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

Literature Overview

The paper by Liu Bin, Liu Yongjian, Yang Yuehua, Cao Mingming, and Wang Kangning published in Bridge Construction (2019, Vol. 49, No. 5, pp. 97-102) presents the design methodology and structural optimization of the Huangyan Bridge, a (24+40+24) m continuous rigid frame PBL-stiffened rectangular steel tube concrete composite truss bridge. The bridge employs a hybrid PBL (Punched and Bent Lattice) connection technology combined with rectangular steel tube concrete (SRC) sections to achieve high load-bearing capacity while optimizing the negative moment region and joint performance. The research was supported by the National Natural Science Foundation of China (Grants 51778058, 51378068) and the Central University Basic Research Business Fee Special Fund (Grant 300102219310).

Core Technical Concepts

The fundamental innovation of this design lies in the integration of PBL stiffening ribs within rectangular steel tubes to form a PBL-stiffened SRC cross-section. PBL connections, originally developed for steel-concrete composite structures, involve punching and bending thin steel plates to create interlocking lattice elements that provide both shear transfer and mechanical interlock between steel and concrete components. In this application, the PBL ribs are longitudinally placed within the rectangular steel tubes of the lower chord members and Y-shaped bridge piers, with concrete subsequently poured into the tubes to create a composite action.

PBL Stiffening Mechanism

The PBL stiffening ribs serve a dual purpose: they enhance the load-bearing capacity of individual structural members and improve the local buckling resistance of the compressed steel tubes. In conventional rectangular SRC sections, the thin-walled steel tubes are susceptible to local buckling under compressive loads, particularly in the negative moment region where the upper flange of the deck experiences tension while the lower chord undergoes compression. The PBL ribs provide additional confinement and geometric stability, effectively distributing the compressive stresses and delaying the onset of local buckling.

Negative Moment Region Optimization

The negative moment region in continuous rigid frame bridges is a critical design consideration. In this zone, the structural behavior differs significantly from the positive moment region: the bottom flange of the main truss experiences tension while the top chord and pier connections undergo compression. The authors implemented two key optimization measures to address joint failure modes:

1. Concrete infilling of the main tube — This provides additional confinement to the steel tube at the joint, improving the ductility and post-yield behavior of the connection.
2. Equal-width design of branch and main tubes — By ensuring that the branch tube width matches the main tube width, the stress concentration at the weld junction is significantly reduced, and the load transfer path becomes more uniform.

Joint Design and Connection Technology

The bridge deck connection system employs embedded steel plates with PBL openings on the top chord closed rectangular section. This approach creates a robust shear connection between the concrete deck slab and the main truss structure. The PBL openings in the embedded plates provide mechanical interlock that supplements the weld strength, offering redundancy in load transfer.

For the pier-to-foundation connection, PBL punched steel plates are arranged in a grid network pattern in both longitudinal and transverse directions. This concentrated arrangement at the pile cap junction ensures that the complex stress state at the pier base is adequately transferred to the foundation. The diamond-shaped pile cap combined with bored pile foundation provides a stable base for the Y-shaped dual-limb rectangular SRC tree-like pier.

Engineering Practice Integration

Key Technical Insights and Reflections

The application of PBL technology in this bridge design represents a meaningful advancement in composite bridge engineering. From a welding and fabrication perspective, the PBL connection technology introduces specific challenges: the punching and bending operations must be precisely controlled to ensure consistent geometric accuracy, and the weld quality at the PBL-to-steel tube interfaces requires careful monitoring. The equal-width branch-to-main tube design simplifies the welding sequence and reduces the complexity of fillet weld preparation, which is beneficial for quality control in field fabrication.

The decision to fill the main tube with concrete at the joint region reflects a practical understanding of the stress redistribution behavior in complex composite joints. This approach effectively transforms what would otherwise be a brittle failure mode into a more ductile one, which is particularly important for seismic resilience in regions with moderate to high seismicity.

The grid-pattern PBL plate arrangement at the pier base is an elegant solution to the multi-axial loading problem inherent in pier-foundation connections. By distributing the connection points in a network pattern, the design accommodates both vertical and horizontal loads while maintaining constructability.

Study Insights

The Huangyan Bridge design demonstrates that PBL technology, when properly integrated with SRC systems, can yield significant structural advantages in composite truss bridges. The key takeaway for engineering practice is that the optimization of the negative moment region and joint design should be considered as an integrated system rather than isolated component improvements. The equal-width tube design philosophy, while seemingly simple, addresses a fundamental issue in tubular joint design — the stress concentration problem — through geometric simplification rather than complex reinforcement detailing. This approach aligns with the principle of achieving structural reliability through rational design rather than excessive material usage.