Dynamic Characteristics Analysis of Steel Tube Concrete Basket-Handle Arch Bridges

Literature Overview

This study, published in the Journal of Highway and Transportation Research in 2002 (Vol. 19, No. 3, pp. 63–65) by Cheng Haigen and Qiang Shizhong from Southwest Jiaotong University, investigates the dynamic characteristics of steel tube concrete (CFT) basket-handle arch bridges. The basket-handle arch configuration, characterized by its inward-inclined arch ribs that converge at the crown, is widely used in highway and urban bridges due to its superior lateral stability and aesthetic appeal. The research examines how the arch rib inclination angle, cross-bracing arrangement, and hanger type influence the overall dynamic behavior of the bridge.

Structural Configuration and Design Parameters

The basket-handle arch bridge is a distinctive structural form where the two arch ribs are inclined inward from the springing to the crown, creating a tapered plan view. This configuration provides inherent lateral stability through geometric interlock, but the dynamic response is sensitive to several geometric and structural parameters:

Dynamic Analysis Methodology

The study employed finite element modeling to compute the natural frequencies and mode shapes of the CFT basket-handle arch bridge under various parameter combinations. The CFT arch ribs were modeled as composite members with the appropriate effective stiffness and mass properties derived from the steel tube and concrete core contributions. The analysis focused on the first several vibration modes, including symmetric and antisymmetric vertical bending, lateral bending, and torsional modes.

Key Findings on Dynamic Characteristics

The parametric analysis revealed the following important relationships:

1. Inclination angle effect: The inward inclination angle of the arch ribs significantly affects the lateral stiffness of the bridge. A larger inclination angle generally increases lateral stability but may also reduce the vertical natural frequency due to the increased arch rib length and associated mass.
2. Cross-bracing configuration: The arrangement and spacing of cross-bracing between the arch ribs plays a critical role in controlling lateral vibration modes. Insufficient cross-bracing can lead to low-frequency lateral modes that are susceptible to wind-induced vibrations and vehicle-induced lateral excitations.
3. Hanger type influence: The choice between rigid hangers and flexible hangers affects the vertical and torsional natural frequencies. Rigid hangers provide additional stiffness but add mass, while flexible hangers reduce mass but may introduce additional flexibility that lowers natural frequencies.
4. Safety and comfort implications: The dynamic characteristics directly relate to bridge safety under seismic and wind loading, as well as to user comfort under traffic loading. The natural frequencies must be sufficiently separated from common excitation frequencies to avoid resonance.

Design Considerations and Recommendations

Based on the dynamic analysis, the following design considerations are recommended for CFT basket-handle arch bridges:

• The inclination angle should be optimized to balance lateral stability with vertical stiffness, typically ranging from 5° to 15° depending on span length and loading conditions.
• Cross-bracing should be designed to ensure that the first lateral natural frequency is sufficiently high to avoid wind-induced instability and to provide adequate separation from vertical modes.
• The hanger system should be selected based on the desired frequency spectrum, with rigid hangers preferred for shorter spans where mass addition is less critical.
• The dynamic response should be verified against serviceability criteria for vehicle-induced vibration and against safety criteria for seismic and wind loading.

Study Insights and Reflections

This study provides valuable parametric insights for the preliminary design of CFT basket-handle arch bridges, where the interplay between geometric configuration and dynamic response is complex. The emphasis on the inclination angle as a key design variable is particularly useful, as this parameter simultaneously affects structural efficiency, aesthetics, and dynamic performance. The study's relatively concise treatment is appropriate for a journal article, but in practice, the dynamic analysis should be complemented with more sophisticated nonlinear time-history analysis for seismic design and aerodynamic analysis for wind-induced vibration assessment. The findings reinforce the importance of early-stage dynamic evaluation in bridge design, as modifications to the inclination angle or bracing arrangement after the preliminary design stage can be costly and disruptive.