Eccentric Compression Performance of Concrete-Filled Steel Tube Members Reinforced Under Load

Literature Overview

The paper by Hu Rui, Du Xinqi, Cai Zongyang, Cheng Xiaoyan, and Yuan Xixin, published in Industrial Construction in 2017 (Volume 47, Issue 2), presents an experimental study on the eccentric compression behavior of concrete-filled steel tube (CFST) members that are reinforced with additional concrete infill under pre-existing load conditions. The research is supported by the Hubei Provincial Natural Science Foundation (Project 2014CFB715) and the National Natural Science Foundation of China (Project 51508424). Nineteen specimens with varying slenderness ratios, eccentricity ratios, and initial stress ratios were tested, and finite element simulations were conducted to validate experimental findings.

Experimental Design and Key Variables

The experimental program systematically varied three critical parameters: slenderness ratio, eccentricity ratio, and initial stress ratio. This multi-variable approach enables the isolation of individual parameter effects on structural response, which is essential for developing reliable design formulas.

The failure mode of flexural buckling instability with a deflection curve closely following the sine curve assumption is significant because it validates the applicability of classical Euler-type buckling theory to these reinforced members. This simplifies the analytical framework for design, as engineers can use established buckling formulas with appropriate modifications for the initial stress condition.

Initial Stress Effects and Reinforcement Behavior

The most critical finding is that the presence of initial stress reduces the ultimate bearing capacity of concrete-filled steel tube members, and this reduction becomes more pronounced as the initial stress increases. This is physically consistent with the concept of residual capacity: when a member is already carrying load, the additional load capacity available for reinforcement is diminished. The nonlinear relationship between initial stress and capacity reduction suggests that reinforcement under high initial stress conditions yields progressively less benefit.

From a practical standpoint, this finding has direct implications for structural retrofitting and strengthening projects. When reinforcing existing CFST members that are already under service loads, engineers must account for the initial stress state in their design calculations. The reinforcement strategy should consider not only the added concrete infill but also the interaction between the pre-existing stress state and the new material's contribution to overall capacity.

Study Insights and Reflections

The research provides essential data for the design of load-bearing reinforcement of CFST members, a scenario commonly encountered in building renovation, bridge strengthening, and industrial facility upgrades. The 19-specimen test matrix offers robust statistical support for developing design provisions. The finite element validation confirms that numerical models can reliably predict the behavior of these members under eccentric compression with initial stress.

Engineers should note that the sine curve deflection assumption, while validated experimentally, may not hold for members with significant initial geometric imperfections or under highly asymmetric loading conditions. In practice, finite element analysis incorporating measured imperfections should supplement analytical calculations for critical applications.

This study contributes valuable experimental data and analytical insights for the strengthening of existing CFST structures under service loads. The findings support the development of more accurate design guidelines that account for initial stress effects, ultimately improving the safety and reliability of retrofitting interventions.