Local Buckling Analysis of Square CFST Columns Under Eccentric Compression Using the Finite Strip Method

Literature Overview

The paper by Long Yueling and Pan Chuanghan (2017), published in Industrial Construction (Vol. 47, No. 2, pp. 158-162), presents a local buckling analysis of square steel tube concrete-filled columns under eccentric compression using the Finite Strip Method (FSM). The research is supported by the National Natural Science Foundation of China (projects 51008085 and 11472084), the Guangzhou Zhujiang Science and Technology New Star Project (2012J2200100), and the China Postdoctoral Science Foundation (projects 2014T70807 and 2012M511810). The study establishes a local buckling model for the steel plate of square CFST columns under eccentric compression, assuming all four edges of the steel plate are fixed, and calculates critical buckling coefficients under different stress gradients.

Core Technical Methodology

The Finite Strip Method is a numerical technique particularly suited for plate buckling analysis, as it discretizes the plate into strips along one direction while using analytical functions in the other. In this study, the steel plate of the square CFST column is modeled as a plate with fixed boundary conditions on all four edges, subjected to a linearly varying compressive stress distribution resulting from eccentric axial loading. The stress gradient is characterized by the ratio of minimum to maximum compressive stress across the plate width.

The analysis proceeds by: (1) defining the stress gradient parameter that represents the eccentricity effect; (2) applying the FSM to compute the critical buckling coefficient for each stress gradient; (3) deriving the local buckling strength of the steel plate from the critical buckling stress; and (4) establishing rational width-to-thickness ratio limits for different stress ratios to guide design practice.

Validation and Results

The model validation involves two comparisons. First, the critical buckling coefficients calculated by the FSM are compared with analytical solutions based on the energy variational method, showing excellent agreement. Second, the local buckling strength predictions are compared with experimental test results from previous studies, demonstrating overall good correlation. The study ultimately provides design-oriented width-to-thickness ratio limits for the steel plate of square CFST columns under eccentric compression, accounting for the varying stress gradient caused by the eccentricity ratio.

Engineering Practice Implications

For steel pipe manufacturers and structural engineers, this study provides critical design parameters for CFST column applications where eccentric loading is inevitable. The width-to-thickness ratio limits derived from this analysis directly inform the selection of steel plate thickness for square tube manufacturing. In practice, square CFST columns are often fabricated by welding flat steel plates, and the welding quality of the longitudinal and transverse seams affects the effective boundary conditions assumed in the buckling model. If the welds are not fully fused or contain defects such as lack of fusion or porosity, the actual boundary condition may be closer to simply supported rather than fixed, reducing the buckling resistance. Therefore, welding quality control per standards such as NB/T 47015 or ASME Section IX becomes essential for ensuring that the theoretical buckling capacity is achieved in practice. The study also underscores the importance of considering stress gradient effects in local buckling design, which is often neglected in simplified design codes.