Dynamic Characteristics of Curved-Chord Through-Truss Steel Tubular Concrete Bridges

Literature Overview

The paper by Wang Yan, Chen Huai, and Li Jie from the School of Civil Engineering, Zhengzhou University, published in World Information on Earthquake Engineering (2017, Vol. 33, No. 1, pp. 216-222), investigates the dynamic behavior of curved-chord through-truss bridges constructed with steel tubular concrete (SRC) members. Funded by the Henan Provincial Science and Technology Project (152102310295), this work is significant for bridge engineers who need to understand the stiffness distribution and vibration modes of this specific bridge configuration. The authors employed the Midas/Civil finite element program to establish spatial models for bridges with spans of 62 m and 122 m, computing natural frequencies and mode shapes to evaluate overall dynamic performance.

Core Technical Findings

The study identifies three critical stiffness characteristics of the curved-chord through-truss SRC bridge. First, the main truss exhibits the weakest lateral stiffness among the three principal directions. Second, the overall vertical stiffness of the complete bridge falls between the lateral and torsional stiffness values. Third, the torsional stiffness of the entire bridge structure is relatively the largest. This stiffness hierarchy has direct implications for seismic design and wind resistance considerations, as the lateral direction becomes the critical failure mode under transverse loading.

The vibration mode analysis reveals that the bridge exhibits complex modal behavior. Multiple principal vibration modes are accompanied by local vertical vibration of the deck system, indicating a coupled dynamic response between the main truss and the deck structure. The deck system demonstrates noticeably weaker vertical and torsional stiffness compared to the main truss, which the authors emphasize requires strengthening measures. Furthermore, local vertical bending vibration of the deck system can readily excite lateral bending vibration in the web members (diagonals), making out-of-plane vibration of these members a critical concern that must be addressed to prevent excessive lateral displacement.

Implications for Steel Tubular Concrete Member Design

From a steel pipe and fitting perspective, this study highlights several important considerations for the selection and detailing of steel tubular concrete members in bridge applications. The steel tubes forming the truss chords and web members must be designed to resist not only axial and in-plane bending forces but also significant out-of-plane lateral bending moments induced by deck system vibration. The steel tube specifications should comply with standards such as GB/T 6725 for structural hollow sections or API 5L for line pipe when used in such applications, with attention to the minimum wall thickness to prevent local buckling under combined loading.

The finding that the deck system has weaker stiffness suggests that connection details between the deck and the main truss require careful engineering. In practice, this means that the steel tubes forming the deck girders or stringers should have adequate torsional rigidity, which for circular tubes depends on the ratio of outer diameter to wall thickness. Thicker-walled tubes or elliptical cross-sections may be advantageous for deck members where torsional stiffness is critical.

Engineering Practice Integration

The conclusions drawn in this paper provide a rational basis for the strengthening design of existing curved-chord through-truss SRC bridges. In my experience with bridge rehabilitation projects, the lateral vibration of web members identified in this study is often underappreciated during initial design. Practical countermeasures include increasing the wall thickness of web member tubes, adding lateral bracing or stiffeners at intermediate points along the diagonals, and ensuring that the deck system connections provide sufficient restraint against out-of-plane displacement. The 122 m span model demonstrates that these concerns become more pronounced with increasing span length, which aligns with the general observation that long-span truss bridges are more sensitive to dynamic effects.

Key Questions and Reflections

A question that arises from this study is whether the finite element model adequately captures the nonlinear interaction between the steel tube and the confined concrete, particularly under cyclic loading conditions. The Midas/Civil model likely employs a linear or simplified nonlinear approach, which may underestimate the actual dynamic response during severe seismic events. Additionally, the study does not appear to address the effect of corrosion or fatigue damage on the long-term dynamic characteristics, which is a practical concern for bridges exposed to harsh environments. The connection between the steel tube and the concrete core, typically achieved through spiral reinforcement or internal stiffeners, also warrants further investigation regarding its influence on the overall dynamic stiffness.

Study Insights and Outlook

This paper serves as a valuable reference for engineers involved in the design and assessment of steel tubular concrete truss bridges. The identification of lateral stiffness as the governing weakness and the recognition of coupled vibration between the deck system and web members are particularly actionable findings. Future research should extend to time-history analysis under realistic seismic inputs, incorporate material nonlinearity and geometric nonlinearity, and validate the numerical predictions through full-scale or component-scale dynamic testing. For steel pipe manufacturers supplying tubes for such bridge applications, this study reinforces the importance of providing products with consistent dimensional accuracy and adequate mechanical properties across the entire length, as localized weakness in wall thickness can trigger premature dynamic instability.