Zhuojin Pipe Fitting Co., LtdAxial Compression Behavior of Square Steel Tube Recycled Concrete Long Columns
Literature Overview
This study by Zhang Zhaqiang, Zhao Junhai, and Xu Yang (2015), published in the journal Concrete, investigates the axial compression performance of eight square steel tube recycled concrete (RAC) long column specimens under monotonic loading. The primary variable is the slenderness ratio, which ranges from relatively stocky to slender configurations. The research is funded by the National Natural Science Foundation of China (Grant 51308479) and the Sichuan Provincial Department of Education (Grant 13ZB0179), and is affiliated with Chang'an University and Southwest University of Science and Technology.
Core Technical Findings
The experimental program reveals that square steel tube RAC long columns undergo three distinct stages under axial compression: elastic stage, yielding stage, and failure stage. The failure mode transitions significantly with increasing slenderness ratio. Specimens with lower slenderness ratios exhibit material strength failure, where the concrete core and steel tube reach their ultimate capacities simultaneously. Conversely, specimens with higher slenderness ratios demonstrate pronounced second-order effects, resulting in overall flexural buckling failure. Notably, all specimens exhibited local buckling of the steel tube wall prior to ultimate failure, regardless of slenderness ratio.
The load-displacement and load-strain curves provide critical insight into the load-bearing capacity degradation mechanism. As slenderness ratio increases, the peak load decreases progressively, and the post-peak ductility diminishes. This confirms that the geometric imperfection amplification effect becomes dominant as the column becomes more slender, reducing the effective load-carrying capacity below the material strength limit.
Standards Applicability Analysis
The authors evaluated the applicability of several Chinese design codes for calculating the axial compression capacity of square steel tube RAC long columns. The following comparison summarizes the findings:
| Standard | Method Type | Applicability Finding |
|---|---|---|
| GB 50017 | Steel structure design | Underestimates capacity for lower slenderness ratios; conservative for higher ratios |
| DBJ 13-51-2003 | Steel tube concrete design | Reasonably accurate across the tested slenderness range |
| CECS 159:2004 | Composite column design | Provides consistent predictions with experimental results |
| GB 50993 | Concrete-encased steel column | Not directly applicable due to different cross-section geometry |
The recommendation to adopt DBJ 13-51-2003 and CECS 159:2004 for design calculations is well-founded, as these standards account for the interaction between the steel tube confinement effect and the concrete core behavior, which is particularly relevant for recycled aggregate concrete where the elastic modulus and Poisson's ratio may differ from natural aggregate concrete.
Engineering Practice Implications
From a manufacturing and construction perspective, several practical considerations emerge from this research. The local buckling phenomenon observed in all specimens highlights the importance of steel tube wall thickness selection and the need for adequate material quality control during steel tube fabrication. For recycled aggregate concrete, the aggregate crushing value, water absorption, and bond strength at the aggregate-cement interface must be carefully controlled, as these parameters directly influence the confinement effectiveness of the steel tube.
In terms of welding considerations for square steel tube fabrication, the seam weld quality along the longitudinal and transverse welds becomes critical, particularly for slender columns where weld defects could initiate premature local buckling. The recommended welding process for square steel tube fabrication should include full penetration butt welds with proper preheating and post-weld heat treatment to minimize residual stresses that could compound the second-order effects.
The study also underscores the importance of initial geometric imperfections and eccentricities. In practice, the out-of-straightness of the steel tube and the eccentricity of concrete placement must be controlled to within specified tolerances, as these factors significantly amplify the second-order moment effects in slender columns.
Key Reflections
This research contributes valuable data for the structural design of composite columns utilizing recycled aggregates, which aligns with sustainability goals in construction. The transition from material strength failure to buckling failure with increasing slenderness ratio mirrors classical column behavior but with the added complexity of composite action and the potentially lower elastic modulus of recycled aggregate concrete. Engineers should note that the confinement effect provided by the square steel tube is less effective at the corners compared to circular sections, which may influence the distribution of plastic deformation and the onset of local buckling.