Eccentric Compression Bearing Capacity of Multi-Chamber Polygonal Steel Tube Concrete Columns

Research Context and Engineering Motivation

This study by Xu Lihua and colleagues from Wuhan University, published in the China Journal of Engineering Mechanics in 2019, investigates the eccentric compression behavior of multi-chamber polygonal steel tube concrete (SC) columns. The research was motivated by the Tianjin Gaoyin 117 Tower, which employs a giant multi-chamber polygonal column as a primary structural element. The column's innovative geometry combines the structural efficiency of steel tube confinement with the load-bearing capacity of concrete, while the polygonal cross-section with internal chambers offers advantages in material distribution and constructability. Funded by the National Natural Science Foundation of China key project program, this research represents a significant advancement in the design of super-tall building columns.

Experimental Program and Specimen Configuration

Eleven specimens were fabricated at a 1/20 scale of the prototype column, representing the Tianjin 117 Tower's giant column. The specimens were subjected to eccentric compression loading under static conditions. The experimental parameters varied to investigate the influence of multiple design variables on structural performance.

The primary failure mode observed across all specimens was bending-type instability, consistent with the behavior expected for slender composite columns under eccentric loading. The load-lateral deflection curves exhibited a distinct pattern: an initial linear elastic stage, a nonlinear hardening stage as the concrete and steel interact, and a post-peak softening stage associated with progressive yielding of the steel tube and crushing of the concrete.

Quantitative Findings and Parametric Analysis

The finite element parametric study, conducted using ABAQUS, provided comprehensive insight into the influence of key design parameters:

• Concrete strength: Higher concrete strength directly increases the ultimate bearing capacity, as expected, since the concrete core carries the majority of the compressive load.
• Steel tube wall thickness: Increasing wall thickness enhances both the ultimate capacity and the confinement effect on the concrete, improving ductility and post-peak performance.
• Internal steel cage: The presence of a reinforcing steel cage inside the chamber significantly improves ductility and late-stage load-bearing capacity, providing additional load paths as the concrete begins to crush.
• Slenderness ratio: Increasing the slenderness ratio from 24 to 70 caused a 38.6% reduction in ultimate bearing capacity, reflecting the increased susceptibility to buckling.
• Eccentricity ratio: Increasing the eccentricity ratio from 0.2 to 1.0 resulted in a 54.1% reduction in ultimate bearing capacity, demonstrating the strong sensitivity of these columns to moment loading.

Based on the finite element results, the authors developed bearing capacity formulas applicable to hexagonal six-chamber and pentagonal four-chamber configurations. These formulas reference existing SC column design methods but incorporate the specific geometric and material characteristics of multi-chamber polygonal sections.

Design Formulas and Code Compliance Considerations

The development of design formulas for non-standard cross-sections is a significant practical contribution. Existing design codes such as GB 50017-2017 for steel structures and GB 50010-2010 for concrete structures provide guidance for conventional circular and rectangular SC columns but do not directly address multi-chamber polygonal configurations. The authors' approach of calibrating formulas against finite element results, while referencing established design methods, provides a pragmatic pathway for code integration.

The formulas should be validated against additional experimental data before widespread adoption in design codes, as the current database of 11 specimens, while valuable, is limited in scope. Future research should expand the specimen database to include more parameter combinations, particularly at higher slenderness ratios and eccentricity ratios where the failure mode transitions from material-dominated to stability-dominated.

Engineering Practice and Welding Implications

From a fabrication standpoint, multi-chamber polygonal columns present unique welding challenges. The internal chamber walls require full-penetration welds at their intersections with the outer shell and internal steel cage, and the polygonal geometry means that weld access is more restricted compared to circular sections. Welding sequence planning is critical to control distortion, as the asymmetric geometry can lead to out-of-plane warping if welds are not properly sequenced. Preheating requirements for thick-walled steel tubes and controlled interpass temperatures are essential to prevent cracking in the heat-affected zone, particularly in high-strength steel grades.

The study's finding that the internal steel cage significantly improves ductility has direct implications for seismic design. In regions subject to earthquake loading, the inclusion of internal reinforcement is not merely a strength consideration but a ductility requirement. The steel cage provides confinement to the concrete core, preventing brittle spalling and maintaining load-carrying capacity under large inelastic deformations.

Study Insights and Outlook

This research bridges the gap between innovative structural concepts and practical design methodology. The 117 Tower's multi-chamber polygonal column represents a new generation of super-tall building structural systems, and the development of validated design formulas is essential for the safe and economical adoption of such systems. The parametric analysis reveals that slenderness and eccentricity are the dominant factors controlling capacity, which should guide preliminary design decisions. Engineers working on similar projects should ensure that their finite element models accurately capture the steel-concrete interaction, including the slip behavior at the interface and the confinement effect of the steel tube on the concrete core.