PBL-Stiffened Rectangular Steel-Concrete Truss Arch Bridge Design and Interface Performance

Literature Overview

The 2017 paper by Long Gang, Liu Yongjian, Zhu Weiqing, and Feng Bowen, published in Journal of Highway and Transportation Research, presents an innovative structural solution for steel-concrete arch bridges: the incorporation of PBL (Perforated Bearing Lug) longitudinal stiffeners within the steel-concrete arch rib. This approach is specifically designed to mitigate the well-known problem of debonding and void formation at the steel-concrete interface, which has been identified as a critical durability concern in conventional steel-concrete composite structures. The study is grounded in a real project in Xining, Qinghai Province, where a PBL-stiffened rectangular steel-concrete truss arch bridge was under construction, providing practical validation of the proposed design methodology.

Core Technical Findings

The authors conducted a two-level structural analysis: first at the level of the lower arch rib, and second at the level of the truss-arch combined system. Finite element numerical simulations were used to establish refined local models of nodes with significant web member forces and typical arch rib segments. The key findings include:

1. The axial compressive force in the beam-rib is maximum near the arch crown and decreases rapidly thereafter.
2. The axial compressive force in the arch rib is minimum at the crown and increases rapidly toward the springing, reaching maximum at the arch foot.
3. Web members serve as force-transfer elements connecting the beam-rib and arch rib into a monolithic truss structure.
4. Nodes near the arch crown with significant web member forces exhibit complex stress states.
5. PBL longitudinal stiffeners significantly reduce force transfer length, alleviate stress concentration, and reduce deformation at nodes.
6. PBL stiffeners effectively mitigate debonding and void formation at the steel-concrete interface under solar radiation effects.

Welding and Fabrication Implications

The introduction of PBL longitudinal stiffeners into the steel-concrete arch rib introduces specific fabrication and welding challenges that must be carefully managed. PBL stiffeners are typically fabricated from high-strength steel plates with regularly spaced circular perforations, and they must be welded longitudinally to the inner surface of the rectangular steel tube before concrete infilling. The welding of PBL stiffeners to the tube walls requires precise control of heat input to avoid distorting the rectangular tube geometry, which would compromise the subsequent concrete pouring quality.

From a welding process perspective, the PBL stiffener attachment welds are typically fillet welds performed using FCAW or GMAW processes. The perforated nature of the stiffener creates a complex weld geometry with alternating weld runs and gaps, which requires careful welding procedure specification (WPS) development. The heat input must be limited to prevent excessive thermal distortion of the rectangular tube, particularly at the stiffener ends where stress concentrations are highest. According to ASME B31.3 and GB 50661, the weld quality level for these stiffener attachments should be at least Grade B, requiring full UT inspection of each weld run.

The concrete infilling operation after PBL installation is equally critical. The perforations in the PBL stiffeners serve as mechanical interlock points that enhance the bond between the steel tube and the infilled concrete. However, if the perforations are partially blocked by weld spatter or slag during the stiffener welding operation, the mechanical interlock effectiveness is reduced. Therefore, a post-weld cleaning and inspection procedure is essential to ensure that all perforations are clear and accessible for concrete penetration. This is a practical quality control point that is often overlooked in field construction.

Interface Performance Under Solar Radiation

The study's finding that PBL stiffeners mitigate solar radiation-induced debonding is particularly important for bridges in high-altitude or arid regions such as Xining, where solar irradiance is intense and diurnal temperature variations are large. The differential thermal expansion between the steel tube (coefficient of thermal expansion approximately 12 x 10^-6 /K) and the concrete (approximately 10 x 10^-6 /K) creates cyclic tensile stresses at the interface. Without mechanical interlock provided by PBL stiffeners, these cyclic stresses can progressively degrade the bond, leading to debonding and void formation that compromises the composite action of the arch rib.

The PBL stiffeners act as dowel bars that mechanically lock the steel and concrete together, preventing relative slip even when the adhesive bond is weakened by thermal cycling. This is analogous to the function of shear connectors in steel-concrete composite beams, but adapted for the unique geometry of a closed steel tube section. The design of the PBL stiffener perforation pattern, including hole diameter, spacing, and number of rows, must be optimized to provide adequate mechanical interlock without creating stress concentrations that could initiate cracking in the surrounding concrete.

Study Insights and Reflections

This paper represents a significant advancement in steel-concrete composite bridge design, addressing a fundamental durability issue that has limited the widespread adoption of steel-concrete arch bridges. The PBL stiffener concept is elegant in its simplicity: a relatively low-cost fabrication addition that provides substantial improvements in interface performance and structural reliability. From a welding and fabrication standpoint, the key challenge is maintaining the geometric precision of the rectangular tube during PBL attachment welding, which requires careful control of welding sequence, current, and travel speed. The paper's recommendation that PBL stiffeners be installed before concrete infilling is sound engineering practice, as it ensures that the stiffeners are properly integrated into the composite section. For future projects, I would recommend incorporating a dedicated inspection step after PBL welding to verify perforation clearance and weld quality, as this is a critical but easily overlooked quality control point that directly affects long-term structural performance.