Zhuojin Pipe Fitting Co., LtdBending Performance of PBL-Stiffened Rectangular Concrete Filled Steel Tube Trusses
Literature Overview
Gao Yimin et al. (Journal of Architecture and Civil Engineering, 2017, Vol. 34, No. 5, pp. 171-180) report on bending performance tests of rectangular concrete filled steel tube (CFST) trusses with and without perforated steel plate (PBL) stiffeners. Funded by the National Natural Science Foundation (Grant No. 51378068), the National Key R&D Program (2016YFC0701202), and the Central University Basic Scientific Research Business Fee Special Fund (310821175015), this research from Chang'an University and Fuzhou University addresses a critical issue in bridge engineering: the joint behavior of CFST truss members.
Test Program and Configuration
Four truss specimens were tested under four-point bending: one conventional rectangular CFST truss and three PBL-stiffened rectangular CFST trusses with different branch-to-chord width ratios (β = 0.5, 0.75, and 0.875). The PBL stiffeners are perforated steel plates embedded within the main chord tube to enhance joint resistance.
| Specimen | Configuration | β Value | Joint Deformation Ratio |
|---|---|---|---|
| T1 | Conventional CFST truss | N/A | 33.43% |
| T2 | PBL-stiffened CFST truss | 0.5 | 24.44% |
| T3 | PBL-stiffened CFST truss | 0.75 | 23.69% |
| T4 | PBL-stiffened CFST truss | 0.875 | 21.44% |
Failure Modes and Structural Behavior
Both conventional and PBL-stiffened CFST trusses failed at the joints, confirming that joint design governs the overall truss performance. The key differences between configurations are:
- Conventional CFST truss: Concrete cracking initiates at the joint and propagates through the chord tube, with the steel tube experiencing significant outward bulging at tension branch connections.
- PBL-stiffened CFST truss: The PBL plates alter the concrete cracking pattern within the chord tube, effectively restraining crack propagation and improving the composite action between steel tube and concrete. The perforated plate provides additional shear transfer area and prevents concrete from being pushed outward through the tube wall.
The PBL stiffeners are particularly effective at tension branch connections where the main chord tube tends to bulge outward. However, at compression branch connections, the PBL effect on chord tube deformation is minimal, as the compressive force naturally resists outward bulging.
Deflection Limit Analysis
The study recommends a vertical deflection limit of span/500 for rectangular CFST and PBL-stiffened CFST composite truss bridge systems, referencing the Highway Steel Structure Bridge Design Code (JTG D64-2015). This recommendation is based on the observation that joint deformation constitutes a significant portion of total truss deflection, particularly in conventional configurations where joint deformation accounts for over one-third of total deformation.
Engineering Practice Integration
From a steel pipe fabrication and welding standpoint, this research highlights several critical aspects:
| Fabrication Aspect | Requirement | Quality Control Focus |
|---|---|---|
| PBL plate installation | Precise positioning within chord tube | Weld geometry, fit-up tolerance |
| Branch-to-chord joints | Full-penetration welds | UT/RT inspection of weld root |
| Tube geometry tolerances | Strict dimensional control | Straightness, ovality, wall thickness |
| Concrete filling | Complete filling without voids | Ultrasonic testing post-fill |
The PBL stiffener concept requires welding perforated steel plates inside the main chord tube before concrete filling. This presents unique welding challenges:
- Internal welding access: Welding inside a confined tube space requires specialized techniques such as GTAW with narrow electrode angles and careful gas shielding management.
- Weld sequence distortion: Multiple welds on the PBL plate can induce significant tube distortion. Pre-compensation and balanced welding sequences are essential.
- Post-weld inspection: Internal welds must be inspected before concrete filling, typically using visual testing (VT) and magnetic particle testing (MT) since radiographic testing (RT) may be impractical for internal configurations.
Key Questions and Reflections
The research raises an important question about the optimal β value. While β = 0.875 provides the lowest joint deformation ratio (21.44%), the improvement from β = 0.75 to β = 0.875 is marginal (23.69% to 21.44%). This suggests that β = 0.75 may represent an economic optimum where the benefit-to-cost ratio is most favorable.
The finding that PBL stiffeners do not significantly affect compression branch connections suggests that selective application of PBL plates only at tension joints could reduce material and fabrication costs without compromising structural performance. This selective stiffening approach warrants further investigation and could significantly simplify fabrication in bridge applications.
The overall message for welding engineers is that joint design in CFST trusses is not merely a structural design issue but directly determines welding requirements, inspection protocols, and fabrication complexity. The PBL concept, while effective, adds significant welding operations that must be carefully planned and executed to maintain structural integrity.