Construction Technology of Steel Tube Concrete Columns and Composite Columns at Nanjing South Station

Literature Overview

This paper by Liu Xiaogang, Dai Yaojun, and Gu Haiyong, published in Construction Technology (2011, Vol. 40, No. 12), documents the construction methodology for steel tube concrete columns and composite columns at Nanjing South Station, a third-generation railway passenger station. The project features innovative structural design with steel tube concrete composite columns at the first floor and steel tube concrete columns from the second floor to the roof. The maximum steel tube diameter reaches 1,600 mm, representing one of the largest diameter steel tube concrete columns in China at the time of construction.

Construction Process and Steel Pipe Fabrication Challenges

The construction of ultra-large diameter steel tube concrete columns presents unique challenges in steel pipe fabrication, welding, and on-site assembly. The 1,600 mm diameter tubes require either spiral welding from steel plate or multi-plate fabrication with extensive welding operations. From a steel pipe manufacturing standpoint, the following technical aspects are particularly noteworthy:

• The large diameter necessitates careful control of ovality and flatness during forming to ensure uniform concrete placement and consistent confinement pressure distribution.
• The longitudinal welds on such large diameter tubes require multi-pass welding with strict control of interpass temperature, heat input, and post-weld treatment to minimize residual stresses and distortion.
• The transverse welds at column joints represent critical structural connections that must accommodate both axial loads and bending moments from lateral forces.

Composite Column Construction Methodology

The composite column system described in this paper combines steel tube concrete columns with reinforced concrete columns through a carefully designed transition zone. The key construction innovations include:

1. Innovative anchor bolt installation techniques that ensure precise alignment of the steel tube columns with the reinforced concrete foundation elements.
2. Steel tube column hoisting procedures that account for the significant self-weight and wind loading during lifting operations.
3. Welding procedures for the field connections that maintain structural integrity while accommodating thermal expansion and construction tolerances.
4. Internal concrete pouring and compaction techniques that ensure void-free concrete placement within the large diameter tubes.

The concrete shrinkage issue is particularly critical in steel tube concrete construction. As the concrete shrinks, differential movement between the steel tube and the concrete can create gaps at the interface, reducing the composite action. The authors address this through careful sequencing of construction activities and the use of appropriate concrete mix designs with controlled shrinkage characteristics.

Welding Quality Assurance for Large Diameter Tubes

The welding of 1,600 mm diameter steel tubes requires specialized procedures. The longitudinal welds, typically executed by submerged arc welding (SAW) or flux-cored arc welding (FCAW), must be designed to achieve full penetration with adequate root and cap preparation. The heat input per pass must be carefully controlled to prevent excessive grain growth in the heat-affected zone, particularly for high-strength steels such as Q345 or Q420 commonly used in structural steel tubes.

The field welding of transverse joints presents additional challenges due to the 6G or 6FG welding position, limited access, and the need to maintain structural continuity during construction. The welding procedure specification (WPS) must account for preheating requirements based on the carbon equivalent of the steel, interpass temperature control, and post-weld heat treatment where specified by the applicable code (typically GB 50661 or ASME B31.3 for process piping applications).

Verification Methods and Quality Control

The authors employed a rigorous verification approach including mock-up testing and scale model cutting to validate the construction quality. This methodology is commendable and should be adopted as a best practice for all large-diameter steel tube concrete column projects. The scale model cutting allows direct visual inspection of the interface bond quality, concrete fill density, and weld integrity at critical locations.

Non-destructive testing (NDT) protocols for such projects should include:

• Full-length ultrasonic testing (UT) of all longitudinal and transverse welds using phased array ultrasonic testing (PAUT) or time-of-flight diffraction (TOFD) for enhanced defect detection capability.
• Magnetic particle testing (MT) of all weld surfaces and heat-affected zones to detect surface and near-surface defects.
• Radiographic testing (RT) of a representative percentage of transverse welds, particularly at the column base connections and critical structural joints.
• Hydrostatic testing of the assembled steel tube sections before concrete placement to verify the structural integrity of the welded joints.

Engineering Practice Implications

The Nanjing South Station project demonstrates that ultra-large diameter steel tube concrete columns can be successfully constructed when appropriate engineering methods are applied. The key lessons for steel pipe manufacturers and welding engineers include the importance of early-stage involvement in construction planning, the necessity of comprehensive welding procedure qualification for large-diameter tube fabrication, and the value of verification testing to confirm construction quality. The composite column transition zone design requires careful coordination between the steel pipe supplier, the welding contractor, and the concrete contractor to ensure that the interface between steel tube concrete and reinforced concrete elements functions as intended under all load combinations.