Construction Technology for Long-Span Steel Tube Trusses at Hangzhou International Expo Center

Overview and Project Background

This paper by Ge Jie, Wang Guiling, Wang Yuling, and Zhang Xiaoyong (China Construction Eighth Engineering Division, 2016) documents the construction technology applied to the large-span steel tube trusses at Zone V of the Hangzhou International Expo Center. The project involves 14 trusses, each spanning 72 meters, installed in the three-story column-free exhibition hall. The trusses are spindle-shaped (spindle-type) steel tube trusses, which means the members vary in cross-section along their length to optimize material usage while maintaining structural efficiency. The trusses were erected using a ground assembly and overall lifting method, which presents unique challenges related to dimensional accuracy, lifting stresses, and the transition from construction state to design state.

Structural and Construction Challenges

The construction of these large-span trusses presents several significant engineering challenges:

• Large span and long members: At 72 meters per span, the trusses are among the longest in Chinese exhibition hall construction. The individual tube members are correspondingly long, requiring careful handling during fabrication, transportation, and assembly.
• Complex geometry: The spindle-shaped configuration means that the truss members are not uniform in cross-section, creating challenges for fabrication, welding, and assembly. The varying member sizes also affect the load distribution and deflection behavior.
• Ground assembly constraints: The "ground assembly, overall lifting" method requires that the entire truss be assembled on the ground at full scale before being lifted into position. This approach minimizes the need for working platforms at height but requires precise ground-level assembly facilities and lifting equipment with adequate capacity.
• Deflection and deformation: The large span and the weight of the truss during lifting create significant deflections and deformations that must be anticipated and compensated for through pre-camber (pre-set camber) of the truss before installation.

Construction State Analysis and Pre-Camber Determination

A key contribution of this paper is the systematic comparison of the structural response in the construction state versus the design state. The analysis process includes:

1. Finite element modeling: A detailed finite element model of the truss was developed to simulate the construction sequence, including ground assembly, lifting, and final installation.
2. Construction state simulation: The model was used to predict the deflection, member forces, and stress distribution during each stage of the construction process, particularly during the lifting operation.
3. Design state comparison: The construction state results were compared with the design state results to identify the differences in structural response. These differences are primarily due to the temporary loading conditions during lifting and the absence of permanent loads (such as roof deck and finishes) during the lifting phase.
4. Pre-camber optimization: Based on the comparison, the optimal pre-camber value was determined to ensure that the truss achieves the design geometry after all permanent loads are applied. The pre-camber compensates for the dead load deflection of the completed structure.

Fabrication and Welding Considerations

From a steel pipe and welding perspective, the fabrication of spindle-shaped steel tube trusses requires careful attention to several quality aspects:

• Tube cutting and beveling: The varying member sizes require precise cutting and beveling of steel tubes at each connection point. The bevel angle and thickness must be consistent with the welding procedure specifications (WPS) to ensure full-penetration welds.
• Welding sequence and distortion control: The welding sequence for each truss node must be carefully planned to minimize angular and longitudinal distortion. Symmetric welding patterns and intermittent welding sequences help to control residual stresses and deformation.
• Heat-affected zone management: For the steel grades typically used in large-span trusses (such as Q345 or Q355), the HAZ must be controlled to avoid excessive hardening or softening that could compromise the fatigue performance of the welded joints.
• Non-destructive testing: Given the criticality of the truss members and the difficulty of inspecting welds after installation, comprehensive NDT (radiographic testing and ultrasonic testing) should be performed on all critical welds before the truss is lifted into position.

Engineering Practice Implications

The construction technology documented in this paper provides valuable lessons for similar large-span steel truss projects:

1. Pre-camber accuracy: The pre-camber value must be determined through rigorous finite element analysis that accounts for the actual construction sequence, temporary loading conditions, and material properties. Field measurements during assembly should be compared with the analytical predictions to verify the pre-camber adequacy.
2. Lifting procedure: The lifting operation must be carefully planned to minimize dynamic effects and asymmetric loading. The lifting points should be selected to create a balanced load distribution, and the lifting speed should be controlled to avoid shock loads.
3. Tolerance management: The dimensional tolerances for the truss members and nodes must be tight enough to ensure proper fit-up during assembly without requiring excessive field modification. The accumulated tolerance along the 72-meter span can be significant and should be managed through a systematic tolerance budget.
4. Quality documentation: Each truss should have a comprehensive quality dossier documenting the material certifications, welding records, NDT results, and dimensional inspection reports. This documentation is essential for traceability and for future maintenance or modification of the structure.

Key Reflections and Study Insights

The most valuable aspect of this paper is the emphasis on the systematic comparison between the construction state and the design state. In practice, many large-span truss projects suffer from unexpected deflections after installation because the pre-camber was estimated rather than calculated. The rigorous finite element analysis approach advocated in this paper provides a rational basis for pre-camber determination that significantly reduces the risk of post-installation geometric deviation.

A practical observation from the steel pipe fabrication standpoint is that the spindle-shaped truss design, while material-efficient, creates challenges for the welding and inspection of variable-diameter tube joints. The transition between different tube diameters at the truss nodes requires careful design of the gusset plates or sleeve connections to ensure adequate weld access and inspection capability. Engineers should consider the constructability of the connection details during the design phase, not merely the structural adequacy.

The paper also implicitly highlights the importance of construction sequencing in large-span truss projects. The order in which the 14 trusses are assembled and lifted can affect the temporary loading on the supporting structure and the cumulative deformation of the already-installed trusses. A systematic construction plan that accounts for these interactions should be developed before the project begins.