Feasibility Study on Design and Construction of 700m-Class CFST Arch Bridge

Literature Overview and Strategic Significance

This paper by Zheng Jialian, Wang Jianjun, Mou Tingmin, Feng Zhi, Han Yu, and Qin Dayan, published in China Engineering Science (2014, Vol. 16, No. 8, pp. 33-37), presents a feasibility study on the design and construction of 700-meter-class steel tube concrete (CFST) arch bridges, building upon the established design and construction technologies developed for 500-meter-class CFST arch bridges. The authors, representing the Guangxi Zhuang Autonomous Region Department of Transportation, Guangxi Highway and Bridge Engineering Corporation, and Sichuan Provincial Highway Planning and Design Institute, argue that no technical barriers remain to the construction of 700-meter CFST arch bridges and that this bridge type should be actively promoted at this span range.

Overall Design and Construction Scheme

The feasibility study establishes an overall design scheme for the 700-meter CFST arch bridge, building on the successful experience of the 500-meter class bridges that have been constructed in China. The design considers the structural system, material selection, arch rib configuration, hanger system, and foundation design. The construction scheme addresses the fabrication of the arch rib tubes, the erection methodology, and the concrete casting process into the arch rib tubes.

The key technical challenges identified for the 700-meter span include:

• The increased weight and geometric complexity of the arch rib tubes
• The logistics of transporting and erecting large-diameter steel tubes at the required elevation and span
• The concrete casting process into the arch rib tubes at significant heights and over large spans
• The structural stability during the construction phases, particularly during the arch rib erection and concrete casting stages
• The thermal effects on the steel tube structure during concrete casting and long-term service

Arch Rib Tube Fabrication for 700m Span

The arch rib tubes for a 700-meter CFST arch bridge are among the largest steel tubes ever fabricated for a bridge application. These tubes typically have diameters in the range of 1000 to 1600 mm with wall thicknesses of 20 to 40 mm, fabricated from high-strength steel plates conforming to GB/T 1591 or equivalent international standards. The fabrication process involves plate rolling, longitudinal welding, and circumferential welding, with extensive non-destructive testing and quality control at every stage.

The fabrication of these large-diameter tubes requires specialized equipment including heavy-duty plate rolling machines, long welding tables or positioners, and automated or semi-automated welding systems capable of maintaining consistent weld quality over long weld lengths. The welding of thick-walled tubes demands careful control of heat input to prevent excessive grain growth in the heat-affected zone and to minimize hydrogen-induced cracking. Multi-pass welding with proper layering sequences is essential to manage residual stresses and prevent distortion.

Concrete Casting Process into Arch Rib Tubes

The concrete casting process into the arch rib tubes is a critical construction activity that directly affects the structural performance of the CFST arch. For a 700-meter span, the concrete volume within the arch rib tubes is substantial, and the casting process must be carefully planned to manage the thermal effects, the pumping pressures, and the structural loading during the casting sequence.

The concrete casting typically employs a bottom-up approach, with the concrete pumped into the tubes from the lowest point. The pumping pressure must be sufficient to overcome the friction losses in the pump lines and the hydrostatic head of the concrete column, but must not exceed the structural capacity of the unfilled or partially filled tubes. The thermal effects of concrete hydration can cause significant temperature rises within the tubes, leading to thermal stresses in the steel tube and potential cracking of the concrete if not properly managed.

The steel tube must be designed to accommodate the thermal expansion during concrete casting without inducing excessive stresses or deformations. Expansion joints or flexible connections may be required at specific locations to allow for thermal movement. The concrete mix design should incorporate low-heat cement or supplementary cementitious materials to limit the peak temperature rise, and cooling pipes may be embedded in the concrete to control the temperature gradient.

Construction Erection Methodology

The erection of the 700-meter CFST arch involves the assembly of prefabricated steel tube segments into the full arch shape, followed by the concrete casting process. The erection methodology typically employs a combination of temporary supports, cable-stayed construction, and segmental assembly. The steel tube segments are fabricated in the workshop, transported to the site, and lifted into position using cranes or cable-stayed systems.

The welding of the steel tube segments in the field is a critical construction activity that requires careful planning and execution. The field welding environment is less controlled than the workshop environment, with factors such as wind, temperature, humidity, and accessibility all affecting the welding quality. Welding procedure qualifications must be obtained for field conditions, and welders must be certified for the specific welding processes and positions required. The inspection of field welds is essential, with 100% radiographic and ultrasonic testing of all circumferential welds and a representative percentage of longitudinal welds.

Stability Analysis During Construction

The structural stability during the construction phases is a critical concern for the 700-meter CFST arch bridge. The arch structure is not self-supporting until the concrete has been cast and has achieved sufficient strength. During the steel tube erection phase, the arch is supported by temporary structures, and the stability analysis must consider the various construction sequences and loading conditions.

The stability during the concrete casting phase is particularly important because the asymmetric loading from the concrete placement can create significant bending moments and torsional stresses in the arch. The steel tube must have adequate stiffness and strength to resist these transient loads, and the construction sequence must be planned to minimize the asymmetric loading. The use of temporary stays or supports during the concrete casting phase may be necessary to ensure structural stability.

Feasibility Conclusion and Industry Implications

The feasibility study concludes that the construction of 700-meter CFST arch bridges is technically feasible, with no insurmountable technical barriers. The study recommends early engineering practice and promotion of this bridge type at the 700-meter span range. This conclusion is significant for the steel pipe manufacturing industry because it signals a demand for the fabrication of extremely large-diameter steel tubes with stringent quality requirements.

The steel pipe industry must develop and demonstrate the capability to fabricate tubes of 1000-1600 mm diameter with 20-40 mm wall thickness, with full-penetration welds, comprehensive NDT, and verified mechanical properties. The welding technology must be advanced to handle the thick sections with controlled heat input, minimal residual stress, and excellent weld toughness. The quality control systems must be capable of ensuring consistent quality across the entire production run, with full traceability and documentation.

The feasibility study also highlights the importance of construction sequencing and temporary support design in ensuring structural stability during the construction phases. The steel pipe manufacturer must collaborate closely with the construction contractor to ensure that the tube fabrication tolerances are compatible with the erection methodology and that the tubes can be transported, lifted, and connected on site without damage or degradation of quality.

Study Insights and Reflections

This feasibility study represents a significant milestone in the advancement of CFST arch bridge technology from the 500-meter class to the 700-meter class. The conclusion that no technical barriers exist is encouraging but should not be interpreted as a signal that the challenges are trivial. The fabrication of large-diameter steel tubes for 700-meter arch bridges requires advanced manufacturing capabilities, sophisticated welding technology, and rigorous quality control systems. The concrete casting process into these large tubes presents unique thermal and structural challenges that must be carefully managed. The steel pipe and welding industry must invest in the development of the necessary capabilities to meet the demands of this emerging market segment, and the engineering community must continue to refine the design and construction methodologies through practical experience and research.