Axial Compression Behavior of Inner-Round Outer-Square Composite Steel-Concrete Short Columns

Literature Overview

The research by Zhang Yang, Qian Jiaru, Ji Xiaodong, and Cao Wanlin, published in the journal World Earthquake Engineering in 2011, investigates the axial compression bearing capacity of a novel composite column configuration featuring an inner circular steel tube and an outer square steel tube with concrete infill. Funded by the Beijing Science and Technology Program Major Project (D09050600370000), the Beijing Municipal Education Commission Science and Technology Program Key Project (KZ200910005008), and the Tsinghua University Independent Research Program (20101081766), this study explores the structural behavior and load-bearing characteristics of this unique composite section under concentric axial compression.

Technical Configuration and Design Rationale

The inner-round outer-square composite section combines the beneficial confinement properties of a circular steel tube with the geometric advantages of a square outer section for architectural and connection purposes. The circular inner tube provides uniform confinement to the concrete core, while the square outer tube offers greater torsional stiffness and easier integration with beam connections. The concrete infill between the two steel tubes creates a sandwich composite action that enhances overall structural performance.

The composite action between the inner and outer steel tubes through the concrete infill is a critical design consideration. The interface between the two tubes must be designed to ensure effective load transfer and prevent relative slip that would compromise the composite behavior. Welding, mechanical fastening, or continuous concrete casting are potential methods for achieving this integration.

Experimental Results and Load-Displacement Behavior

The experimental program involved testing multiple specimens with varying parameters including steel tube wall thickness, concrete strength grade, and slenderness ratio. The load-displacement curves exhibited the characteristic three-stage behavior observed in composite steel-concrete columns: an initial elastic stage, a yielding stage with progressive plastic deformation, and a strain-hardening or plateau stage before ultimate failure.

The ultimate bearing capacity of the composite columns exceeded the sum of the individual component capacities by 15 to 25 percent, demonstrating the beneficial confinement effect of the steel tubes on the concrete. This enhancement factor, often denoted as alpha in design equations, reflects the triaxial stress state induced in the confined concrete. The inner circular tube provided more effective confinement than the outer square tube due to its continuous curvature, which eliminates stress concentrations at corners.

Failure Mode Analysis

The failure modes observed in the tested specimens provide valuable insights into the structural behavior of the composite section. In specimens with adequate confinement, the failure was characterized by concrete crushing accompanied by local buckling of the outer square tube walls. The concrete in the corner regions of the square outer tube exhibited earlier crushing due to stress concentration effects, while the concrete adjacent to the inner circular tube maintained higher residual strength due to more uniform confinement.

In specimens with insufficient steel tube wall thickness, premature buckling of the outer tube occurred before full concrete confinement was achieved, resulting in a more brittle failure mode. The authors identified a critical wall thickness ratio below which the confinement benefit is significantly reduced. This finding has direct implications for the design of composite columns in seismic applications, where ductile behavior is essential for energy dissipation.

Standards and Design Code Implications

The research findings have implications for the design of composite columns under standards such as GB 50017 (Chinese standard for steel structures), GB 50010 (Chinese standard for concrete structures), and relevant international codes including Eurocode 4 and AISC 360. The inner-round outer-square configuration is not explicitly addressed in most current design codes, which typically provide provisions for circular or square/rectangular concrete-filled steel tubes (CFST). The study provides experimental data that can inform the development of design provisions for this novel section type.

The axial compression design formula proposed by the authors incorporates a confinement enhancement factor that accounts for the dual-tube configuration:

The proposed formula includes terms for the steel tube contribution, the confined concrete contribution with an enhancement factor dependent on the confinement ratio, and an interaction term that accounts for the composite action between the two steel tubes. The confinement ratio is defined as the ratio of the total steel tube cross-sectional area to the concrete cross-sectional area, adjusted for the geometric configuration.

Engineering Practice and Constructability

From a practical construction standpoint, the inner-round outer-square composite section presents several challenges. The fabrication of the square outer tube requires precision cutting and welding of flat plates, with attention to corner weld quality and dimensional accuracy. The insertion of the inner circular tube requires careful alignment and fixing within the square tube before concrete placement. The concrete placement process must ensure complete filling of the space between the two tubes without voids, which may require the use of self-consolidating concrete or specialized vibration methods.

The connection design between the composite column and adjacent structural elements is another critical consideration. The square outer tube provides a convenient interface for beam-column connections, while the inner circular tube may require special connection details if continuity is required. The authors recommend that connection designs be developed and tested separately, as the interaction between the connection details and the composite section behavior can significantly affect the overall structural performance.

Study Insights and Reflections

This research contributes to the broader field of composite structural engineering by demonstrating the potential of innovative section geometries to enhance structural performance. For engineers working in the cladding and bimetallic products field, the principles of composite action, interface behavior, and confinement effects are directly relevant to the design of clad pressure vessels and bimetallic components. The study highlights the importance of experimental validation in developing design provisions for novel structural configurations, as numerical models alone may not capture all aspects of the complex interaction between different materials and geometries.

The findings also underscore the value of multi-institutional collaboration in advancing structural engineering knowledge, bringing together expertise from Tsinghua University and Beijing University of Technology to address a technically challenging research question. The practical implications for seismic-resistant design of composite structures are significant, particularly in regions with high seismic risk where ductile and energy-absorbing structural systems are essential for life safety.