Zhuojin Pipe Fitting Co., LtdNonlinear Study of CFST Composite Lattice Column High-Pier Continuous Rigid Frame Bridge
Literature Overview
The paper by Zhan Yulin et al. (2009), published in Sichuan Building Science, presents the nonlinear analysis of the Labajintea Bridge on the Ya'an-Luzhou Highway, which was the first bridge in China to employ circular steel tube concrete (CFST) composite lattice columns for high piers in a large-span continuous rigid frame bridge. The study, supported by Southwest Jiaotong University and Sichuan Provincial Transportation Department, established a three-dimensional nonlinear numerical model that considered the composite section behavior of the CFST lattice columns and conducted analysis for both construction stages and the completed bridge state. The research was motivated by the need to evaluate the lateral deformation control provided by transverse connections in this innovative structural system.
Core Technical Content
The Labajintea Bridge represents a pioneering application of CFST composite lattice columns in bridge engineering. The lattice column system consists of multiple circular CFST members interconnected by transverse and diagonal braces, forming a spatial truss-like structural system that provides high axial stiffness while maintaining reasonable lateral flexibility. The key innovation lies in the combination of CFST members (which provide high compressive capacity and ductility) with a lattice configuration (which provides efficient material utilization and lateral stability).
Nonlinear Analysis Results
| Analysis Aspect | Key Finding | Engineering Significance |
|---|---|---|
| Construction stage nonlinearity | Progressive load redistribution during staged construction | Construction sequence must be carefully planned |
| Completed bridge stiffness | CFST lattice columns provide significantly higher axial stiffness than conventional RC columns | Reduced pier height and foundation size |
| Transverse connection effect | Transverse braces significantly reduce lateral deformation | Essential for wind and seismic resistance |
| Long-term deformation | Time-dependent effects (creep, shrinkage) significantly influence long-term deflection | Must be accounted for in design |
| Lateral deformation control | Lateral deformation reduced by 30-40% with transverse connections compared to independent CFST columns | Critical for bridge alignment and serviceability |
Structural Configuration Parameters
The study examined the influence of several key geometric and material parameters on the nonlinear behavior of the bridge:
- CFST column diameter and wall thickness: Determined by axial load capacity requirements, with a typical slenderness ratio of 15-25 for the individual CFST members within the lattice.
- Lattice spacing: The spacing between CFST members in the lattice arrangement, which affects both the load distribution efficiency and the constructability.
- Transverse connection type: Welded gusset plate connections versus bolted connections, each with different stiffness and ductility characteristics.
- Concrete strength: Typically C40-C60 for bridge applications, with higher strength providing increased composite action through improved confinement.
Welding and Fabrication Considerations for Bridge CFST Lattice Columns
The fabrication of CFST lattice columns for bridge applications presents several welding and fabrication challenges that are distinct from building structural applications:
Welding Challenges
- Circumferential welds for CFST segments: The individual CFST members are fabricated in segments that are connected by circumferential butt welds. These welds must achieve full penetration and maintain the circular geometry of the tube. The recommended welding method is submerged arc welding (SAW) with automatic or semi-automatic equipment, using a two-pass technique (root pass + fill/cap pass) with flux cored wire (E71T-8 or equivalent).
- Transverse connection welds: The welded gusset plate connections between CFST members and transverse braces require fillet welds that must achieve adequate throat thickness while minimizing distortion of the CFST tube. Gas metal arc welding (GMAW) or flux-cored arc welding (FCAW) is typically used, with careful preheating and interpass temperature control.
- Geometric accuracy: The lattice configuration requires precise geometric alignment of all CFST members, as misalignment introduces unintended bending moments that can significantly affect the structural performance. The dimensional tolerances for the CFST members should be within ±1 mm per meter of length for straightness and ±0.5% of diameter for circularity.
Quality Control Requirements
| Quality Parameter | Acceptance Criteria | Test Method | Standard Reference |
|---|---|---|---|
| Weld penetration | 100% full penetration | UT (PAUT) | ISO 17635-2 Level B |
| Surface defects | No cracks, no undercut > 0.5 mm | MT/PT | ISO 17637 |
| HAZ hardness | ≤ 350 HV (for Q345/Q355 steel) | Microhardness | API 5L Table 4.4.2 |
| Impact toughness (HAZ) | ≥ 27 J at -20°C | Charpy V-notch | EN 10216-3 |
| Hydrostatic test | 1.5 × design pressure, 2 min hold | Pressure test | GB/T 21836 |
| Dimensional tolerance | ±0.5% diameter, ±1 mm/m straightness | Dimensional survey | EN 10216-1 |
Residual Stress Management
The lattice column configuration creates a complex residual stress field resulting from multiple sequential welding operations. The residual stresses from circumferential welds, transverse connection welds, and any post-fabrication straightening operations interact in a complex manner that can significantly affect the buckling behavior of the CFST members under compressive loading. The following measures are recommended:
- Welding sequence optimization: Plan the welding sequence to minimize accumulated distortion, using symmetric welding patterns where possible.
- Post-weld stress relief: Apply vibration stress relief (VSR) to reduce residual stresses to below 100 MPa, or local PWHT at 580-620°C for critical weld regions.
- Residual stress measurement: Use hole-drilling strain gauge method or X-ray diffraction method to measure residual stresses at critical locations and verify that they are within acceptable limits.
Engineering Practice Integration
The Labajintea Bridge demonstrates the practical viability of CFST composite lattice columns for high-pier bridge applications. The nonlinear analysis results confirm that the system provides superior stiffness and lateral deformation control compared to conventional reinforced concrete piers, with the added benefit of reduced construction time due to the prefabrication potential of CFST members. However, the analysis also reveals that the construction sequence has a significant influence on the final structural behavior, requiring careful planning and monitoring during construction.
For welding and fabrication engineers, this case study highlights the importance of achieving high geometric accuracy and weld quality in CFST lattice columns. The performance of the entire bridge structure depends on the precise interaction between individual CFST members, which in turn depends on the quality of the welds connecting them. Any deficiency in weld quality or geometric accuracy can compromise the load distribution efficiency of the lattice system and reduce the overall structural capacity.
Summary and Reflection
The nonlinear analysis of the CFST composite lattice column bridge presented in this paper demonstrates the structural efficiency and practical viability of this innovative system for high-pier bridge applications. The key finding that transverse connections significantly reduce lateral deformation underscores the importance of the welded connections between CFST members as critical structural elements. For fabrication and welding engineers, this translates into a requirement for exceptional weld quality and geometric accuracy, as the performance of the entire lattice system depends on the integrity of every weld joint. The construction of CFST lattice columns should be approached with the same rigor as pressure vessel fabrication, with comprehensive procedure qualification, skilled welder certification, 100% volumetric NDT of all welds, and careful residual stress management to ensure the long-term structural integrity of these ambitious bridge structures.