Zhuojin Pipe Fitting Co., LtdRobustness Design of Concrete-Filled Steel Tube Arch Bridges
Literature Overview
This paper by Chen, Fan, Yu, Wu, and Huang (2016), published in Bridge Construction (Vol. 46, No. 6, pp. 88-93), addresses the critical issue of structural robustness in medium and lower-bearing CFST arch bridges. Funded by the National Natural Science Foundation of China (Grants 51678154 and 51178118), the study proposes robustness design principles from both conceptual and structural design perspectives, with particular focus on the deck system and hanger rod systems that are most vulnerable to progressive collapse.
Conceptual Framework for Structural Robustness
Structural robustness refers to the ability of a structure to withstand local damage or failure of individual components without suffering disproportionate collapse. For CFST arch bridges, this concept is particularly critical because the deck system of medium and lower-bearing arch bridges relies heavily on hanger rods to support the deck weight and transfer loads to the arch ribs. The failure of even a single hanger rod can trigger a cascade of failures if the deck lacks adequate redundancy.
The authors classify hanger rod failure incidents into three severity levels:
| Severity Level | Description | Consequence |
|---|---|---|
| Level 1 | Single hanger rod failure | Local damage, deck remains stable |
| Level 2 | Multiple adjacent hanger rod failures | Partial deck collapse, adjacent sections affected |
| Level 3 | Progressive collapse of entire deck system | Catastrophic structural failure |
The robustness design requirements are defined as: (1) hanger rods should not break under credible accident scenarios, and (2) the deck system should not collapse even if hanger rod failure occurs.
Classification of Deck Systems by Robustness
The study categorizes deck systems into five types based on their inherent robustness characteristics:
- Type 1: Fully integral deck with continuous longitudinal girders providing inherent redundancy.
- Type 2: Deck with stiffening longitudinal beams of sufficient stiffness and load-carrying capacity.
- Type 3: Semi-integral deck with partial continuity.
- Type 4: Deck with stiffening longitudinal beams requiring enhanced design.
- Type 5: Simply supported deck segments with minimal redundancy.
For Type 4 deck systems, the stiffening longitudinal beams must possess adequate stiffness and load-carrying capacity to redistribute loads following hanger rod failure. This has direct implications for the design and fabrication of the steel girders forming the stiffening beams, including requirements for material toughness, weld integrity, and connection detailing.
Robustness Design Recommendations
The paper proposes specific robustness design recommendations for different structural components:
- Hanger rods: Design should include a robustness-specific ultimate limit state calculation to ensure that the rods maintain sufficient residual capacity under overload conditions, including vehicle impact, fatigue, and corrosion scenarios.
- Type 4 deck systems: The stiffening longitudinal beams require enhanced design with increased section modulus, higher material toughness requirements, and detailed connection design to ensure load redistribution capability.
- Tied-arch bridges: The tie rods require robustness design with adequate safety margins against horizontal thrust, and the substructure must provide sufficient reserve capacity to resist horizontal forces.
- Arch ribs: The CFST arch ribs themselves generally exhibit good robustness due to the composite action between steel and concrete, but the connection details at the arch feet and springing points require careful attention.
Engineering Practice and Quality Control Implications
From a steel pipe and welding engineering perspective, the robustness design philosophy translates into several practical requirements:
- Material selection for hanger rods and stiffening beams: Materials should be selected with adequate toughness at service temperatures, with Charpy V-notch impact energy requirements specified at the relevant temperatures. For Q345 steel components, minimum impact energy of 47 J at -20°C is a common requirement.
- Welding quality for critical connections: Welds at hanger rod anchorages, stiffening beam splices, and arch rib joints must be designed and inspected to the highest standards. Full-penetration butt welds with 100% UT or RT examination are recommended for critical load paths.
- Fatigue design: Hanger rods and stiffening beam connections are susceptible to fatigue under traffic loading. Fatigue detail categories should be selected conservatively, and fatigue assessment should be included in the robustness design.
- Corrosion protection: The robustness of CFST arch bridges depends on the long-term integrity of both steel and concrete components. Coating systems, cathodic protection, and drainage design must be specified to ensure adequate service life.
The robustness design approach advocated in this paper represents a paradigm shift from traditional limit state design, emphasizing the need for structural systems that can survive local failures without catastrophic consequences. This philosophy should be integrated into the design, fabrication, and quality control processes for all CFST arch bridge components, with particular attention to the hanger rod systems and deck stiffening elements that are most vulnerable to progressive collapse.