Zhuojin Pipe Fitting Co., LtdNonlinear Seismic Analysis of Steel Tube Concrete Arch Bridges with Cross Bracing
Literature Overview
The paper by Du Qian and colleagues from Lanzhou Jiaotong University, published in the Journal of Lanzhou Jiaotong University in 2019 (Vol. 38, No. 4, pp. 7-11), presents a systematic nonlinear dynamic analysis of steel tube concrete (STC) arch bridges incorporating cross bracing systems. The research was supported by the National Natural Science Foundation of China (Grants 51368033 and 51668035) and the Science and Technology Research and Development Program of China Railway Corporation (2015G002-B). The study addresses a critical gap in understanding the seismic vulnerability of cross bracing members in large-span STC arch bridges, which have become increasingly prevalent in transportation infrastructure projects across China.
Core Methodology and Modeling Approach
The researchers employed the OpenSees (Open System for Earthquake Engineering Simulation) platform to construct a finite element dynamic analysis model of a medium-deck STC arch bridge. A critical step in the methodology was the validation of the OpenSees model against a Midas Civil model, specifically comparing the first five natural vibration periods to ensure fidelity in capturing the structural dynamic characteristics. This cross-verification approach is commendable because it mitigates the risk of model-specific artifacts influencing the analytical conclusions.
The seismic input was provided by a single strong-motion record subjected to Incremental Dynamic Analysis (IDA), which progressively scales ground motion intensity to trace the structural response from the elastic range through yielding into the plastic deformation regime. The IDA approach is particularly well-suited for identifying the sequence of member yielding and the evolution of damage patterns under increasing seismic demand.
Constitutive Model Comparison
A central technical contribution of this study is the comparative analysis of two constitutive models applied to cross bracing members: linear elastic and bilinear (elastic-perfectly plastic) models. The following table summarizes the key response comparisons at critical arch rib sections:
| Section Location | Parameter | Linear Elastic Model | Bilinear Model | Relative Change |
|---|---|---|---|---|
| Arch foot | Internal force | Baseline | Slightly reduced | Minor difference |
| Quarter-span (1/4 L) | Internal force | Baseline | Reduced | Moderate reduction |
| Quarter-span (1/4 L) | Displacement | Baseline | Reduced | Moderate reduction |
| Arch crown | Internal force | Baseline | Reduced | Significant reduction |
| Arch crown | Displacement | Baseline | Reduced | Significant reduction |
The study found that as the number of yielded cross bracing elements increases, the reduction in internal forces and displacements at the quarter-span and arch crown sections becomes progressively more pronounced. This observation has direct implications for seismic design practice: using a bilinear constitutive model for cross bracing provides a more realistic representation of energy dissipation capacity and yields more conservative estimates of demand on the arch ribs.
Key Technical Findings and Engineering Implications
The IDA results revealed that cross bracing elements located near the 3/8 and 5/8 span positions of the arch ribs yield first under transverse seismic loading. This yielding sequence is consistent with the structural mechanics of medium-deck STC arch bridges, where cross bracing members at these locations experience amplified displacements due to the combined effects of arch rib flexure and the kinematic coupling between the arch ribs and the main girder. The study further confirmed that cross bracing members in the vicinity of the arch crown and at the arch rib-to-main girder connection points also yield under strong transverse seismic action.
From a materials and welding perspective, the yielding of cross bracing members has significant implications for fabrication and field erection quality. Cross bracing members are typically fabricated from steel tubes that are welded to arch rib nodes and main girder connection points. The weld quality at these connection details directly influences the ductility and energy dissipation capacity of the bracing system. Poor weld quality, such as insufficient weld penetration, under-cutting, or lack of fusion at the root pass, can lead to premature brittle fracture of the connection before the parent material yields, thereby negating the intended ductile behavior assumed in the bilinear constitutive model.
Welding Process Considerations for Cross Bracing Connections
The connection details between cross bracing tubes and arch rib nodes typically involve fillet welds and groove welds depending on the joint configuration. For transverse seismic loading, the critical weld details are those subjected to cyclic bending and shear. The following process considerations are recommended:
| Weld Type | Location | Recommended Process | Key Quality Parameter |
|---|---|---|---|
| Full-penetration groove | Bracing-to-node T-joint | SAW or GTAW | Root penetration, HAZ microstructure |
| Fillet weld | Bracing-to-girder connection | SMAW or FCAW | Throat thickness, toe geometry |
| Butt weld | Bracing tube splices | SMAW with backing | Full fusion, absence of slag inclusion |
The heat-affected zone (HAZ) of the connection welds is particularly susceptible to low-temperature embrittlement and hydrogen-induced cracking, especially when the base material is a high-strength steel grade such as Q345 or Q390 commonly used in bridge construction. Preheating at 80-120 degrees Celsius, interpass temperature control below 200 degrees Celsius, and post-weld heat treatment where feasible are essential measures to mitigate these risks.
Study Insights and Independent Reflections
The most valuable contribution of this study is the demonstration that the choice of constitutive model for cross bracing significantly affects the predicted seismic demand on arch rib critical sections. In engineering practice, there is a tendency to simplify bracing members as linear elastic elements due to computational convenience or lack of detailed material data. However, this simplification can lead to overestimation of internal forces in the arch ribs, resulting in unnecessarily heavy and costly designs, or conversely, to underestimation of displacement demands, which may compromise serviceability after seismic events.
I note that the study relies on a single seismic record for the IDA, which introduces record-to-record variability into the results. Future work should employ multiple ground motion records and consider the spatial variability of seismic excitation across the bridge span. Additionally, the study does not address the interaction between cross bracing yielding and the concrete confinement behavior of the STC arch ribs, which could be significant under severe seismic demand. The nonlinear interaction between the steel tube confinement and the core concrete under cyclic loading is a complex phenomenon that warrants further investigation using advanced concrete constitutive models such as the Kent-Park or Mander models.
From a quality control standpoint, the findings reinforce the importance of ensuring that cross bracing members and their connections are designed and fabricated to achieve the assumed ductility. Non-destructive testing (NDT) of all critical welds, including ultrasonic testing (UT) for volumetric defects and magnetic particle testing (MT) for surface-breaking defects, should be mandated for seismic-design bridges. Mechanical property testing of weld specimens, including tensile and bend tests, provides additional assurance of weld integrity.
This study provides a solid foundation for improving the seismic design of STC arch bridges with cross bracing, and its recommendations should be integrated into design codes and standards governing the fabrication and erection of such structures.