Multi-Cavity Steel Tube Concrete Giant Bifurcated Column Axial Compression Strain Test Analysis

Literature Overview

This paper by Yang Guang, Cao Wanlin, Dong Hongying, Yang Weibiao, and Tian Shichuan (2016) reports on the axial compression strain test analysis of an irregular cross-section multi-cavity steel tube concrete (CFST) giant bifurcated column. The research is directly linked to the "China Zun" project in Beijing, one of the tallest buildings in the world, which employs this unique structural form to achieve both architectural elegance and structural efficiency at the base of the superstructure. The study was funded by the National Natural Science Foundation of China (51578020) and published in the Journal of Natural Disasters (Vol. 25, No. 6, pp. 138–149).

Core Technical Content

The irregular multi-cavity cross-section bifurcated column represents a highly innovative structural solution where the transition from a single large-diameter column to multiple smaller columns is achieved through a bifurcated geometry with multiple concrete-filled cavities. The cross-section geometry is far from regular, and the interaction between the steel plates and the confined concrete within each cavity creates a complex stress-strain behavior that differs significantly from conventional circular or rectangular CFST members.

Key Experimental Parameters and Findings

The test specimens were scaled models of the actual giant bifurcated column used in the China Zun project. Strain gauges were installed on both the longitudinal and transverse directions of the steel plates to capture the full strain field during axial compression loading. The section geometric properties were systematically analyzed in conjunction with the strain data.

Confinement Effect Mechanism

A critical insight from this study is the differentiation between the confinement effect provided by inner steel plates versus outer steel plates. In a multi-cavity configuration, the inner plates (shared between adjacent cavities) experience a different stress state compared to the outer plates. The inner plates receive lateral pressure from concrete on both sides simultaneously, while the outer plates receive pressure from only one side. This asymmetry leads to:

• Differential hoop stress development between inner and outer plates
• Non-uniform concrete confinement pressure distribution within the cavity
• Potential for localized buckling at regions where the outer plate is less effectively confined

Section Geometry and Post-Peak Behavior

The paper emphasizes that for irregular cross-sections, the geometric properties of the section (including the distribution of steel plate thickness, cavity dimensions, and the transition geometry at the bifurcation) play a dominant role in governing the post-peak deformation capacity. This is particularly critical for seismic design, where ductility and energy dissipation capacity are paramount. The bifurcation region, where the column splits into multiple branches, represents a geometric discontinuity that can act as a stress concentration zone.

Engineering Practice Implications

For engineers involved in the design of super-tall building core structures, this study provides several actionable insights:

1. Stiffener design: Longitudinal stiffeners should not be treated as purely local buckling prevention measures. They actively modify the global strain distribution and can either enhance or degrade the confinement efficiency depending on their spacing and geometry.
2. Differential confinement modeling: Standard confinement models (such as Mander's model) assume uniform lateral confinement. For multi-cavity sections, a modified model that distinguishes between inner and outer plate contributions is necessary for accurate prediction of concrete strength enhancement and ductility.
3. Bifurcation detail design: The transition geometry at the bifurcation point requires careful detailing to ensure smooth load transfer between the single column and the multiple branches. The strain data indicate that this region is sensitive to geometric variations.

Study Insights and Reflections

Having worked extensively with steel tube structures in industrial applications, I find this research particularly illuminating because it bridges the gap between conventional CFST design principles and the demands of architectural megastructures. The China Zun project demonstrates that structural engineering can accommodate extreme architectural ambition, but only when the mechanical behavior of unconventional geometries is thoroughly understood through experimental investigation.

The finding that inner and outer steel plates provide different levels of confinement has direct implications for material selection and thickness optimization. In a cost-conscious design, one might consider using thicker outer plates (which are less efficiently confined) and thinner inner plates (which benefit from bilateral concrete pressure), thereby achieving equivalent structural performance at lower material cost. However, this must be balanced against fabrication complexity and the need for adequate local buckling resistance at the outer plates.

The post-peak deformation sensitivity to section geometry is a cautionary note for engineers who rely heavily on finite element analysis without experimental validation. For irregular cross-sections, the assumptions embedded in standard material and interaction models may not capture the true behavior, particularly in the post-peak regime where geometric nonlinearity dominates. This reinforces the importance of physical testing for novel structural systems.