Prestressing Technology and Structural Analysis of Steel Tube Concrete Tied-Arch Bridge - A Study on Nanjing Qixia Bridge

Literature Overview

The paper by Liu Zhao, Meng Shaoping, Lu Wenhua, and Liu Borun, published in the journal Bridge Construction in 1999 (Volume 29, Issue 3, pages 14-17), presents a comprehensive investigation into the prestressing construction technology and structural behavior of the Nanjing Qixia Bridge. This bridge is a large-span, wide-deck steel tube concrete tied-arch bridge with a main span of 86.5 meters. The authors, affiliated with Southeast University's Prestressing Engineering Research Institute and Nanjing Railway Bureau No.1 Engineering Company, provide both theoretical analysis and field measurement data to evaluate structural safety throughout the construction stages. This work represents an important milestone in the application of prestressing techniques to steel tube concrete arch structures in China during the late 1990s.

Core Technical Content

The Nanjing Qixia Bridge is notable for its combination of steel tube concrete (SRC) construction with a tied-arch system and prestressing technology. The prestressing system is applied to the tie beam to control deflection and stress distribution during and after construction. The authors describe the prestressing construction sequence, which involves careful coordination between the erection of the steel tube concrete arch ribs, the casting of the deck, and the application of prestress to the tie members.

The structural analysis encompasses multiple construction stages, each with distinct loading conditions and load paths. The authors analyzed the force characteristics of the bridge at each construction phase, including the temporary support stage, the arch rib erection stage, the deck casting stage, and the prestress application stage. Field measurements of tie beam stresses were conducted and compared against theoretical predictions to validate the structural safety throughout construction.

A key finding highlighted by the authors is the unique mechanical behavior of large-span, wide-deck tied-arch bridges. The wide deck introduces additional torsional and lateral bending effects that are not present in conventional narrow-deck arch bridges. The steel tube concrete construction provides high strength and stiffness while allowing the use of prestressing to manage internal forces effectively.

Technical Points and Engineering Insights

The prestressing technology applied to this bridge serves multiple purposes: controlling long-term deflection of the tie beam, reducing tensile stresses in the concrete tie members, and improving the overall stiffness of the tied-arch system. The steel tube concrete arch ribs provide excellent load-carrying capacity through the confinement effect of the steel tube on the concrete core, which enhances both the compressive strength and ductility of the composite section.

From an engineering practice perspective, several lessons emerge from this study. First, the coordination between prestressing and the steel tube concrete construction sequence is critical. The prestress force level and application timing must be carefully designed to account for the time-dependent behavior of the concrete, including shrinkage and creep, which affect the long-term stress state of the tie beam. Second, the wide deck configuration requires attention to lateral stability during construction, particularly during the asymmetric erection of the deck and arch ribs. The field measurement program described in the paper demonstrates the importance of monitoring structural response during construction, especially for complex bridges where the load path changes significantly between construction stages.

The unique mechanical characteristics of wide-deck tied-arch bridges include increased sensitivity to wind-induced lateral loads, torsional coupling between the deck and the arch, and the need for robust transverse bracing systems. The steel tube concrete construction offers advantages in terms of constructability, as the steel tubes can be erected and welded in segments before concrete placement, allowing for faster construction cycles compared to conventional reinforced concrete arch construction.

Study Insights and Implications for Practice

This 1999 study predates many of the more recent advances in steel tube concrete bridge engineering, but its fundamental principles remain highly relevant. The integration of prestressing with steel tube concrete construction represents a hybrid approach that leverages the advantages of both systems. The prestressing compensates for the tensile limitations of concrete in the tie members, while the steel tube concrete arch ribs provide high compressive capacity with good ductility.

For modern engineering practice, this study reinforces the importance of stage-by-stage structural analysis for complex bridge construction. The construction sequence directly influences the final stress state, and any deviation from the planned sequence can lead to unexpected structural behavior. The field measurement approach adopted in this project, combining strain gauges and displacement monitoring, provides a template for construction monitoring programs that should be implemented on similar structures.

The study also highlights the value of combining theoretical analysis with experimental validation. The comparison between calculated and measured tie beam stresses provides confidence in the structural safety assessment methodology. Engineers working on similar steel tube concrete tied-arch bridges should adopt a similar integrated approach, ensuring that construction monitoring data is systematically compared against predictive models throughout the construction process.