PBL-Stiffened Rectangular Steel Tube Concrete Composite Truss Bridge Design Study Note

Literature Overview

The paper published in Bridge Construction (2019, Vol. 49, No. 5) by Liu Bin and colleagues from Chang'an University, Zhejiang Communications Group, and China Railway Major Bridge Engineering Group Research Institute presents the design methodology for the Huangyan Highway Overpass, a (24+40+24) m continuous rigid-frame PBL-stiffened rectangular steel tube concrete (SRC) composite truss bridge. This structure represents a notable advancement in hybrid steel-concrete bridge engineering, where Partially-Through-Box (PBL) shear connectors are integrated into rectangular steel tube members to enhance load-bearing capacity and local buckling resistance. The research is funded by the National Natural Science Foundation of China (Grants 51778058, 51378068) and the Central Universities Basic Research Fund (300102219310).

Core Design Configuration and Structural System

The bridge adopts a composite truss system where the main girders consist of rectangular steel tube trusses combined with a concrete deck. The piers are Y-shaped twin-leg rectangular SRC columns supported by a diamond-shaped pile cap and bored pile foundations. The key innovation lies in the application of PBL longitudinal ribs within the rectangular steel tubes in the negative moment zone of the lower chord members and the Y-shaped piers, with concrete infill to form a PBL-stiffened SRC cross-section.

The following table summarizes the principal structural parameters and design features:

Technical Interpretation of PBL Hybrid Connection Technology

PBL shear connectors, originally developed by Japan Road Engineering Corporation (JREC), have become a mainstream solution for composite steel-concrete connections. In this bridge design, the PBL technology serves a dual function: it acts as a shear connector between the concrete deck and the steel truss, and simultaneously functions as an internal stiffener within the rectangular steel tubes to improve local buckling resistance in the negative moment zone.

The critical design consideration is the optimization of the negative moment zone. In a continuous rigid-frame truss bridge, the lower chord members and pier top regions experience significant negative bending moments during service and seismic loading. The rectangular steel tubes in these zones are susceptible to local flexural buckling of the compression flange. By inserting PBL longitudinal ribs and filling the tube with concrete, the composite section achieves several benefits:

1. The concrete infill provides lateral confinement to the steel tube walls, significantly delaying local buckling onset.
2. The PBL ribs act as internal stiffeners, increasing the effective buckling resistance of the compression flange.
3. The composite action between steel and concrete through the PBL connectors ensures efficient stress transfer and load sharing.

Node Optimization and Load Path Analysis

The paper highlights two key node optimization measures to improve the bearing capacity and failure mode of PBL-stiffened SRC joints:

• Main tube concrete infill: Filling the main tube (chord member) with concrete at connection nodes enhances the joint stiffness and prevents premature local buckling of the chord member walls under concentrated loads from diagonal members.
• Equal-width branch and main tubes: Designing the branch members (diagonal and web members) with the same width as the main tube eliminates the width discontinuity at the connection, thereby reducing stress concentration and preventing local yielding at the junction.

These optimization measures are particularly important because joint failure in truss structures is typically a brittle, sudden event that can lead to catastrophic collapse. By ensuring that the joint capacity exceeds the member capacity, the design promotes a more ductile failure mode where members yield before the joints fail.

The connection between the concrete deck and the main truss employs closed upper chord members with PBL-embedded steel plates. This arrangement provides a robust shear transfer mechanism while maintaining the structural integrity of the closed upper chord, which contributes to the overall torsional stiffness of the truss. Similarly, the pier-to-pile-cap connection utilizes a grid-pattern arrangement of PBL steel plates, distributing the concentrated loads from the pier legs across the pile cap in a controlled manner.

Welding and Fabrication Considerations

From a welding and fabrication perspective, this design presents several technical challenges that are worth noting for engineering practice:

The welding of rectangular steel tubes requires careful control of residual stress and distortion. The longitudinal seam welds of rectangular tubes, typically fabricated using HFW (High-Frequency Welding) or SAW (Submerged Arc Welding), must achieve full fusion with acceptable dilution ratios. The transverse welds at node connections, where multiple members intersect, require multi-pass welding with controlled interpass temperatures to prevent hydrogen-induced cracking and minimize residual stress accumulation.

Engineering Practice Implications

This case study demonstrates that the PBL-stiffened SRC concept can be effectively applied to highway overpass bridges with moderate spans. The continuous rigid-frame system eliminates the need for bearings and expansion joints at the pier tops, reducing maintenance requirements. The rectangular steel tube sections provide good torsional resistance and aesthetic appeal compared to conventional I-section trusses.

However, engineers should note that the PBL connector spacing, rib thickness, and concrete strength must be carefully calibrated to ensure that the composite action is fully mobilized. The shear capacity of PBL connectors depends on the concrete strength, steel yield strength, and the geometry of the rib (typically the rib thickness should be at least 6 mm for structural applications). The concrete infill within the tubes must be properly consolidated to avoid voids that could compromise the composite action and create corrosion risks.

A key insight from this study is the systematic approach to node design optimization. Rather than treating joints as secondary elements, the design philosophy elevates joint performance to a primary design concern, which aligns with the well-established principle in structural engineering that the weak link in a structure determines its overall performance.

Summary and Reflection

The Huangyan Overpass design represents a mature application of PBL hybrid connection technology in the context of a composite truss bridge system. The integration of PBL stiffeners within rectangular SRC members to address negative moment zone vulnerabilities is an elegant engineering solution that simultaneously improves bending capacity and local buckling resistance. The node optimization measures of concrete infill and equal-width connections demonstrate a thorough understanding of joint failure mechanisms in truss structures. For practitioners in steel and composite bridge engineering, this case study provides valuable reference for the design of similar structures, particularly regarding the detailed connection design and the systematic approach to ensuring joint ductility through member-joint capacity matching.