Seismic Isolation of Steel Tube Concrete Tied-Arch Bridges Under Multi-Dimensional Consistent Seismic Excitation

Literature Overview

The paper by Gao Zhonghu, Di Shengkui, Wu Yun, and Wu Zhongtie, published in 2021 in the journal "Engineering Earthquake Resistance and Reinforcement" (Volume 43, Issue 1, pages 118–124), investigates the seismic behavior of a through-type steel tube concrete (SRC) tied-arch bridge with and without seismic isolation. The research is funded by the Central Universities Basic Scientific Research Business Fee Special Fund (31920170068), Northwest Minzu University teaching reform project (2019XJJG-47), the National Natural Science Foundation of China (51868067), and the Gansu Provincial Department of Construction Science and Technology Project (JK2019-10). The authors employed MIDAS finite element software to establish both non-isolated and isolated models, then subjected them to three types of multi-dimensional consistent seismic excitations to compare dynamic characteristics and seismic responses.

Core Technical Approach and Methodology

The study adopts a comparative modeling approach, constructing two finite element models of the same through-type SRC tied-arch bridge: one without seismic isolation and one with lead rubber bearings (LRBs) installed at the bridge piers. The finite element model captures the interaction between the steel tube arch ribs, the concrete infill, the tie rods, the hangers, and the pier supports. The seismic input is characterized as multi-dimensional consistent excitation, meaning that multiple seismic components (typically longitudinal, transverse, and vertical) are applied simultaneously with spatial coherence, rather than treating each direction independently. This is a critical distinction because in reality, earthquake ground motion is inherently three-dimensional, and the structural response is coupled across all directions.

The dynamic characteristics analysis includes natural frequencies, mode shapes, and effective participation factors for both models. The seismic response evaluation encompasses arch rib displacement, velocity, absolute acceleration, relative acceleration, and internal forces in both the arch ribs and the piers. The comparison covers three different multi-dimensional seismic wave combinations to ensure robustness of conclusions across varying ground motion characteristics.

Key Findings and Technical Interpretation

The study reveals several important patterns in the seismic response of isolated versus non-isolated SRC tied-arch bridges:

A particularly noteworthy finding is that the maximum absolute and relative accelerations in the isolated model are not only reduced in magnitude but also distributed more uniformly along the arch rib. This uniformity is significant for engineering practice because uneven acceleration distribution can create local stress concentrations that lead to fatigue damage or brittle fracture, especially at weld connections and bolted joints in steel tube structures.

The observation that longitudinal and transverse displacements increase in the isolated model while vertical displacement decreases is consistent with the fundamental mechanism of seismic isolation. Lead rubber bearings are designed to provide horizontal flexibility while maintaining vertical stiffness. The increased horizontal displacement reflects the intentional period elongation of the structure, which shifts the natural frequency away from the dominant frequency range of seismic ground motion. The reduced vertical displacement indicates that the isolation system effectively decouples vertical ground motion from the superstructure.

Relevance to Steel Pipe and Structural Engineering Practice

From the perspective of steel pipe and structural engineering, this research has direct implications for the design of steel tube concrete arch bridges. The arch ribs in such bridges are typically fabricated from large-diameter steel pipes, often with wall thicknesses ranging from 12 mm to 40 mm or more, depending on the span and load requirements. The steel tubes are filled with concrete to form composite members that leverage the compressive strength of concrete and the tensile and buckling resistance of steel.

The finding that seismic isolation reduces internal forces in both arch ribs and piers means that the steel tube arch ribs can potentially be designed with slightly thinner wall thicknesses or lower steel grades, resulting in material savings. However, the increased displacement demands must be accommodated by the connection details, including the saddle bearings at the arch haunches, the hanger connections, and the tie rod anchorages. These connections involve welding of steel pipe fittings such as tees, reducers, and custom-shaped transition pieces, and their fatigue performance under cyclic displacement must be carefully evaluated.

The multi-dimensional consistent seismic excitation approach adopted in this study is more rigorous than the single-direction analysis commonly used in preliminary design. Engineers should recognize that for critical SRC bridge structures, especially those in high-seismicity regions, multi-directional seismic analysis is essential to capture the true response envelope. The steel tube concrete arch ribs, being slender members with significant geometric nonlinearity, are particularly sensitive to multi-directional loading because biaxial bending and torsion can develop simultaneously.

Study Insights and Reflections

This paper contributes meaningfully to the understanding of seismic isolation for SRC tied-arch bridges, but several aspects merit further investigation. The study focuses on elastic seismic response, whereas in severe earthquakes, the lead rubber bearings themselves may enter inelastic deformation, and the steel tube concrete arch ribs may experience local buckling of the steel shell or crushing of the confined concrete. A nonlinear time-history analysis incorporating material and geometric nonlinearities would provide more realistic predictions of damage states.

Additionally, the long-term durability of lead rubber bearings in bridge environments, particularly their sensitivity to temperature, UV radiation, and ozone, should be considered in the design life assessment. The study does not address the replacement interval or maintenance requirements of the isolation bearings, which is a practical concern for bridge authorities.

For steel pipe fabrication and welding engineers, the key takeaway is that the connection details in isolated SRC bridges must be designed to accommodate larger cyclic displacements. This means that welded joints at arch rib connections, hanger anchorages, and pier caps require careful attention to weld quality, including full-penetration butt welds, adequate heat-affected zone toughness, and proper residual stress management. Non-destructive testing protocols, particularly phased array ultrasonic testing (PAUT) and time-of-flight diffraction (TOFD), should be specified for critical welds in these locations.

In summary, this research provides valuable guidance for the seismic design of steel tube concrete tied-arch bridges, demonstrating that lead rubber bearing isolation can significantly reduce acceleration demands and internal forces while increasing displacement demands. Engineers involved in the design, fabrication, and welding of steel pipe components for such bridges must ensure that connection details and weld quality are adequate to accommodate the increased displacement envelope while maintaining structural integrity under seismic loading.