Zhuojin Pipe Fitting Co., LtdKey Technology Research on Drag Construction of Curved Steel Tube Concrete Continuous Truss Bridge
Literature Overview
This study by Wang Hailiang, Yang Xinlei, Ren Quanchang, Dong Peng, and Wang Zhenyu, published in Railway Engineering (Vol. 52, No. 9, 2012, pp. 5-7), addresses the construction technology challenges of a curved steel tube concrete (SRC) spatial continuous truss bridge with a span arrangement of (40.7 + 9×45.5 + 40.7) m. The research focuses on the drag-in construction method, examining guide beam design, slider local stress distribution, and construction process stability. The work was supported by the Tianjin Natural Science Foundation Key Project (12JCZDJC28900), the Ministry of Housing and Urban-Rural Development Research Project (2011-K2-18), and the Tianjin Higher Education Science and Technology Development Fund (20100904).
Core Technical Content and Key Findings
Drag-in construction (拖拉施工) is a method where prefabricated bridge segments are pulled across the span using a guide beam and slider system, eliminating the need for extensive falsework over navigable waterways or traffic lanes. For a curved SRC truss bridge, this method presents unique challenges due to the spatial curvature of the structure, the significant self-weight of the concrete-filled steel tubes, and the complex stress states in the truss members during the incremental construction phases.
Guide Beam Design
The guide beam is the critical temporary structure that spans the gap ahead of the advancing bridge segment. Its design must account for:
- Geometric complexity: The curvature of the bridge introduces lateral and torsional loads on the guide beam that are absent in straight-span applications.
- Loading configuration: The guide beam must support the weight of the bridge segment, the construction equipment, and the pulling forces, all while maintaining adequate clearance above the construction zone.
- Material and section selection: Given the span lengths involved (up to 45.5 m per segment), the guide beam typically requires box girder or truss configurations with high bending stiffness.
Slider Local Stress Analysis
The slider system, which carries the bridge segment on rollers or sliders moving along guide rails, is subjected to highly concentrated contact stresses. The authors conducted finite element analysis of the slider local stress distribution, identifying critical stress concentrations at the slider-guide rail contact zones. Key findings include:
| Slider Parameter | Typical Value | Design Limit | Safety Factor |
|---|---|---|---|
| Contact pressure | 120–180 MPa | 250 MPa | 1.4–2.1 |
| Local bearing stress at roller | 150–220 MPa | 280 MPa | 1.3–1.9 |
| Slider deflection | 3–8 mm | 10 mm | 1.3–2.0 |
| Guide rail web local buckling stress | 80–130 MPa | 200 MPa | 1.5–2.5 |
Construction Process Stability
The stability analysis during drag-in construction is particularly critical for curved SRC bridges because:
- Eccentric loading: The curved geometry causes the center of gravity of the bridge segment to deviate from the slider centerline, creating additional overturning moments.
- Lateral stability: As the bridge segment is pulled across the span, the unsupported overhang creates a cantilever condition that must be checked for lateral-torsional buckling.
- Concrete pouring sequence: The steel tubes are filled with concrete incrementally during construction, and the timing of concrete pouring relative to the drag-in position significantly affects the structural stability.
Welding Quality Requirements for SRC Truss Members
The steel tube concrete truss members used in this bridge type are typically large-diameter seamless or welded steel tubes, and their welding quality directly affects the structural integrity of the completed bridge.
Steel Tube Manufacturing Specifications
| Parameter | Seamless Pipe | HFW Welded Pipe | LSAW Welded Pipe |
|---|---|---|---|
| Typical diameter | 300–600 mm | 400–1200 mm | 600–2000 mm |
| Wall thickness | 8–25 mm | 8–30 mm | 12–40 mm |
| Weld inspection | N/A | UT + MT + ET | RT + UT + MT |
| Tensile strength | ≥400 MPa | ≥390 MPa | ≥345 MPa |
| Impact energy (0°C) | ≥47 J | ≥47 J | ≥47 J |
| Hydrostatic test | 1.5× yield | 1.5× yield | 1.5× yield |
Field Welding Considerations
The field splicing of steel tubes during drag-in construction requires careful planning:
- Weld preparation: The tube ends must be prepared with precise bevel angles (typically 30°±2° for wall thicknesses above 20 mm) to ensure full penetration.
- Positional welding: Field welds are often performed in the 5G or 6G position (horizontal or overhead), requiring qualified welders with specific positional certifications.
- Weld procedure qualification (WPQ): Each welding process (SAW, SMAW, GTAW) must be qualified for the specific material grade, thickness range, and welding position used in the field.
- Post-weld heat treatment (PWHT): For thick-walled tubes (typically >30 mm) or high-carbon equivalent steels (CE >0.45%), PWHT is required to relieve welding residual stresses and improve HAZ toughness.
PDCA Cycle Application in Construction Quality Control
Applying the Plan-Do-Check-Act cycle to the drag-in construction process:
Plan: Develop detailed construction sequences, welding procedures, and quality inspection plans. Define acceptance criteria for steel tube dimensions, weld quality, concrete properties, and structural geometry.
Do: Execute the construction sequence with real-time monitoring of slider loads, guide beam deflections, and structural deformations. Maintain welding logs and concrete batch records.
Check: Conduct non-destructive testing of all field welds (UT for volumetric defects, MT or PT for surface defects), verify concrete strength through cylinder tests, and monitor structural geometry with total stations and theodolites.
Act: Analyze inspection results, identify any deviations from specifications, implement corrective actions, and update construction procedures for subsequent segments.
Study Insights and Conclusions
This research provides valuable technical guidance for the drag-in construction of curved SRC truss bridges, a construction method that is increasingly important for large-span bridges over navigable waterways. The guide beam design, slider stress analysis, and stability assessment presented in the study offer practical tools for construction engineers. The key insight from a materials and welding perspective is that the quality of the steel tube manufacturing and field welding directly determines the structural performance of the completed bridge. Any defect in the longitudinal or circumferential welds of the steel tubes can propagate under the complex stress states experienced during and after construction, potentially leading to catastrophic failure. Therefore, rigorous welding quality control, including weld procedure qualification, welder certification, and comprehensive non-destructive testing, is not optional but essential for the safety and durability of SRC truss bridges constructed by the drag-in method.