Failure Modes of Rectangular Steel Tube Concrete Continuous Composite Truss Beams

Literature Overview

Published in the Journal of Chang'an University in 2024, this study by Ma Guogang, Pu Beichen, Jiang Lei, Liu Yongjian, and Liu Bin addresses the failure mechanisms of rectangular steel tube concrete (RSTC) continuous composite truss beam bridges. The research was supported by multiple funding sources including the Gansu Provincial Department of Housing and Urban-Rural Development and the Northwest Design and Research Institute of China Municipal Engineering. The authors employed finite element (FE) analysis validated against experimental results to conduct a full-process failure analysis of the main truss, concrete deck, deck reinforcement, core concrete, member yielding progression, and plastic deformation.

Core Findings and Design Parameter Optimization

The study proposes a rational failure mechanism based on the cross-section plastic hinge model and investigates the reasonable ranges of several design parameters. The following table summarizes the key design parameter recommendations:

The finding that deck thickness has a more pronounced effect on negative moment zone capacity than positive moment zone capacity is consistent with the structural behavior of continuous trusses, where the top chord at interior supports experiences compression while the bottom chord at mid-span experiences compression. The negative moment zone at interior supports is governed by the compressive behavior of the top chord and the tensile behavior of the bottom chord, making the deck thickness—particularly in the tension zone—a critical design variable.

Interpretation of Technical Points

From a pipe fitting and welding engineering perspective, the fabrication of rectangular steel tube concrete truss beams involves several critical process steps:

• Rectangular tube fabrication: The parent rectangular tubes are typically fabricated from flat steel plates through bending and longitudinal welding. The wall thickness ratio between web members and chord members (α = 0.667–1.0) directly influences the welding sequence and heat input management. When α approaches unity, the uniform wall thickness simplifies welding procedures, but when α is at the lower bound of 0.667, the differential thermal expansion between thicker chord walls and thinner web walls may induce distortion at the joint.
• Joint fabrication: The connection between chord members and web members in a truss beam typically involves coped or flush-end connections. The welding of these joints must ensure full penetration and adequate throat thickness to transmit both axial and shear forces. For RSTC members, the presence of concrete infill further complicates the welding sequence—welding must be completed before concrete placement, and the concrete placement process must not damage the weld integrity.
• Concrete infill quality: The strength grade of the infill concrete in the chord members is identified as a significant factor in stiffness and capacity enhancement. In practice, concrete placement into rectangular tubes requires careful attention to compaction, particularly in the upper corners where air entrapment is common. The use of vibratory equipment and controlled pouring sequences is essential.

Process and Standards Analysis

The design of RSTC composite truss beam bridges must comply with relevant standards for both the steel tube components and the composite structural system. The following standards are pertinent:

The study's emphasis on the full-process failure analysis—from elastic behavior through yielding to plastic hinge formation and ultimate failure—is methodologically sound and provides engineers with a comprehensive understanding of the structural response envelope. The plastic hinge model, which assumes that plastic deformation concentrates at specific cross-sections rather than being distributed, is a practical simplification that enables rational design while capturing the essential nonlinear behavior.

Engineering Practice Integration

For engineers involved in the fabrication and erection of steel tube concrete truss bridges, the following practical considerations emerge from this study:

1. Wall thickness optimization: The recommended web-to-chord wall thickness ratio of 0.667–1.0 provides a clear design guideline. In practice, this ratio should be selected considering not only structural efficiency but also fabrication economy. Thicker chord walls increase material costs and welding heat input, while thinner web walls may require additional welding passes to achieve full fusion.
2. Concrete strength selection: The finding that increasing concrete strength in the bottom chord at mid-support and the top chord at mid-span yields more significant stiffness and capacity improvements than uniform strength upgrades across all members suggests a differentiated concrete strength strategy. This approach can reduce material costs while maximizing structural performance.
3. Deck reinforcement design: The recommended reinforcement ratio of 1.39%–2.31% for the concrete deck must be balanced against constructability. Higher reinforcement ratios increase congestion at construction joints and may compromise concrete placement quality.

Key Questions and Reflections

The study focuses on the static failure mechanism of RSTC truss beams but does not address fatigue behavior under repeated traffic loading, which is a critical serviceability concern for bridge structures. Additionally, the study does not examine the effect of corrosion on the long-term performance of the steel tubes, which is particularly relevant for bridges in harsh environmental conditions. The interaction between the concrete infill and the steel tube under thermal cycling—caused by diurnal temperature variations and solar radiation—also warrants further investigation, as differential thermal expansion between steel and concrete can induce internal stresses that may accelerate fatigue crack initiation at weld joints.

The FE model validation against experimental results is essential for credibility, and the authors' approach of calibrating the model before conducting parametric studies is methodologically rigorous. However, the accuracy of the FE model depends heavily on the constitutive models used for both steel and concrete, and the sensitivity of the results to these model assumptions should be acknowledged in any engineering application.

Study Insights and Implications

This study provides a comprehensive framework for understanding and designing RSTC continuous composite truss beam bridges. The systematic investigation of design parameters—deck reinforcement ratio, deck thickness, wall thickness ratio, and concrete strength—equips engineers with practical guidelines for optimizing structural performance while controlling fabrication complexity. The identification of the negative moment zone as the critical design region, governed primarily by deck thickness, offers a valuable insight that can guide targeted design improvements. For the pipe fabrication and welding industry, the study reinforces the importance of precise wall thickness control, quality weld execution, and concrete infill quality assurance in composite steel tube structures. The rational failure mechanism proposed based on the plastic hinge model provides a clear basis for performance-based design, enabling engineers to predict and control structural behavior under extreme loading conditions.