Elastic-Plastic Seismic Performance of Steel Tube Concrete High-Pier Continuous Beam Bridges

Literature Overview

This paper, published in the Journal of Wuhan University (Engineering Science Edition) in 2021 by Ouyang Zhijie, Xie Mingqin, Lin Jianmao, and Wang Jinze from Fujian University of Technology and the Fujian Institute of Transportation Science and Technology, investigates the elastic-plastic seismic behavior of steel tube concrete (STC) high-pier continuous beam bridges. The study is grounded in the engineering practice of the Ganhaizi Grand Bridge in Sichuan Province, a three-span single-continuous bridge supported by STC lattice piers. The research is supported by the National Natural Science Foundation of China (No. 51408128), the Fujian Provincial Natural Science Foundation (No. 2017J01471), and the Fujian Provincial Transportation Science and Technology Program (No. 202003).

Core Methodology and Technical Approach

The authors employed the Midas Civil finite element software to construct a three-dimensional computational model of the three-span continuous beam bridge. Two ground motion records, designated E1 and E2, were selected to represent different seismic intensity levels. The analysis proceeded through two complementary methods:

1. Elastic-Plastic Time History Analysis — This method was used to evaluate the internal force distribution, yield mechanism, and structural ductility under the action of E1 and E2 earthquakes. The model incorporated nonlinear material properties for both the steel tube and the infill concrete, allowing the simulation of progressive yielding and plastic hinge formation.
2. Incremental Dynamic Analysis (IDA) — This method progressively scaled the input ground motion to identify the sequence of structural failure and the underlying damage mechanisms of the STC lattice piers. The IDA approach provides a capacity curve that maps structural response (e.g., displacement ductility) against seismic intensity (e.g., peak ground acceleration), offering a more comprehensive picture of structural vulnerability than a single time history analysis.

Key Technical Findings

The study revealed several important observations regarding the seismic performance of STC high-pier continuous beam bridges:

The failure progression is particularly noteworthy. The concrete infill within the steel tube initially cracks under cyclic loading, which reduces the effective composite action. As the seismic intensity increases, the longitudinal lacing tubes that provide lateral restraint to the steel tube limbs begin to yield partially. Ultimately, the steel tube cross-sections of the pier limbs reach their yielding capacity, leading to a loss of load-carrying capacity. This sequence suggests that the lacing tubes play a critical role in maintaining the structural integrity of the lattice pier during seismic events.

Process and Standards Analysis

From a standards perspective, the findings of this study have direct implications for the seismic design of STC bridges. The Chinese code JTG/T B02-01 (Specifications for Seismic Design of Highway Bridges) and GB 50011 (Code for Seismic Design of Buildings) provide general seismic design requirements, but they do not specifically address the unique failure mechanisms of STC lattice piers. The research highlights the need for:

• Explicit consideration of the concrete cracking stage in nonlinear analysis models
• Attention to the yielding capacity of lacing tubes as a secondary failure mode
• Verification that the fixed pier, which typically attracts the largest seismic forces, is designed with adequate ductility

The yield mechanism identified in this study — progressive concrete cracking followed by lacing tube yielding and then steel tube limb yielding — suggests a potential design strategy of allowing controlled yielding in the lacing tubes while maintaining the integrity of the main steel tube limbs. This concept is analogous to the "weak link" design philosophy used in buckling-restrained braces, where a designated element yields first to dissipate energy while protecting other structural components.

Integration with Engineering Practice

In practical bridge engineering, the STC high-pier configuration is often adopted for bridges crossing deep valleys or rivers where traditional reinforced concrete piers would be excessively tall and heavy. The composite action between the steel tube and the infill concrete provides superior strength-to-weight ratio and improved ductility compared to plain concrete piers. However, the lattice configuration introduces additional complexity in terms of load path continuity and failure modes.

For engineers involved in the design and review of similar bridges, the following practical recommendations emerge from this study:

1. The pier base should be detailed with adequate confinement reinforcement and steel tube thickness to resist the concentrated bending moments identified as the controlling internal force.
2. The lacing tubes should be designed with sufficient yield strength to ensure they do not fail prematurely, yet the design should acknowledge that partial yielding of lacing tubes is an acceptable intermediate damage state.
3. Nonlinear time history analysis, rather than linear response spectrum analysis, should be employed for the seismic verification of STC high-pier bridges, as the failure mechanisms are inherently nonlinear.

Key Questions and Reflections

Several questions arise from this study that merit further investigation. First, the study focuses on two specific ground motion records, and the conclusions may be sensitive to the selection of input motions. A probabilistic seismic hazard analysis incorporating a broader suite of ground motions would provide more robust conclusions. Second, the study does not address the effect of soil-structure interaction, which can be significant for tall piers in soft soil conditions. Third, the long-term durability of the concrete infill within the steel tube under cyclic seismic loading is not examined, yet this could influence the residual seismic capacity of the bridge after a major earthquake.

From a personal perspective, the most significant insight from this paper is the recognition that the failure sequence of STC lattice piers is governed not only by the primary structural elements (the steel tube limbs) but also by the secondary elements (the lacing tubes). This reinforces the principle that in composite structures, the weakest link — not necessarily the most heavily loaded element — often dictates the overall structural performance. Engineers should therefore pay equal attention to the design and detailing of all structural components, not just the primary load-bearing members.

Study Insights and Implications

This research contributes valuable theoretical and practical insights for the seismic design of STC high-pier continuous beam bridges. The identification of the pier base as the critical section and the fixed pier as the location of maximum displacement response provides clear guidance for design emphasis. The progressive failure mechanism — concrete cracking, lacing tube yielding, steel tube limb yielding — offers a roadmap for understanding how these structures behave under increasing seismic demand. The use of IDA in addition to conventional time history analysis represents a methodological advancement that provides a more complete picture of structural vulnerability. For future work, extending this research to include parametric studies on steel grade, concrete strength, pier geometry, and soil conditions would further enhance the applicability of these findings to a wider range of bridge configurations. The study serves as an important reference for both academic researchers and practicing engineers working on the seismic design and evaluation of composite steel tube concrete bridges.