Failure Modes of Rectangular Steel Tube Concrete Continuous Composite Truss Girders

Literature Overview

Published in the Journal of Chang'an University in 2024, this comprehensive study by Ma Guogang, Pu Beichen, Jiang Lei, Liu Yongjian, and Liu Bin examines the failure mechanisms and design parameter optimization of rectangular steel tube concrete (RSTC) continuous composite truss girders. The research was supported by multiple funding sources including Gansu Provincial Housing and Urban-Rural Development Department projects (JK2023-31, JK2023-39), Northwest Municipal Engineering Design Institute projects, Shaanxi Provincial Returned Overseas Personnel Science and Technology Activity Program (2021-11), and Central University Basic Research Business Fee Special Fund (300102213207). The paper presents a finite element analysis validated against experimental results, covering the entire failure process and proposing reasonable design parameter ranges.

Finite Element Methodology and Failure Process Analysis

The researchers developed a validated finite element model capable of simulating the complete failure process of RSTC continuous composite truss girders. The model captures stress distributions in the main truss, concrete deck, deck reinforcement, and core concrete, as well as member yielding progression and plastic deformation.

The failure process analysis reveals a progressive collapse mechanism: initial yielding occurs in the most critically stressed members, followed by plastic hinge formation at specific cross-sections, and ultimately structural failure through the redistribution of internal forces. The study proposes a reasonable failure mechanism based on the cross-sectional plastic hinge model, which is essential for limit state design methodology.

Design Parameter Optimization Results

The study systematically evaluated several design parameters using three evaluation criteria: cross-sectional stiffness, load-bearing capacity, and whether the structure achieves a reasonable failure mode. The results provide practical design guidance:

The finding that deck thickness has a more pronounced effect on negative moment zone capacity is particularly relevant for continuous truss bridges, where the mid-support region experiences significant negative bending. This insight guides engineers to allocate additional deck thickness at critical negative moment locations rather than uniformly increasing thickness across the entire span.

The wall thickness ratio α between web members and chord members is a critical design parameter. The recommended range of 0.667 to 1.0 ensures that web members yield before or simultaneously with chord members, creating a ductile failure pattern rather than a brittle chord failure. This aligns with the design philosophy of ensuring a controlled, progressive failure mechanism.

Engineering Practice Considerations

From a steel pipe and welding engineering perspective, several practical aspects deserve attention:

• Fabrication of rectangular steel tubes: The rectangular cross-section requires either roll-forming or plate-welded construction. For welded rectangular tubes, the longitudinal and transverse welds must achieve full penetration with adequate root preparation. Typical wall thicknesses for chord members range from 8-16 mm, requiring multi-pass welding with careful control of interpass temperature to prevent excessive grain growth in the HAZ.
• Welded connections in truss nodes: The gusset plate or bolted connections at truss nodes represent critical fabrication points. The stress concentrations at welded joints can initiate fatigue cracks under traffic loading. Quality control should include full UT inspection of critical node welds per ASME B31.3 or GB/T 12467 standards.
• Concrete infill process: The concrete infill of steel tubes requires careful placement to ensure complete filling without voids. Vibration access is limited in rectangular tubes, so self-compacting concrete or low-slump concrete with adequate workability should be specified. The concrete-steel bond is critical for composite action, and the steel tube inner surface should be roughened or mechanically prepared to enhance bond strength.
• Coating and corrosion protection: The internal concrete provides some corrosion protection to the steel tube, but moisture ingress through cracks or at tube ends can cause internal corrosion. The design should specify protective coatings at tube ends and incorporate drainage provisions.

Study Insights and Implications

This research makes a significant contribution to the design methodology of RSTC continuous composite truss bridges by providing quantitative parameter ranges rather than qualitative recommendations. The integration of finite element analysis with the plastic hinge model creates a practical design framework that balances computational efficiency with accuracy.

The emphasis on achieving a "reasonable failure mode" reflects a mature structural engineering philosophy: a bridge should fail progressively with warning rather than catastrophically. This principle is particularly important for bridge engineering, where the consequences of sudden failure are unacceptable. The recommended wall thickness ratio range ensures that failure initiates in less critical members (web members) before the primary load-carrying chords, providing a safety margin and warning time for emergency response.

The study's focus on the negative moment zone highlights an area often underemphasized in preliminary design. Engineers should pay particular attention to the mid-support regions of continuous truss bridges, where the combination of negative bending, shear, and the transition from positive to negative moment creates complex stress states that demand careful detailing and adequate reinforcement.

Future research should extend these findings to consider long-term performance under traffic fatigue loading, the effects of environmental degradation on the concrete-steel composite action, and the seismic behavior of these bridge systems. The finite element framework developed in this study provides a valuable platform for such extensions.