Zhuojin Pipe Fitting Co., LtdNovel Dumbbell-Section CFST Short Columns with H-Section Internal Stiffeners
Literature Overview
This study by Sheng Ye, Chen Baochun, Huang Fuyun, and Xiao Zerong, published in the Journal of Rail and Transportation Engineering in 2005, addresses a significant practical challenge in steel tube-confined concrete (CFST) arch rib construction. Traditional dumbbell-section CFST arch ribs require concrete to be poured into the enclosed belly cavity formed by the two steel tubes connected by web plates. The authors identify that the stress concentration at the intersection of the steel tube and the web plate during concrete pouring can cause explosive bursting of the tube wall, a serious construction hazard. Their proposed solution replaces the concrete-filled belly with H-section internal stiffeners, eliminating the need for cavity concrete filling.
Problem Identification and Design Innovation
The construction of traditional dumbbell-section CFST arch ribs involves a critical risk: when concrete is pumped into the enclosed belly cavity, the internal pressure exerted on the steel tube walls at the tube-web plate junction can exceed the local yield strength of the steel tube, causing sudden bursting. This phenomenon, known in Chinese construction practice as "bao guan" (tube bursting), has resulted in multiple construction accidents and significant safety concerns.
The novel design proposed in this paper retains the outer dumbbell-shaped steel tube configuration but replaces the concrete-filled belly cavity with internal H-section stiffeners. This modification eliminates the need for cavity concrete filling during construction, thereby removing the primary cause of tube bursting. The H-section stiffeners provide structural continuity between the two steel tubes while maintaining the overall dumbbell cross-sectional geometry that provides high bending stiffness.
Axial Compression Test Results
The authors conducted axial compression tests on short column specimens of the novel dumbbell-section CFST configuration and compared the results with traditional concrete-filled dumbbell-section specimens. The test program included measurement of load-displacement relationships, strain distribution on the steel tube walls, and observation of failure modes.
| Test Parameter | Traditional Dumbbell CFST | Novel H-Stiffener Dumbbell |
|---|---|---|
| Concrete filling | Belly cavity filled with concrete | No cavity concrete; H-section stiffeners only |
| Failure mode | Steel tube outward local buckling with concrete crushing | Steel tube outward local buckling with stiffener yielding |
| Ultimate bearing capacity | Reference baseline | Comparable to traditional type |
| Stress concentration at tube-web junction | High during pouring; moderate during service | Eliminated during construction; moderate during service |
| Construction risk | High risk of tube bursting during concrete placement | Minimal risk; no cavity filling required |
The test results demonstrated that the ultimate bearing capacity of the novel dumbbell-section short columns was comparable to that of traditional concrete-filled dumbbell-section columns. The stress-strain curves showed similar stiffness and ductility characteristics, indicating that the H-section stiffeners effectively transfer shear forces between the two steel tubes and maintain structural integrity.
Bearing Capacity Calculation Method
The authors concluded that the ultimate bearing capacity of the novel dumbbell-section CFST columns can be calculated using the same formula as traditional dumbbell-section CFST columns. This finding is significant because it means that existing design codes and calculation methods for CFST members remain applicable without modification, facilitating the adoption of the novel design in engineering practice.
The bearing capacity calculation for dumbbell-section CFST columns typically employs the superposition method, where the total capacity is expressed as the sum of the individual steel tube capacities plus the confined concrete capacity. For the novel design, the contribution of the H-section stiffeners to the overall bearing capacity should be included in the steel component capacity calculation. The confinement effect on the concrete within the outer steel tubes remains unchanged since the concrete is still confined by the outer steel tubes.
Engineering Practice Implications
From a steel pipe fabrication and welding perspective, this study offers several important practical insights. First, the elimination of cavity concrete filling simplifies the construction sequence and reduces the risk of construction accidents. This is particularly beneficial for large-span arch bridges where dumbbell-section CFST arch ribs are commonly used.
Second, the welding details at the H-section stiffener-to-steel tube junction require careful attention. These welds must transfer shear forces between the stiffener and the outer tubes and must be designed for fatigue resistance in structures subjected to cyclic loading. The weld configuration should follow AWS D1.1 or EN ISO 3834 standards for structural steel welding.
Third, the fabrication of the novel dumbbell-section members involves welding H-section stiffeners to the inner surface of the steel tubes. This internal welding operation presents access challenges and requires specialized welding procedures. In practice, TIG (GTAW) welding with filler wire insertion through access holes, or robotic welding with manipulator arms, may be employed. The weld quality must be verified by ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) to detect internal defects.
Study Insights and Reflections
This paper addresses a practical construction problem with an elegant engineering solution. The replacement of cavity concrete with H-section stiffeners eliminates the tube bursting risk while maintaining comparable structural performance. The finding that existing bearing capacity formulas remain applicable is a significant practical advantage, as it reduces the barrier to code adoption.
However, several aspects warrant further investigation. The long-term durability of the H-section stiffeners when exposed to environmental conditions, particularly in bridge applications where chloride ingress and carbonation may affect the internal steel elements, should be evaluated. Additionally, the fire resistance of the novel configuration, where the absence of concrete in the belly cavity may reduce the thermal mass and compromise the structural performance under fire loading, requires assessment.
The study also raises questions about the fatigue performance of the H-section stiffener welds under repeated traffic loading in bridge applications. The stress concentration at the stiffener-to-tube weld junction could be a fatigue crack initiation site, and fatigue life assessment using fracture mechanics methods may be necessary for critical bridge components.
Summary
This study presents a practical and innovative solution to the tube bursting problem in traditional dumbbell-section CFST arch ribs. By replacing cavity concrete with H-section internal stiffeners, the construction risk is substantially reduced while maintaining comparable ultimate bearing capacity and structural performance. The applicability of existing bearing capacity calculation methods simplifies the design process. Engineers considering this novel configuration should pay attention to the welding quality of internal stiffener connections, long-term durability of internal steel elements, and fire resistance performance. This approach represents a valuable contribution to the safe and efficient construction of CFST arch structures.