Full-Scale Test of Rectangular CFST Composite Truss Girder Continuous Rigid-Frame Bridge

Literature Overview and Experimental Design

This paper by Liu Yongjian, Ma Yinping, Tian Zhijuan, Yuan Zhuoya, Xiong Zhihua, and Yang Jian, published in China Journal of Highway and Transport (2018, Vol. 31, No. 5, pp. 53-62), reports the first full-scale load test of a rectangular steel tube concrete (CFST) composite truss girder bridge in China. The test bridge has a span arrangement of 24+40+24 meters with a continuous rigid-frame structural system. The loading test employed three 400-kN loading trucks, executing three load cases with twelve loading steps, achieving load efficiencies between 1.90 and 3.05. The experimental program investigated the overall mechanical behavior, the mechanical performance of individual rectangular CFST members, and the effective width of the bridge deck. The research was supported by the National Key R&D Program of China (2016YFC0701202) and the National Natural Science Foundation of China (51778058, 51378068).

Key Experimental Results and Technical Significance

The test results demonstrate that all control members maintained linear stress-strain and load-displacement relationships throughout the loading process, indicating that the bridge remained in a well-defined elastic working state even under overload conditions. This is a significant finding because it confirms that the design assumptions regarding linear elastic behavior are valid within the operational load range, providing confidence for future engineering applications.

The load distribution between the steel tube and the core concrete in the compressed lower chord members followed the axial compressive stiffness ratio of the two materials, confirming the theoretical composite action model. This finding validates the design methodology that allocates loads between steel and concrete based on their respective stiffness contributions, which is fundamental to the structural analysis of CFST members.

Perhaps the most remarkable finding concerns the tensile stiffness of the rectangular CFST members equipped with PBL (Perforated Bridge Lamella) stiffeners. The PBL stiffeners create concrete keys in the perforated regions, substantially increasing the tensile stiffness of the members to approximately 80% of the compressive stiffness. This is a transformative result because conventional CFST members typically exhibit significantly lower tensile stiffness due to the absence of effective composite action in tension.

PBL Stiffener Technology and Steel Tube Fabrication

The PBL stiffener, or perforated bridge lamella, is a longitudinal steel plate with regularly spaced circular perforations installed inside the rectangular steel tube before concrete casting. The concrete filling the perforations creates mechanical interlocks, or concrete keys, that provide shear transfer between the steel tube wall and the core concrete. This technology is critical for rectangular CFST members because rectangular tubes are more susceptible to local buckling than circular tubes, and the PBL stiffeners provide additional lateral support to the tube walls.

From a steel pipe manufacturing perspective, the fabrication of rectangular CFST members with PBL stiffeners presents several challenges. The rectangular tubes must be fabricated with precise dimensional accuracy to ensure proper fit of the PBL stiffeners within the tube interior. The welding of PBL stiffeners to the tube walls introduces additional weld joints that must be inspected for quality. The longitudinal welds of the rectangular tubes, typically made by LSAW or SAW processes, must be executed with particular care because the rectangular cross-section is more sensitive to weld-induced distortion than circular cross-sections.

The welding of rectangular tubes for this application typically involves:

• Longitudinal welds: LSAW or SAW with flux-cored or solid wire, requiring back gouging and back welding for full-penetration joints
• Transverse welds: SMAW with GTAW root pass for thin sections, or SAW for thicker sections
• PBL stiffener welds: fillet welds connecting the stiffener plate to the tube inner wall, typically made by SMAW or FCAW with appropriate access provisions
• Cross-member connection welds: full-penetration butt welds with backing bars or backing strips

Bridge Deck Effective Width and Shear Lag

The test results on bridge deck effective width show good agreement with existing literature, confirming that the shear lag effect is more pronounced at joint locations than in the field between joints. This finding has direct implications for the design of the steel tube members at the joints, where the connection details must accommodate higher local stresses. The shear lag effect means that not all of the bridge deck width participates equally in resisting flexural stresses, with the outer regions of the deck contributing less than the regions directly above the steel tube members.

For the steel pipe engineer, the joint region represents a critical area where multiple welds converge: the longitudinal welds of the main tube, the transverse welds connecting the tube to the cross members, and the PBL stiffener welds all interact in this region. The stress concentration at these weld intersections must be carefully managed through proper weld sequencing, weld geometry optimization, and post-weld inspection. Fatigue performance at these joint regions is particularly important for bridges subject to repeated traffic loading.

Engineering Practice Implications

The full-scale test results provide valuable validation data for the design methodology of rectangular CFST composite truss girder bridges. Several practical recommendations emerge for the steel pipe supply chain:

1. Rectangular tube fabrication must maintain dimensional tolerances within +/-3 mm for length and width, with corner radius uniformity within +/-1 mm, to ensure proper PBL stiffener installation and structural performance.
2. All longitudinal and transverse welds must undergo 100% radiographic testing, with acceptance criteria per GB/T 3323 or ISO 17636, supplemented by ultrasonic testing for volumetric defect detection.
3. PBL stiffener welds should be inspected by magnetic particle testing on the accessible surfaces, with spot ultrasonic testing for weld root quality verification.
4. Post-weld dimensional verification is essential for rectangular tubes, as welding distortion can significantly alter the cross-sectional geometry and compromise the structural performance.
5. Material traceability must be maintained from the steel plate mill through the tube fabrication and PBL stiffener welding processes, with mill certificates and welding procedure qualifications linked to each tube serial number.

Study Insights and Reflections

This full-scale test represents a landmark achievement in the engineering application of rectangular CFST composite truss girder bridges. The confirmation of elastic behavior under overload conditions, the validation of composite action in compression, and the remarkable tensile stiffness enhancement from PBL stiffeners collectively demonstrate the structural viability and efficiency of this bridge type. For the steel pipe and welding industry, this work highlights the growing demand for high-quality rectangular steel tubes with integrated PBL stiffener systems. The welding quality requirements are stringent, requiring qualified welding procedures, certified welders, and comprehensive non-destructive testing programs. The engineering community should recognize that the structural performance of these innovative bridge systems depends fundamentally on the quality of the steel tube fabrication and welding processes, and that any compromise in these areas could undermine the structural safety and service life of the completed bridge.