Zhuojin Pipe Fitting Co., LtdImpact Coefficient Analysis for Partially Filled Rectangular Steel Tube Concrete Composite Truss Girder Bridges
Literature Overview
This paper, published in Science, Technology and Engineering (Vol. 23, Issue 33, 2023, pp. 14428–14436), investigates the dynamic amplification effects in partially filled concrete rectangular steel tube concrete composite truss girder bridges. The study, conducted by researchers from Taiyuan University of Science and Technology, Shanxi Road and Bridge Construction Group, and other institutions, examines a three-span continuous truss girder bridge and analyzes the influence of vehicle speed, bridge deck roughness, vehicle weight, and concrete filling ratio on the impact coefficient. The work was supported by the Shanxi Provincial Basic Research Program and Shanxi Transportation Science and Technology Group innovation projects.
Methodology and Parameter Study
The study employs vehicle-bridge coupled dynamic analysis to compute both deflection impact coefficients and axial force impact coefficients. A total of 867 impact coefficient data points were generated and subjected to goodness-of-fit testing using MATLAB. The analysis considers four key variables:
| Parameter | Variation Range | Influence Trend |
|---|---|---|
| Vehicle speed | 30–120 km/h | Unpredictable; no monotonic relationship |
| Deck roughness | Low to severe | Strong positive correlation; up to 400% increase |
| Vehicle weight | Light to heavy | Negative correlation; up to 200% increase at low weights |
| Concrete filling ratio | Partial to full | Slight increase in impact coefficient with more filling |
Core Findings
Partial Concrete Filling Effect
The study finds that partially filling the lower chord steel tubes with concrete effectively reduces the dynamic response of the lower chord members. However, this beneficial effect on local dynamic response does not translate into a reduction in the overall dynamic amplification effect of the truss girder bridge under vehicle loading. This distinction is important for design: partial filling improves local performance but does not alter the fundamental dynamic characteristics of the bridge system.
Impact Coefficient Statistical Value
At a 95% assurance rate, the statistical impact coefficient for the studied bridge is 0.223. This value is compared against international codes:
| Code/Standard | Impact Coefficient Value | Comparison |
|---|---|---|
| China (JTG) | < 0.223 | Lower than study result |
| US AASHTO | > 0.223 | Higher than study result |
| Australia AS | > 0.223 | Higher than study result |
| UK BS | > 0.223 | Higher than study result |
| Canada CSA | > 0.223 | Higher than study result |
The fact that the Chinese code value is lower than the computed 95% assurance rate value raises significant concerns about the adequacy of current Chinese design codes for this bridge type.
Axial Force vs. Deflection Impact Coefficients
The study notes that the axial force impact coefficient at critical sections is greater than the deflection impact coefficient. This is critical for the design of steel tube concrete truss members, where axial force governs the buckling and stability design of the lower chord tubes.
Engineering Practice Implications
Design Recommendations
- Impact coefficient selection: For partially filled steel tube concrete truss girder bridges in China, designers should consider using an impact coefficient of at least 0.223 (95% assurance rate) rather than the lower code-specified value, particularly for bridges with poor deck roughness or expected light vehicle traffic.
- Deck roughness control: Since deck roughness is the most influential parameter (up to 400% increase), strict control of deck surface quality during construction and maintenance is essential. This includes proper paving specifications, joint design, and periodic resurfacing.
- Concrete filling strategy: Partial filling of lower chord tubes provides local dynamic benefits but should not be relied upon as a global dynamic improvement measure. The filling ratio should be optimized considering both static strength and dynamic performance.
- Axial force design: The higher axial force impact coefficient compared to deflection impact coefficient means that steel tube concrete truss members must be designed for amplified axial loads, not just amplified deflections.
Steel Tube Concrete Interface Considerations
For the steel tube concrete members in the truss, the interaction between the steel tube and concrete core is governed by confinement effects and interfacial bond. The partial filling creates a non-uniform cross-section with different dynamic characteristics in the filled and unfilled portions. The transition region between filled and unfilled sections is a potential stress concentration zone that should be carefully detailed with transverse stiffeners or internal diaphragms.
Study Insights and Reflections
This study makes an important contribution to the dynamic design of steel tube concrete truss bridges, a bridge type that is gaining popularity in China for its material efficiency and aesthetic appeal. The finding that the Chinese code impact coefficient may be insufficient for this bridge type is a significant practical concern. In my experience, code-specified impact coefficients are often based on empirical data from conventional bridge types, and their direct application to composite steel tube concrete truss bridges may not capture the unique dynamic behavior of these structures. The unpredictable relationship between vehicle speed and impact coefficient is also noteworthy—it suggests that resonance phenomena or parametric excitation may be at play, and further research into the frequency characteristics of these bridges would be valuable. The 400% increase in impact coefficient due to deck roughness is a stark reminder that construction quality control and long-term maintenance are as important as structural design in ensuring bridge safety.