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

Literature Overview

This paper by Liu Yongjian, Ma Yinping, Tian Zhijuan, Yuan Zhuoya, Xiong Zhihua, and Yang Jian, published in the China Journal of Highway and Transport (2018, Vol. 31, No. 5, pp. 53-62), reports the results of full-scale loading tests conducted on China's first rectangular concrete-filled steel tube (CFST) composite truss girder bridge. The test bridge has a span arrangement of 24 m + 40 m + 24 m in a continuous rigid frame system. The research was supported by the National Key R&D Program (2016YFC0701202) and the National Natural Science Foundation of China (Grants 51778058 and 51378068).

Test Configuration and Methodology

The loading test employed three 400 kN loading trucks, conducting three load cases comprising 12 loading steps in total. The test was designed to evaluate three aspects of the bridge's mechanical behavior:

1. Overall structural mechanical performance: Global load-displacement response, deflection patterns, and overall stiffness.
2. Rectangular CFST member mechanical performance: Axial force-strain relationships in individual truss members, load distribution between steel tube and concrete core.
3. Deck slab effective width: Shear lag effects and effective width factors at nodes and between nodes.

The load efficiency ratio ranged from 1.90 to 3.05, indicating that the test loads exceeded the design loads by a significant margin. This is a deliberate practice in bridge loading tests to verify the safety margin of the structure under overload conditions.

Key Test Results

The finding that the bridge maintained elastic behavior even under overload conditions with load efficiency ratios up to 3.05 is a strong indicator of the structural robustness of the rectangular CFST composite truss girder system. The load distribution between the steel tube and the concrete core following the axial stiffness ratio confirms that the composite action is fully mobilized, which is a critical design assumption.

PBL Stiffener Effect and Its Significance

The paper highlights the role of longitudinal PBL (Perforated Steel Plate Lug) stiffeners installed inside the rectangular CFST tube walls. These PBL stiffeners feature a series of circular openings that, after concrete pouring, form concrete keys or tenons. This configuration provides two critical functions:

1. Enhanced tensile stiffness: The concrete tenons formed in the PBL openings create a mechanical interlock between the steel tube and the concrete core, significantly improving the tensile stiffness of the members. The measured tensile stiffness reaching 80% of the compressive stiffness is a remarkable result, as traditional CFST members typically exhibit significantly lower tensile stiffness due to the absence of effective shear transfer in tension.
2. Shear transfer mechanism: The PBL stiffeners provide a reliable shear transfer mechanism between the steel tube and the concrete core, ensuring that the composite action is maintained under both compressive and tensile loading conditions.

Welding Quality and Fabrication Considerations

From a welding and fabrication standpoint, the PBL stiffeners introduce several critical welding challenges that must be carefully managed:

• PBL-to-tube wall welds: The PBL stiffeners are attached to the inner surfaces of the rectangular tube walls by fillet welds. These welds must be designed and executed to withstand the high shear stresses developed at the steel-concrete interface during loading. The weld leg size, weld length, and welding process parameters must be carefully controlled to ensure full penetration and adequate weld strength.
• Heat input control: The welding of PBL stiffeners inside the tube walls can be challenging due to restricted access. Excessive heat input can lead to distortion of the tube walls and degradation of the base metal properties in the HAZ. Preheating and interpass temperature control are essential for thicker tube walls, particularly when using high-strength steels.
• NDT requirements: The PBL welds should be subject to rigorous NDT, including ultrasonic testing (UT) for volumetric defects and magnetic particle testing (MT) for surface-breaking defects. Given the confined access inside the tube walls, alternative NDT methods such as phased array ultrasonic testing (PAUT) or time-of-flight diffraction (TOFD) may be advantageous.

The fabrication of rectangular CFST truss members requires precise dimensional control to ensure proper fit-up during assembly. The longitudinal and circumferential welds of the rectangular tubes must be executed with high quality to ensure the structural integrity of the truss members. For members subjected to cyclic loading (as in bridge applications), the weld details should be designed to minimize stress concentrations, particularly at the weld toes where fatigue crack initiation is most likely.

Engineering Practice Implications

The successful full-scale test results provide strong empirical support for the design methodology of rectangular CFST composite truss girder bridges. The following practical implications can be drawn:

1. Design validation: The test results confirm that the theoretical assumptions regarding composite action, load distribution, and effective width are reasonable for practical design.
2. Safety margin verification: The elastic behavior under overload conditions demonstrates a substantial safety margin, which is particularly important for bridges in seismic zones or areas subject to heavy traffic loading.
3. PBL stiffener design: The significant improvement in tensile stiffness due to PBL stiffeners validates their use in tension members, which was previously a concern for CFST members.

For welding engineers, the key implication is that the quality of PBL welds is critical to the overall structural performance. Any weld defect—such as lack of fusion, porosity, or undercut—can reduce the effective shear transfer capacity and compromise the composite action. Therefore, rigorous weld inspection and qualification procedures should be implemented throughout the fabrication process.

Study Insights and Reflections

This full-scale test represents a significant milestone in the development of rectangular CFST composite truss girder bridges. The comprehensive test program and the positive results provide valuable empirical data for the calibration of analytical models and the development of design codes. The finding that the shear lag effect is more pronounced at nodes than between nodes is an important observation that should be incorporated into future design guidelines.

From a welding perspective, the successful test results validate the welding quality achieved in the fabrication of the test bridge. However, it is important to recognize that full-scale testing represents a single set of fabrication and welding conditions. The welding procedures, welder qualifications, and quality control measures used in the test bridge fabrication should serve as benchmark practices for future projects.

In summary, this paper provides compelling evidence that rectangular CFST composite truss girder bridges are structurally sound and possess adequate safety margins under overload conditions. The PBL stiffener technology is a key enabler of the composite action, and its welding quality is critical to the overall structural performance. Engineers involved in the fabrication and welding of such structures should pay particular attention to the PBL weld details and ensure rigorous quality control throughout the fabrication process.