Zhuojin Pipe Fitting Co., LtdVehicle Vibration Performance of CFST Truss Girder-Lattice Pier Lightweight Bridge
Literature Overview
This paper by Huang Yufan, Wu Qingxiong, and Yuan Huihui from Fuzhou University, published in the Journal of Guangxi University (Natural Science) in 2018 (Vol. 43, No. 4, pp. 1640-1650), investigates the dynamic response characteristics of a lightweight bridge system composed of a steel tube concrete (CFST) truss girder supported on lattice-type high piers. The study uses the Ganhaizi Extra-Large Bridge as the case study, employing a vehicle-bridge interaction model that accounts for road surface irregularities. The research was supported by the National Natural Science Foundation of China (Grants 51608125, 51678154, 51508104) and Fuzhou University research startup funds.
Structural System Description
The Ganhaizi Extra-Large Bridge represents a distinctive structural system that combines several innovative features:
- CFST truss girder: The main span uses a truss configuration with steel tube concrete members, providing high strength-to-weight ratio and good ductility
- Lattice-type high piers: The substructure employs an open lattice design with steel tube concrete columns, reducing structural weight while maintaining stability
- Lightweight design philosophy: The overall system minimizes dead load relative to live load, creating a structure with relatively high live-load-to-dead-load ratio
| Structural Component | Material System | Key Dimensional Parameters |
|---|---|---|
| Main girder | CFST truss | Span length, truss depth, member sizes |
| Piers | Lattice CFST | Height, column spacing, member sizes |
| Deck system | Composite or orthotropic | Thickness, width |
| Bearings | Elastomeric or pot | Dimensions, stiffness |
The lightweight nature of this bridge system creates unique dynamic characteristics that differ significantly from conventional reinforced concrete bridges, particularly in terms of the proportion of live load effects and the dynamic amplification factors.
Analytical Methodology
Vehicle-Bridge Interaction Model
The researchers developed a coupled vehicle-bridge interaction model that accounts for:
- Two-axle vehicle model: A half-car model representing a typical highway vehicle with sprung and unsprung masses
- Road surface irregularity: Random road profile generated according to ISO 8608 or equivalent standard, characterized by road roughness coefficient
- Vehicle parameters: Mass, suspension stiffness and damping, tire stiffness, wheelbase
- Bridge model: Finite element model with appropriate boundary conditions and material properties
Analysis Parameters
| Analysis Category | Parameters | Purpose |
|---|---|---|
| Static analysis | Dead load, live load distribution | Determine static response and load ratio |
| Modal analysis | Natural frequencies, mode shapes | Identify dynamic characteristics |
| Dynamic analysis | Vehicle speed, number of vehicles, road roughness | Evaluate dynamic response |
| Comfort evaluation | Sperling index, Ogata-Kagawa index | Assess ride quality |
Key Findings and Technical Analysis
Dynamic Characteristics
The modal analysis revealed that the Ganhaizi Extra-Large Bridge has dominant frequencies of 2.501 Hz (vertical) and 0.275 Hz (lateral). The relatively low lateral frequency reflects the flexibility of the lattice pier system, which provides lateral stability through geometric configuration rather than mass or stiffness.
The vertical frequency of 2.501 Hz places the bridge in a frequency range that may interact with typical vehicle bounce frequencies (typically 1-2 Hz), potentially leading to resonance conditions at certain vehicle speeds. This frequency-vehicle interaction is a critical consideration for dynamic response evaluation.
Dynamic Response Under Moving Vehicle Load
The dynamic analysis demonstrated several important findings:
- Speed dependency: Dynamic response increases with vehicle speed, as expected from the increased excitation frequency and reduced vehicle-bridge interaction time
- Multi-vehicle effect: The dynamic response under multiple vehicles is significantly greater than under a single vehicle, due to the superposition of dynamic effects and potential resonance conditions
- Vibration modes: The bridge primarily experiences vertical and lateral vibrations under vehicle loading, with minimal torsional response
- Impact factor: The numerically calculated impact factor is slightly lower than the code-specified value, suggesting that the code formula may be conservative for this type of lightweight bridge
Live Load Ratio
A particularly significant finding is that the live load effect ratio for this lightweight bridge is substantially higher than for conventional reinforced concrete bridges. This elevated live-load proportion has several implications:
- Dynamic effects become more significant relative to static response
- Fatigue considerations may be more critical due to higher stress ranges from live loading
- Serviceability criteria (deflection, vibration) may be more easily exceeded
- The design must account for the possibility of dynamic amplification in the live load effects
Ride Comfort Evaluation
The researchers recommend using both the Sperling index and the Ogata-Kagawa index for ride comfort evaluation. These indices provide complementary assessments:
| Index | Basis | Applicable Range | Limitation |
|---|---|---|---|
| Sperling index | Perceived acceleration | 0.3-5 m/s² | Subjective correlation varies by individual |
| Ogata-Kagawa index | Frequency-weighted acceleration | Broad range | Standardized but complex calculation |
The use of both indices provides a more comprehensive assessment of ride comfort, accounting for both the magnitude and frequency content of the vibration response.
Engineering Practice Integration
Design Implications
The findings from this study have direct implications for the design of lightweight CFST bridge systems:
- Dynamic load factors: The impact factor from code provisions may need adjustment for lightweight bridge systems, potentially requiring project-specific dynamic analysis rather than reliance on simplified code formulas
- Serviceability verification: The elevated live-load ratio necessitates careful verification of deflection limits, vibration criteria, and fatigue life under service loading
- Monitoring requirements: Lightweight bridges with low natural frequencies may require more extensive vibration monitoring during service to detect any degradation in dynamic characteristics
Quality Control for Steel Tubes
From a manufacturing and welding quality perspective, the dynamic performance of CFST truss members depends critically on:
| Quality Parameter | Effect on Dynamic Performance | Inspection Method |
|---|---|---|
| Steel tube straightness | Affects member stiffness and buckling resistance | Visual and measurement inspection |
| Weld quality at truss joints | Influences joint stiffness and load transfer | UT/MT inspection |
| Concrete fill density | Determines composite action and member mass | Core testing, density measurement |
| Steel tube dimensional accuracy | Affects assembly fit and connection quality | Dimensional inspection |
| Surface finish | Influences fatigue crack initiation | Visual inspection, roughness measurement |
The fatigue performance of CFST truss members under repeated vehicle loading is particularly sensitive to the quality of weld connections and the presence of stress concentrators. Any defect at welded joints can significantly reduce fatigue life, potentially leading to premature structural failure under the elevated dynamic loading conditions identified in this study.
Study Insights and Reflections
This research makes an important contribution to understanding the dynamic behavior of lightweight bridge systems that combine CFST truss girders with lattice piers. The key insight is that the lightweight design philosophy, while providing material efficiency and construction advantages, introduces unique dynamic challenges that must be carefully addressed in design.
The finding that the code-specified impact factor may be conservative for this bridge type is particularly noteworthy. While conservatism in design is generally acceptable, excessive conservatism can lead to unnecessary material usage and cost. The ability to perform project-specific dynamic analysis using validated vehicle-bridge interaction models enables more rational and efficient design of lightweight bridge systems.
For engineers involved in the design, construction, and maintenance of CFST truss bridges, this paper provides valuable guidance on dynamic analysis methodology, critical performance parameters, and quality control requirements. The emphasis on vehicle-bridge interaction modeling represents a mature analytical approach that should be adopted for all lightweight bridge designs where dynamic effects are significant.