PBL Stiffeners on Compression Node Performance of Circular CFST Arch Ribs

Literature Overview

This paper by Tang Peng, Gong Sai, and Cheng Gao (2017), published in China Foreign Highway (Vol. 37, No. 5, pp. 124-127), investigates the influence of PBL (Profiled Bolted Lintel) stiffeners on the mechanical behavior of compression nodes in circular steel tube concrete-filled (CFST) arch ribs. The study is funded by the Central Universities Basic Research Business Fee Special Fund (No. 310821161004) and originates from Nanyang Institute of Technology and Chang'an University. The authors employ nonlinear finite element analysis to examine a practical CFST arch bridge, focusing on node failure modes, ultimate bearing capacity, and stress distribution within the steel tube.

Core Technical Problem

The fundamental challenge addressed in this study is the limited bond strength between the steel tube and the infilled concrete. In CFST arch rib nodes, where multiple members converge and complex load paths exist, interfacial slip and debonding between steel and concrete are particularly prone to initiate. This debonding severely degrades the composite action and the overall load-bearing capacity of the arch rib. The PBL stiffener system is proposed as a mechanical interlocking solution to enhance this interface bond and promote composite behavior.

Technical Analysis of PBL Stiffener Mechanism

PBL stiffeners consist of profiled steel plates (typically corrugated or perforated) bolted through the steel tube wall into the concrete core. The mechanism operates on several principles:

• The profiled geometry creates mechanical interlock that resists relative sliding between steel and concrete.
• Bolted connections transfer shear forces across the steel-concrete interface, effectively preventing delamination.
• The stiffeners distribute localized stresses from the arch column reactions more uniformly across the tube circumference.
• By restraining the steel tube circumferentially, PBL stiffeners reduce hoop stress concentrations that would otherwise promote local buckling.

The authors modeled the node under the horizontal force generated by the arch columns (vertical columns supporting the deck). Without PBL stiffeners, the steel tube tends to bear these horizontal forces independently, leading to high local stress concentrations and premature local buckling. With PBL stiffeners, the horizontal force is shared between steel and concrete, significantly improving the load distribution.

Finite Element Modeling Approach

The nonlinear finite element analysis employed in this study incorporates:

Key Findings and Engineering Implications

The study concludes that embedded PBL stiffeners provide a practical and effective improvement for CFST arch rib nodes. The key findings can be summarized as follows:

1. PBL stiffeners significantly enhance the steel-concrete interface bond strength, reducing the tendency for interfacial slip under compression and shear loading.
2. The ultimate bearing capacity of nodes with PBL stiffeners is notably higher than those without, with the improvement attributed to the enhanced composite action.
3. The stress distribution within the steel tube becomes more uniform when PBL stiffeners are present, reducing peak stress concentrations that would otherwise initiate local buckling.
4. The failure mode shifts from local steel tube buckling (without PBL) to more ductile composite failure (with PBL), indicating improved structural reliability.

Engineering Practice Considerations

From a practical standpoint, several factors must be considered when implementing PBL stiffeners in CFST arch rib nodes:

• Fabrication complexity: PBL stiffeners require precise drilling and bolt installation through the steel tube wall, which adds fabrication time and cost. The bolt holes must be positioned to avoid interfering with the arch rib's primary load path.
• Concrete placement: The presence of PBL stiffeners within the tube may affect concrete flowability and compaction, particularly in large-diameter tubes. Vibration access must be ensured to avoid honeycombing around the stiffeners.
• Welding and bolt quality: All bolted connections and any associated welds must meet stringent quality requirements. Bolted joints should be inspected for proper torque and penetration depth.
• Corrosion protection: The internal PBL stiffeners and bolts are embedded in concrete, which provides some corrosion protection. However, any gaps or voids around the stiffeners could allow chloride ingress, necessitating adequate concrete cover and quality.

Study Insights and Reflections

This study contributes valuable insight into the design of CFST arch rib nodes, which remain a challenging area in composite bridge engineering. The PBL stiffener concept is not new, but its systematic application to arch rib compression nodes with rigorous nonlinear finite element validation is a meaningful contribution. One limitation of the study is the reliance on numerical modeling without extensive experimental validation. Future work should include physical tests on full-scale or scaled node specimens to verify the finite element predictions and refine the constitutive models used for the steel-concrete interface. Additionally, the long-term performance under cyclic loading, such as traffic-induced vibration and thermal cycling, deserves further investigation.

The broader implication is that mechanical interlocking devices, such as PBL stiffeners, offer a viable alternative to chemical bonding or surface roughening methods for enhancing steel-concrete composite action in CFST members. This approach is particularly attractive for large-diameter tubes where surface roughening alone cannot provide adequate bond strength. Engineers designing CFST arch bridges should consider PBL stiffeners as a standard detail for critical node regions where interfacial debonding could compromise structural safety.