Zhuojin Pipe Fitting Co., LtdDynamic Characteristics Analysis of Curved-Chord Through-Truss Concrete-Filled Steel Tube Truss Bridges
Literature Overview
This paper by Wang Yan, Chen Huai, and Li Jie, published in "World Information on Earthquake Engineering" in 2017 (Volume 33, Issue 1, pages 216-222), presents a finite element analysis of the dynamic characteristics of curved-chord through-truss bridges utilizing concrete-filled steel tubes (CFST). The study, supported by the Henan Provincial Science and Technology Key Project (152102310295), employs Midas/Civil software to model bridges with 62 m and 122 m spans, calculating natural frequencies and mode shapes. The research provides valuable insights into the structural behavior of CFST truss bridges, which is directly relevant to pipe engineers supplying tubes for such applications.
Core Technical Content
Finite Element Modeling Approach
The authors developed spatial finite element models for two bridge spans (62 m and 122 m) using Midas/Civil. The modeling considerations include:
- Member modeling: CFST members are modeled as beam elements with composite section properties, accounting for the interaction between steel tube and concrete core.
- Boundary conditions: Realistic support conditions representing the actual bridge bearings and connections are implemented.
- Mass distribution: Both steel and concrete masses are included, with appropriate density values (7850 kg/m³ for steel, 2400 kg/m³ for concrete).
- Stiffness properties: The composite section stiffness is calculated based on the transformed section method, considering the elastic modulus ratio between steel and concrete.
Dynamic Characteristics Results
The analysis reveals several important findings regarding the bridge's dynamic behavior:
| Parameter | 62 m Span | 122 m Span | Observation |
|---|---|---|---|
| Main truss lateral stiffness | Relatively weakest | Relatively weakest | Lateral stability is critical |
| Vertical stiffness | Moderate | Moderate | Adequate for service loads |
| Torsional stiffness | Relatively largest | Relatively largest | Good torsional resistance |
| Floor system vertical stiffness | Weak | Weak | Requires strengthening |
| Floor system torsional stiffness | Weak | Weak | Requires strengthening |
Key Findings
- Lateral stiffness deficiency: The main truss lateral stiffness is the weakest among the three principal stiffness directions. This is attributed to the relatively slender nature of the truss in the lateral direction compared to the vertical and torsional directions.
- Complex vibration modes: The bridge exhibits complex mode shapes where multiple structural components vibrate simultaneously. Higher-order modes often include local vertical vibrations of the floor system coupled with global truss movements.
- Floor system vulnerability: The floor system (deck) has significantly weaker vertical and torsional stiffness compared to the main truss. This creates potential for local vibrations that may fatigue the floor system components.
- Web member out-of-plane vibration: The local vertical bending vibration of the floor system can excite lateral (out-of-plane) bending vibration of the web members (diagonal and vertical truss members). This coupled vibration mode is particularly concerning because web members are typically designed for in-plane loading only.
Engineering Practice Implications for Pipe Supply
From a steel pipe supply perspective, this analysis has several important implications:
- Material quality requirements: CFST truss bridges require high-quality steel tubes with consistent mechanical properties. The dynamic loading conditions mean that fatigue resistance is as important as ultimate strength. Tubes should meet or exceed the requirements of GB/T 14975 or GB/T 14976 for mechanical properties.
- Dimensional precision: Tight tolerances on tube diameter, wall thickness, and straightness are essential for proper fit-up and welding of truss members. Deviations can affect both structural integrity and constructability.
- Welding quality: Truss member connections are critical load paths. Weld quality directly affects fatigue performance. SAW or FCAW processes with appropriate filler metals (such as E7018 or equivalent) are typically required.
- Surface quality: Clean, scale-free tube surfaces are important for proper concrete placement and bonding within the tube. Surface defects can create stress concentrations under cyclic loading.
Quality Control Recommendations
Based on the dynamic analysis findings, the following quality control measures are recommended for CFST truss bridge pipe supply:
- Ultrasonic testing (UT): Full-length UT inspection of all tubes to detect internal defects that could initiate fatigue cracks.
- Dimensional inspection: Laser scanning or manual measurement at multiple stations along each tube to verify dimensional compliance.
- Mechanical property verification: Tensile testing and hardness testing on each heat lot to confirm material conformance.
- Weld qualification: Pre-qualification of welding procedures and regular qualification testing of welders for truss member connections.
Study Insights and Reflections
This paper provides valuable structural engineering insights that are directly relevant to pipe engineers. The finding that web member out-of-plane vibration is a concern is particularly important because it suggests that tube straightness and dimensional accuracy are not merely cosmetic requirements but structural necessities. A slightly bowed tube could initiate or amplify out-of-plane vibrations under dynamic loading. The recommendation to strengthen the floor system stiffness also implies that pipe engineers should be aware of the broader structural context in which their products are used. This paper exemplifies how structural dynamics analysis can inform pipe manufacturing requirements, and I believe it should be studied by engineers involved in bridge pipe supply to better understand the performance expectations placed on their products.