Zhuojin Pipe Fitting Co., LtdStatic and Seismic Study of Double Steel Tube-Concrete Composite Structure Bridge Towers for Multi-Span Cable-Stayed Bridges
Literature Overview
This study note addresses a 2013 publication by Song Xiaohui, Xu Feiping, Chen Renshan, and Yang Ze'ying, published in the journal World Bridges (Vol. 41, Issue 1, pp. 43–48). The authors are affiliated with the Shandong Expressway Co., Ltd. Jinan Yellow River Bridge Management Office and the School of Civil and Hydraulic Engineering at Shandong University. The paper investigates the static and seismic performance of double steel tube-concrete composite structure bridge towers applied in a multi-span cable-stayed bridge, using a 7-tower, 8-span cable-stayed bridge model as the background case study.
Structural Configuration and Design Approach
The double steel tube-concrete composite structure bridge tower configuration involves two steel tubes filled with concrete, arranged in parallel to form the tower legs. This configuration combines the advantages of steel tube-concrete (STC) composite action with the redundancy and stability benefits of a double-tube arrangement. The design approach employs the ultimate limit state design method to verify the safety of the bridge tower under both static and seismic loading conditions.
The structural behavior of double steel tube-concrete towers is governed by several key factors:
| Factor | Description | Engineering Significance |
|---|---|---|
| Concrete confinement | Steel tubes provide lateral confinement to the core concrete | Enhances concrete compressive strength and ductility |
| Steel tube contribution | Steel tubes carry axial and bending loads independently | Provides significant load-carrying capacity |
| Interface bonding | Bond between steel tube inner surface and concrete | Enables composite action and load transfer |
| Connection details | Connections between tower legs and cross beams | Critical for structural integrity and seismic performance |
| Foundation interaction | Tower-foundation connection conditions | Influences overall structural stiffness and seismic response |
Static Performance Analysis
The static analysis examined the effects of live load arrangement on the bridge tower and main girder bending moments. A key finding was that the staggered (alternating span) arrangement of live loads produces larger bending moments in both the main girder and the bridge tower compared to the full-span load arrangement. This is attributed to the asymmetric loading pattern creating larger differential deflections between adjacent spans, which in turn induces greater bending in the tower structure.
The study also found that taller bridge towers produce smaller displacements and bending moments under the same loading conditions. This counterintuitive result can be explained by the increased structural stiffness of taller towers, which are typically designed with larger cross-sectional dimensions and more robust connection details to accommodate the increased height.
Seismic Performance Analysis
The seismic analysis compared three types of main girder-to-tower cross beam connection conditions:
| Connection Type | Description | Seismic Response Characteristics |
|---|---|---|
| Longitudinally movable support | Main girder can move freely in the longitudinal direction relative to the tower cross beam | Maximum tower top displacement and base bending moment |
| Linear elastic connection | Main girder connected to tower with linear spring elements | Intermediate seismic response |
| Bi-linear elastic connection | Main girder connected to tower with bi-linear spring elements (initial stiffness followed by reduced stiffness) | Minimum tower top displacement and base bending moment |
The bi-linear elastic connection provides superior seismic performance because it introduces a controlled yielding mechanism that dissipates seismic energy through inelastic deformation of the connection elements. This effectively decouples the main girder from the tower during strong seismic events, reducing the seismic forces transmitted to the tower structure.
The seismic analysis was conducted under both moderate intensity and super-strong seismic wave conditions. The results consistently showed that:
- Tower top displacement and base bending moment are largest under the longitudinally movable support condition.
- Tower top displacement and base bending moment are smallest under the bi-linear elastic connection condition.
- The bi-linear elastic connection provides the most effective seismic isolation for the tower structure.
- The double steel tube-concrete composite structure bridge tower is suitable for application in multi-span cable-stayed bridges.
Connection Detail Design Considerations
The bi-linear elastic connection concept deserves particular attention from a design and construction perspective. The connection typically involves:
- Sliding bearings: Installed at the tower cross beam to allow controlled longitudinal movement of the main girder.
- Energy dissipation devices: Bi-linear dampers or friction devices that provide initial stiffness for service loads and reduced stiffness for seismic loads.
- Restraint systems: Limit displacement devices that prevent excessive movement of the main girder relative to the tower during extreme seismic events.
The design of these connection details requires careful consideration of the following:
- Serviceability requirements: The connection must maintain adequate stiffness under traffic and wind loads to prevent excessive vibration and discomfort.
- Seismic energy dissipation: The connection must be designed to dissipate sufficient seismic energy to protect the tower structure from damage.
- Constructability: The connection details must be practical to construct and maintain during the service life of the bridge.
- Inspection and maintenance: The connection components must be accessible for periodic inspection and replacement as needed.
Engineering Practice Integration
For bridge engineers and designers, this study provides valuable guidance on the application of double steel tube-concrete composite towers in multi-span cable-stayed bridges. The key practical recommendations include:
- Adopting bi-linear elastic connections between the main girder and tower cross beams for optimal seismic performance.
- Performing detailed seismic analysis under multiple seismic wave scenarios to ensure robustness of the design.
- Conducting dynamic testing or shake table testing of the connection details to validate the analytical models.
- Implementing a comprehensive inspection and maintenance program for the connection details and tower structure.
The study also highlights the importance of considering live load arrangement patterns in the static design of cable-stayed bridges, as the staggered load pattern can produce significantly larger bending moments than the conventional full-span load pattern.
Study Insights and Implications
This study demonstrates that double steel tube-concrete composite structure bridge towers are a viable and effective solution for multi-span cable-stayed bridges, particularly when combined with bi-linear elastic connections for seismic protection. The research provides both analytical and practical insights that can guide the design and construction of similar structures. The emphasis on connection detail design is particularly valuable, as it addresses a critical aspect of seismic performance that is often overlooked in preliminary design stages. Engineers should note that the specific findings are based on a particular bridge configuration, and that the results should be validated through additional analysis and testing for each specific project. The work contributes to the advancement of composite steel-concrete bridge engineering and promotes the safe and economical use of steel tube-concrete structures in bridge applications.