Case Study on Large-Span Steel Pipe Bridge for Water Conveyance

Overview of the Liuzhou Zhu-He Stream Crossing Project

The paper by Xiao Ruishu and Yan Liguo from Guangxi Architectural Comprehensive Design Institute, published in China Water Affairs & Drainage (2003, Vol. 19, No. 11), presents a practical engineering case of a large-span steel pipe bridge used for water conveyance across the Zhu-He Stream in Liuzhou City. This case study is particularly valuable for engineers working at the intersection of water supply infrastructure and structural engineering, as it addresses both the structural design of long-span steel pipe crossings and the specialized corrosion protection required for buried or exposed water mains. The project demonstrates how steel pipe bridges can serve as efficient alternatives to traditional culvert or trench-laying methods when crossing rivers, valleys, or other obstacles that would otherwise require excessive excavation or bridge construction.

Structural Design and Load Analysis

The core technical contribution of this paper lies in the structural calculation methodology applied to the over-stream steel pipe. Unlike conventional buried pipelines, a steel pipe bridge must simultaneously resist internal water pressure, its own self-weight, live loads from traffic or pedestrians on the bridge deck, wind loads, and seismic effects. The authors describe the force analysis for the crossing pipe, which involves determining the bending moments, shear forces, and axial forces acting on the pipe under combined loading conditions.

For a large-span steel pipe bridge, the pipe itself acts as a structural member in addition to its primary function of conveying water. The key design parameters include the span length, pipe diameter and wall thickness, allowable stress under combined loads, deflection limits, and buckling resistance. The structural design must comply with relevant standards such as GB 50013 (Code for Design of Water Supply and Drainage Engineering) and applicable structural steel design codes. The following table summarizes the typical design considerations:

The structural analysis reveals that the maximum bending moment occurs at mid-span for simply supported configurations or at the supports for continuous spans. The combined stress state—axial stress from internal pressure plus bending stress from external loads—must remain within the allowable limits for the selected steel grade. For water supply applications, carbon steel grades such as Q235 or Q345 are commonly used, with the wall thickness calculated to satisfy both the hydrostatic pressure requirement and the structural bending requirement.

GZ-2 Special Coating Technology for Corrosion Protection

One of the most distinctive aspects of this project is the application of GZ-2 special paint for corrosion protection and seepage prevention. This coating system is specifically formulated for water conveyance applications and must satisfy dual requirements: external corrosion resistance against soil and atmospheric exposure, and internal water-tightness to prevent seepage through the pipe wall and joints.

The GZ-2 coating technology addresses several critical engineering challenges:

• External corrosion resistance: The coating must provide long-term protection against soil corrosion, which can be particularly aggressive in the humid subtropical climate of Guangxi. The coating system typically consists of a primer layer, intermediate coat, and topcoat, with total dry film thickness ranging from 250 to 400 micrometers.
• Internal seepage prevention: Unlike standard industrial coatings, the GZ-2 system must be water-compatible and maintain adhesion in continuous immersion conditions. This is achieved through specialized resin formulations that provide excellent water resistance and low permeability.
• Energy saving effect: The authors analyze the energy-saving benefits of applying the special coating, noting that reduced seepage losses translate directly to lower pumping energy consumption over the system lifetime.

The application quality of the coating is critical to its long-term performance. Surface preparation to SA 2.5 grade (near-white metal blast cleaning) is essential, and the coating must be applied within the recommended environmental window of 5-40°C temperature and below 85% relative humidity. Post-application inspection using holiday detection (spark test) and dry film thickness measurement (DFT gauge) is mandatory.

Engineering Practice Implications

From a practical standpoint, this case study highlights several lessons for engineers involved in water supply pipeline design. First, the structural design of steel pipe bridges requires a holistic approach that considers both pressure vessel design principles and structural engineering requirements simultaneously. Second, the selection of corrosion protection systems must be tailored to the specific service environment rather than applying generic industrial coating specifications. Third, the energy-saving analysis demonstrates that investing in superior corrosion protection yields significant long-term economic benefits through reduced water losses and lower maintenance costs.

The project also underscores the importance of interdisciplinary collaboration between water supply engineers, structural engineers, and corrosion specialists. The success of such installations depends on the seamless integration of hydraulic design, structural analysis, and material protection strategies. Engineers should note that the GZ-2 coating technology, while effective, requires strict adherence to application protocols to achieve its designed service life, which is typically 15-20 years for external protection and potentially longer for internal water-tightness when properly maintained.

Summary and Key Takeaways

This literature provides a valuable practical reference for the design and construction of large-span steel pipe bridges in water supply systems. The combination of rigorous structural analysis, specialized GZ-2 coating technology, and energy-saving evaluation offers a comprehensive framework that can be adapted to similar projects across China and other regions with comparable climate conditions. The key takeaway is that successful water conveyance infrastructure requires not only sound structural design but also meticulous attention to corrosion protection and long-term operational efficiency, with every design decision evaluated through the lens of lifecycle cost and reliability.