Experimental Study on Axial Compression Ultimate Bearing Capacity of L-Shaped Perforation-Free Steel Plate Connected Square Steel Tube Concrete Composite Special-Shaped Columns

Literature Overview

This paper, authored by Xu Minyang, Chen Zhihua, Zhou Ting, and Li Yanbo from Tianjin University and published in Industrial Construction (2016, Vol. 46, No. 7), presents an experimental investigation into the axial compression behavior of long composite special-shaped columns formed by connecting two square steel tube concrete (SRC) columns with perforation-free L-shaped steel plates. The research was supported by the National Natural Science Foundation of China (NSFC 51308387). Four specimens with different slenderness ratios were tested under pure axial compression loading to evaluate their ultimate bearing capacity, failure modes, lateral deflection behavior, and strain distribution characteristics. This work addresses a practical structural engineering challenge: how to construct compact, space-efficient composite columns in irregular building layouts where conventional circular or rectangular columns cannot be accommodated.

Core Technical Findings

The experimental results reveal several important mechanical behaviors that merit careful attention from practicing engineers. First, the ultimate bearing capacity of the perforation-free steel plate connected SRC composite special-shaped columns decreases with increasing slenderness ratio, following a trend analogous to that observed in individual square steel tube concrete columns. This confirms that the slenderness ratio remains the dominant geometric parameter governing the compressive capacity of such composite members. Second, the failure mode transitions systematically with slenderness ratio: shorter specimens exhibit local buckling and concrete crushing, while longer specimens fail through overall flexural buckling. Third, and perhaps most significantly, the lateral deflection measurements and strain distribution patterns demonstrate that the specimens buckle about an axis (X'-X') inclined at 45 degrees to the X-X axis, rather than about the principal axes of the individual square tubes. This 45-degree buckling axis reflects the asymmetric cross-sectional geometry of the L-shaped composite section and has profound implications for moment of inertia calculations and stability design.

Strain Analysis and Load Transfer Mechanism

The strain analysis of the connecting L-shaped steel plates provides critical insight into the load transfer mechanism within the composite column. The experimental data confirm that the connecting steel plates actively participate in the overall force-bearing behavior of the composite column throughout the entire loading process. This is a crucial finding because it validates the design assumption that the perforation-free plate connection functions as a rigid coupling element, effectively transferring axial loads between the two square steel tubes and ensuring composite action. In contrast, if the connecting plates were to behave passively or if their contribution were negligible, the composite column would essentially behave as two independent SRC columns with no interaction benefit. The measured strain distribution indicates that the connecting plates undergo significant deformation, particularly in the mid-span region where bending moments are maximum, which further confirms their integral role in the structural system.

Engineering Practice Implications

From a practical standpoint, this research carries several important implications for the design and construction of composite special-shaped columns in building structures. The 45-degree buckling axis finding means that standard stability calculations based on principal axis bending may be non-conservative or simply incorrect for this type of composite section. Designers must explicitly identify and calculate about the actual buckling axis, which requires careful determination of the cross-sectional properties of the composite section including the contribution of the connecting steel plates. The confirmation that the connecting plates actively bear load also has implications for connection design: the perforation-free plate connections must be designed to withstand the actual strain levels observed experimentally, and weld details at the plate-to-tube interfaces must be qualified accordingly. Furthermore, the progressive nature of the failure modes with slenderness ratio suggests that engineers should exercise particular caution when designing slender composite special-shaped columns, as the transition from local to global buckling may be less predictable than for conventional column sections.

Study Insights and Reflections

Having reviewed this paper carefully, I find the experimental approach sound and the conclusions well-supported by the measured data. The decision to use perforation-free connecting plates is particularly interesting from a fabrication standpoint, as it avoids the stress concentration issues associated with drilled holes while maintaining a continuous load path. However, I would note that the study is limited to four specimens and pure axial compression loading; eccentric loading conditions, which are almost inevitable in real structures, would likely produce different buckling axis orientations and strain distributions. Additionally, the long-term behavior under sustained loads, including creep and shrinkage effects of the concrete core, was not investigated. For future work, I would recommend expanding the experimental matrix to include eccentrically loaded specimens and conducting parametric finite element analyses to explore the effects of plate thickness, tube size, and concrete strength grade on the composite column performance. The practical value of this research lies in providing the first systematic experimental evidence for the design of this specific type of composite special-shaped column, filling an important gap in the structural engineering literature.