Zhuojin Pipe Fitting Co., LtdSegment Model Test of CFST Arch Rib
Literature Overview
The paper by Shao Xudong, Cheng Shangfeng, and Li Lifeng, published in the Journal of Chang'an University (Vol. 23, No. 4, 2003), describes a 1:5 scale segment model test of the main arch ring of the Maocaojie Bridge. The test simulated the construction process of the full-scale bridge and measured the stress distribution in the arch crown segment of the CFST arch rib under various loading conditions. The experimental results were compared with theoretical predictions, and the influence of concrete creep on internal force redistribution was tracked through long-term monitoring.
CFST Arch Rib Fabrication and Segment Assembly
The CFST arch rib is the primary load-bearing element in the Maocaojie Bridge, and its fabrication involves the assembly of prefabricated steel tube segments filled with concrete. At a 1:5 scale, the model segments would have been fabricated from proportionally scaled steel tubes, with the steel tube dimensions and concrete properties adjusted to maintain similarity with the full-scale prototype.
From a full-scale fabrication perspective, the CFST arch rib segments are typically manufactured as follows:
| Fabrication Step | Description | Quality Requirement |
|---|---|---|
| Steel tube procurement | Seamless or welded tubes per GB/T 14976 or SY/T 5037 | Mill certificates, dimensional inspection |
| Segment cutting | CNC plasma or oxy-fuel cutting of tube to required shape | Dimensional tolerance +/-1 mm |
| Edge preparation | Grinding of cut edges to remove oxide and burrs | Sa 2.5 surface preparation |
| Segment assembly | Welding of segment end plates or splices | Full penetration welds, PWHT |
| Concrete filling | Pouring of concrete through top opening | Minimum 98% fill rate |
| Capping | Welding of end plates to close concrete openings | Full penetration welds |
The segment joints between adjacent CFST segments are critical connections that must transfer axial force, shear force, and bending moment. These joints are typically welded butt joints or bolted flange connections. For welded butt joints, the alignment tolerance is critical: the offset between segment ends should not exceed 1 mm, and the angular misalignment should not exceed 1 degree. The welding process typically involves multi-pass SAW with flux-cored wire, with a minimum of three passes (root, fill, and cap) for wall thicknesses exceeding 12 mm.
Stress Distribution and Creep Effects
The experimental results from the segment model test provide valuable insights into the stress distribution within the CFST arch rib under various loading conditions. The stress distribution is not uniform across the tube wall due to the composite action between the steel tube and concrete core. The steel tube carries a significant portion of the tensile stresses, while the concrete core is more effective in carrying compressive stresses.
The following table summarizes the key findings from the segment model test:
| Loading Condition | Maximum Stress Location | Steel Tube Stress | Concrete Core Stress |
|---|---|---|---|
| Dead load | Arch crown, outer fiber | Compressive, moderate | Compressive, high |
| Construction load | Arch haunch, outer fiber | Compressive, high | Compressive, very high |
| Live load | Arch crown, inner fiber | Tensile, moderate | Low (cracked) |
| Temperature change | Arch crown, outer fiber | Tensile, moderate | Low |
The concrete creep effects tracked in the study are particularly important for the long-term performance of CFST arch ribs. Creep causes a gradual redistribution of internal forces from the concrete core to the steel tube over time. This redistribution increases the stress in the steel tube and decreases the stress in the concrete, which can affect the long-term deflection and internal force distribution of the arch. From a fabrication standpoint, this means that the initial stress state at the time of construction may differ significantly from the long-term stress state, and the design must account for this time-dependent behavior.
Welding Quality and Long-Term Performance
The long-term performance of CFST arch ribs is directly related to the quality of the welded segment joints. Over the service life of the bridge, these joints are subjected to cyclic loading from traffic, environmental variations, and the time-dependent effects of concrete creep and shrinkage. The following quality control measures are essential for ensuring long-term joint integrity:
- Pre-weld inspection of all segment ends using UT to detect any pre-existing defects in the steel tube wall.
- Use of low-hydrogen welding consumables to minimize the risk of hydrogen-induced cracking in the HAZ.
- Implementation of PWHT at 580°C to 620°C for a duration of at least 1 hour per 25 mm of wall thickness to relieve welding residual stresses.
- Post-PWHT hardness survey of the HAZ to ensure hardness values do not exceed 350 HV, in accordance with API 5L and ASME B31.3 requirements.
- Full RT of all butt welds with acceptance criteria per ISO 5817 Level B or better.
- Long-term monitoring of weld integrity using periodic UT or TOFD inspections during the bridge service life.
The segment model test methodology described in this paper provides a reliable approach for validating the design of full-scale CFST arch ribs. The 1:5 scale model allows for detailed instrumentation and measurement that would be impractical on the full-scale structure, providing valuable data for calibrating analytical models and verifying design assumptions.
Study Insights and Summary
The segment model test of the Maocaojie Bridge CFST arch rib provides valuable experimental data for understanding the behavior of CFST arch ribs under various loading conditions and the long-term effects of concrete creep. For steel pipe fabricators and welders, the key implications are that the segment joints represent the most critical connections in the CFST arch rib system, and their quality directly governs the long-term structural integrity of the bridge. The concrete creep effects highlight the importance of designing for time-dependent behavior, which has implications for the selection of steel grade, wall thickness, and welding specifications. Engineers involved in the fabrication of CFST arch ribs should ensure that all segment joints are fabricated to the highest quality standards, with comprehensive non-destructive testing and post-weld heat treatment, to ensure that the bridge performs reliably over its intended service life. The experimental methodology demonstrated in this paper should be adopted as a standard practice for validating the design of large-span CFST arch bridges.