancy standpoint. The arch action provides inherent load redistribution capacity, which is a significant advantage of the tied arch configuration. However, the deck system remains vulnerable, and the weld connections identified as critical failure points must be addressed through appropriate design, material selection, and quality control measures.

Key Questions and Reflections

A question that arises is whether the dynamic amplification factor of three times the initial axial force is applicable to all hanger configurations or is specific to the geometry and span of the studied bridge. The dynamic response is sensitive to the natural frequencies of the structure, the location of the fractured hanger, and the stiffness of the adjacent members. Parametric studies varying these factors would provide more comprehensive guidance for design.

Additionally, the study does not address the long-term fatigue implications of the dynamic loading induced by hanger fracture. Repeated minor hanger failures or partial cable breaks could accumulate fatigue damage in the deck weld connections, potentially leading to progressive degradation of structural capacity.

Study Insights and Reference Value

This paper provides valuable insight into the dynamic behavior of CFST tied arch bridges under hanger fracture scenarios. For steel pipe manufacturers and welding engineers, the key takeaway is that the deck system weld connections in CFST arch bridges must be designed and inspected with awareness of dynamic overload potential. The study's identification of beam-to-beam weld connections as critical failure points provides a clear target for enhanced quality control during construction. The dynamic analysis methodology using LS-DYNA contact modeling offers a practical tool for engineers evaluating similar bridge configurations.