Key Technologies for Drag Construction of Curved Steel Tube Concrete Continuous Truss Bridge

Literature Overview

This 2012 study by Wang Hailiang, Yang Xinlei, Ren Quanchang, Dong Peng, and Wang Zhenyu, published in Railway Engineering, addresses the construction technology challenges of a curved steel tube concrete (CFT) spatial continuous truss bridge with a span configuration of (40.7 + 9 × 45.5 + 40.7) meters. The research was supported by the Tianjin Natural Science Foundation Key Project, the Ministry of Housing and Urban-Rural Development Research Project, and the Tianjin University Science and Technology Development Fund. The study focuses on drag construction (also known as pull-in construction) techniques for this complex curved geometry.

Core Technical Content

Drag construction is an efficient method for building long-span bridges where the superstructure is assembled on a launch yard and then pulled across the supports. For curved CFT truss bridges, this method introduces unique challenges related to the three-dimensional curvature of the structure, the interaction between the launching nose (guide beam) and the curved geometry, and the stability of partially assembled structures during the pulling process.

The study addresses three critical technical areas through finite element simulation of the construction process: guide beam design, slider local stress analysis, and construction stability assessment. Each of these areas requires careful engineering analysis to ensure safe and efficient construction.

Key Technical Findings

The guide beam design must accommodate the curved geometry of the bridge while providing sufficient structural capacity to support the self-weight of the assembled truss during launching. The curvature introduces additional bending moments and torsional stresses in the guide beam that are not present in straight bridge applications. The slider system, which supports the trailing end of the structure during launching, must be designed to handle localized contact stresses that can be significantly amplified by the curvature-induced eccentricities.

The construction stability analysis reveals that the partially assembled bridge structure is most vulnerable during the mid-span crossing phase, where the unsupported cantilever length is maximum. The curved geometry exacerbates this condition by introducing lateral components of gravity loading that must be resisted by the structural system.

Engineering Practice Implications

For steel pipe manufacturing and structural engineering, the drag construction of curved CFT truss bridges presents several practical considerations. The steel tubes used in the truss members must be manufactured with precise geometric tolerances to accommodate the curved alignment, particularly at the nodes where multiple members intersect. Any deviation from the designed curvature can lead to fit-up problems during assembly and residual stresses in the completed structure.

The concreting process for CFT members during drag construction requires careful coordination with the launching schedule. The concrete must reach sufficient strength before the member is subjected to significant structural loads during subsequent launching stages. This may require accelerated curing methods or staged concreting with strength monitoring at each stage.

The slider design must account for the combined effects of vertical gravity loads, lateral curvature-induced forces, and dynamic effects from the launching process. The local stress concentrations at the slider contact surfaces can lead to premature wear or damage if not properly managed through bearing plate design and lubrication systems.

Study Insights and Reflections

This research highlights the importance of construction sequence analysis in the design of complex bridge structures. The structural behavior during construction can differ significantly from the as-built condition, and design assumptions based solely on the final structural configuration may lead to inadequate safety margins during the construction phase. The curved geometry of this bridge introduces additional complexity that requires three-dimensional analysis rather than simplified two-dimensional approximations.

For engineers involved in steel pipe fabrication, the key insight is that the manufacturing tolerances for curved CFT truss members must be tighter than for straight members, as accumulated geometric deviations along the curved alignment can lead to significant fit-up problems. The study also underscores the need for close coordination between structural design, construction engineering, and manufacturing to ensure that the construction method is compatible with the structural design requirements. The drag construction method, while efficient, demands meticulous planning and execution to manage the inherent risks associated with partially assembled structures spanning large distances.