Bending Performance of PBL-Strengthened Rectangular CFST Trusses Experimental Study

Structural Context and Research Significance

This study by Gao Yimin and colleagues from Chang'an University and Fuzhou University, published in Journal of Architecture and Civil Engineering in 2017, investigates the bending performance of rectangular concrete-filled steel tube (CFST) trusses with and without perforated bolted connections (PBL) stiffening. The research was supported by the National Natural Science Foundation of China (51378068), the National Key Research and Development Program (2016YFC0701202), and the Central University Basic Scientific Research Business Fee Special Fund (310821175015). CFST trusses are increasingly used in bridge engineering because they combine the high strength and stiffness of steel tubes with the compressive capacity of concrete, while PBL stiffening is a widely used technique in steel plate girders to enhance shear capacity and limit web buckling.

Test Specimens and Loading Configuration

Four truss specimens were tested: one conventional rectangular CFST truss and three PBL-strengthened rectangular CFST trusses with different chord-to-brace width ratios (β = 0.5, 0.75, and 0.875). The β parameter is defined as the ratio of the brace tube width to the chord tube width, which governs the load transfer efficiency at the joints.

All specimens were subjected to four-point bending loading, simulating the primary loading condition for bridge truss members. The loading was applied quasi-statically to capture the full load-deformation response and identify the failure mechanism.

Failure Mode Analysis

A critical finding of this study is that both the conventional CFST truss and all PBL-strengthened CFST trusses experienced joint failure rather than member failure. This observation is significant because it indicates that the joints are the governing design element for CFST trusses, and the load-carrying capacity of the truss is limited by the joint strength rather than the strength of the individual chord or brace members.

The PBL stiffening was found to alter the concrete cracking pattern within the chord tube. In the conventional truss, concrete cracks propagate freely and can extend through the full depth of the chord tube, leading to a loss of composite action between the steel tube and the concrete core. In the PBL-strengthened trusses, the PBL plates act as internal shear connectors that restrict the crack propagation and maintain the composite action between the steel tube and the concrete throughout the loading process. This results in better stress distribution and more uniform strain development across the chord tube cross-section.

Joint Deformation and Overall Deformation Relationship

The study quantifies the proportion of joint deformation relative to the overall truss deformation, which is a critical parameter for serviceability assessment. The results show that joint deformation accounts for a substantial portion of the total truss deflection:

The data reveals that PBL stiffening effectively reduces the joint deformation ratio from 33.43% to as low as 21.44%, representing a reduction of approximately 36% in the relative contribution of joint deformation to total truss deflection. However, the PBL effect is primarily observed at the tension-side brace locations, where it effectively limits the outward bulging of the chord tube. At the compression-side brace locations, the PBL has a negligible effect on chord tube deformation, likely because the compression load causes inward deformation of the chord tube, which is already restrained by the internal concrete.

Vertical Deflection Limit and Serviceability

The study recommends that the vertical deflection limit for CFST and PBL-strengthened CFST composite truss bridges should be taken as 1/500 of the truss span, referencing the Highway Steel Structure Bridge Design Specification (JTG D64-2015). This recommendation is consistent with existing steel bridge design practice and provides a clear serviceability criterion for engineers designing CFST truss bridges.

Engineering Practice Implications and Welding Considerations

For engineers designing CFST truss bridges, this research provides several practical recommendations. The PBL stiffening technique should be considered for CFST trusses where joint deformation is a concern, particularly for long-span applications where deflection control is critical. The β ratio should be selected to optimize the joint performance, with higher β values (0.75 to 0.875) providing the best joint deformation control. However, higher β values also mean larger brace-to-chord width ratios, which may increase fabrication complexity and cost.

From a welding perspective, the PBL stiffening introduces additional weld details that require careful quality control. The PBL plates are typically welded to the inner surface of the chord tube, creating internal welds that are difficult to inspect. The weld type is typically a fillet weld or a plug weld, and the weld quality directly affects the effectiveness of the PBL as a shear connector. Any lack of fusion or insufficient weld penetration at the PBL-to-tube interface would reduce the shear transfer capacity and compromise the PBL stiffening effect. Engineers should specify appropriate welding procedures, require radiographic or ultrasonic inspection of internal welds, and consider using external weld access holes to facilitate inspection and repair of internal welds.

Study Insights and Design Recommendations

This research provides valuable experimental data on the bending performance of CFST trusses and demonstrates that PBL stiffening is an effective technique for improving joint performance and reducing deflection in CFST truss bridges. The finding that joint failure governs the truss capacity underscores the importance of joint design in CFST truss bridges, and engineers should allocate appropriate design effort to the joint detail design rather than focusing exclusively on member sizing. The quantification of joint deformation ratios provides a practical tool for evaluating the serviceability of CFST truss designs, and the recommended deflection limit of L/500 provides a clear design criterion consistent with existing bridge design codes. The study also highlights the importance of considering both the tension-side and compression-side behavior of chord tubes, as the PBL effect is asymmetric and engineers should not assume uniform improvement at all brace locations.