Axial-Compression Combined Performance Analysis of Steel Tube Reinforced Concrete Core Columns

Literature Overview

This paper by Nie Jianguo, Bai Yu, Li Shengyong, Zhao Jie, and Yan Xiao from Tsinghua University, Guangzhou Rong Baisheng Engineering Design Institute, and the University of Southern California, published in the China Civil Engineering Journal in 2005, presents an analytical study on the axial-compression combined performance of steel tube reinforced concrete core columns. Funded by the National Science Fund for Distinguished Young Scholars Overseas Young Scholars Cooperation Fund (Grant No. 50128807), the study addresses a critical design issue in the application of CFST core columns, where the core CFST and the surrounding conventional concrete exhibit significantly different compressive performance. The authors analyze the influence of the stirrup volumetric ratio of the surrounding concrete on the collaborative behavior of the column and derive the critical stirrup volumetric ratio at the limit state, which is then compared with experimental values to validate the analytical model.

Core Technical Findings

The fundamental challenge addressed in this study is the differential compressive behavior between the core CFST and the surrounding conventional concrete in a composite column. The core CFST, due to the confinement provided by the steel tube, exhibits a significantly higher compressive strength and ductility compared to the surrounding conventional concrete, which is confined only by the stirrups. This differential behavior can lead to incompatibility of deformations between the two components, potentially resulting in premature failure of the surrounding concrete before the full capacity of the core CFST is mobilized.

The study derives an analytical expression for the critical stirrup volumetric ratio of the surrounding concrete at the limit state, which represents the minimum stirrup confinement required to ensure that the surrounding concrete deforms compatibly with the core CFST and does not fail prematurely. The analytical results are compared with experimental values, and the agreement is reported to be good, validating the analytical model and providing a practical design tool for engineers.

Key Design Parameters

The analytical model developed in this study provides a clear design criterion for the stirrup volumetric ratio of the surrounding concrete, which is essential for ensuring the collaborative behavior of the composite column. By comparing the analytical critical stirrup ratio with experimental values, the authors validate the model and provide confidence in its application to practical design. The model also highlights the importance of the steel tube confinement effect in enhancing the compressive performance of the core CFST, which is the primary source of the differential behavior between the two components.

Analytical Model Development

The analytical model is based on the principle of deformation compatibility between the core CFST and the surrounding conventional concrete. At the limit state, the strain in the surrounding concrete must be equal to the strain in the core CFST to prevent premature failure of the surrounding concrete. The model accounts for the enhanced compressive strength and ductility of the core CFST due to the steel tube confinement, as well as the confinement effect of the stirrups on the surrounding concrete.

The critical stirrup volumetric ratio is derived by equating the strain capacity of the surrounding concrete, which depends on the stirrup confinement, to the strain capacity of the core CFST, which depends on the steel tube confinement and the concrete strength. The resulting expression provides a direct relationship between the critical stirrup ratio and the material properties of the core CFST and the surrounding concrete, as well as the axial compression ratio.

Comparison of Analytical and Experimental Results