Working Performance of Hollow Steel Tube Concrete Axial Compression Members

Literature Overview

This paper by Zhong Shantong and Xu Guolin, published in the Journal of Harbin Institute of Technology (2006, Vol. 38, No. 9, pp. 1479–1482), presents research on the axial compression behavior of hollow steel tube concrete members produced by the centrifugal casting method. The study covers circular, square, and polygonal cross-sections and develops a unified strength design formula applicable to various cross-sectional geometries. The work represents a significant advancement in the structural design of SRC members with hollow cores, which offer weight reduction while maintaining load-bearing capacity.

Core Technical Content

The centrifugal method of producing hollow SRC members involves rotating a mold containing the steel tube and concrete mix at high speed, causing the concrete to compact against the inner wall of the steel tube while air and excess water migrate toward the hollow core. This method produces dense, homogeneous concrete with improved workability and reduced void content compared to conventional pouring methods.

Key Performance Parameters

The study demonstrates that the working performance of hollow SRC members varies continuously with both the confinement coefficient and the hollow ratio, exhibiting a systematic and predictable relationship that conforms to unified structural theory. This continuity is a crucial finding because it validates the applicability of analytical models to hollow SRC members, enabling engineers to interpolate or extrapolate performance for intermediate parameter values.

Strength Design Formula

The authors derived a universal strength design formula for axial compression that applies to circular, square, and polygonal cross-sections. The formula incorporates the confinement effect of the steel tube on the concrete core while accounting for the reduced concrete volume due to the hollow section. The experimental results from short column tests showed excellent agreement with calculated results, confirming the formula's reliability.

From a welding and fabrication standpoint, the steel tubes used in these members must satisfy several requirements:

• Dimensional accuracy: The inner diameter must be uniform along the length to ensure consistent centrifugal concrete compaction.
• Surface quality: The inner surface should be free of scale, rust, or weld deposits that would impair concrete adhesion.
• Weld integrity: For welded tubes, the longitudinal seam must be free of defects that could propagate under combined compressive and confining stresses.

Process and Standards Analysis

Centrifugal Casting Process Window

The centrifugal method requires careful control of several process parameters:

1. Rotational speed: Must be sufficient to generate adequate centrifugal force for concrete compaction but not so high as to cause concrete segregation or tube deformation.
2. Concrete mix design: The mix must have adequate workability for centrifugal casting while maintaining target strength. Typical slump values range from 100–180 mm for centrifuged concrete.
3. Vibration and duration: The casting duration must be sufficient for complete compaction and air removal from the concrete core.
4. Curing conditions: Post-casting curing must be controlled to prevent differential shrinkage between the tube wall and concrete core.

Applicable Standards

Integration with Engineering Practice

The hollow SRC concept finds applications in bridge piers, building columns, and marine structures where self-weight reduction is critical. The centrifugal method eliminates the need for internal formwork, reducing construction costs and construction time. However, from a quality assurance perspective, the following concerns must be addressed:

• Hollow ratio control: Excessive hollow ratios may lead to insufficient concrete volume for effective confinement, particularly at the tube corners for polygonal sections.
• Interface quality: The concrete-steel tube interface must be dense and free of voids; centrifugal casting inherently promotes this, but process deviations can create weak zones.
• Long-term durability: The hollow core may be susceptible to moisture ingress if not properly sealed, potentially leading to internal corrosion of the steel tube.

Study Insights and Reflections

The derivation of a unified formula applicable to multiple cross-section shapes represents a significant simplification of design practice. In my experience reviewing structural designs, the lack of standardized formulas for non-circular SRC sections has often led engineers to adopt overly conservative approaches or avoid such geometries altogether. This research removes that barrier.

However, I would note that the study focuses on short columns under pure axial compression. Practical structures involve eccentric loading, shear forces, and combined stress states. The transition from short column to long column behavior in hollow SRC members—where buckling becomes the governing failure mode—requires additional investigation. The hollow core reduces the second moment of area, potentially making these members more susceptible to flexural buckling under eccentric loads.

The centrifugal casting method also raises questions about the long-term durability of the concrete-tube interface. Unlike cast-in-place concrete that cures in situ with the tube, centrifugally cast concrete experiences significant mechanical compaction forces during manufacturing. The residual stresses from this process may evolve over time under service loading, potentially affecting the long-term confinement effectiveness.