Axial Compression Mechanical Properties of Ribbed Square Steel Tube Concrete Long Columns - A Literature Study Note

Literature Overview

The paper by Li Bin, Luan Wenlong, and Gao Chunyan from the School of Civil Engineering at Inner Mongolia University of Science and Technology, published in Concrete (2020, No. 9, pp. 19-22), presents experimental and analytical research on the axial compression mechanical properties of ribbed (stiffened) square steel tube concrete (SRC) long columns. The study was supported by the National Natural Science Foundation of China (Grant No. 51768056) and the Inner Mongolia Science Foundation (Grant No. 2018MS05041). The authors employ orthogonal experimental design to investigate the effects of slenderness ratio, width-to-thickness ratio, and rib height-to-thickness ratio on the load-bearing capacity of SRC long columns. This work is directly relevant to structural engineers and steel pipe fabricators involved in the design and manufacturing of steel tube concrete columns for building and infrastructure applications.

Technical Background and Motivation

Steel tube concrete (SRC) columns are composite structural members in which concrete is confined within a steel tube, combining the compressive strength of concrete with the ductility and confinement capacity of steel. Square SRC columns are widely used in multi-storey buildings, bridges, and industrial structures due to their ease of connection to beam members and their efficient use of material. However, conventional square SRC long columns (those with slenderness ratios exceeding the critical slenderness) suffer from premature local buckling of the steel tube wall before the full composite action between steel and concrete is achieved. This local buckling manifests as outward bulging (drumming) of the steel tube walls, which not only reduces the load-bearing capacity but also compromises the structural integrity by causing weld cracking at the tube-to-plate connections.

The introduction of internal stiffening ribs (stiffeners) is a practical solution to delay local buckling and improve the composite action. The ribs are typically welded to the inner walls of the square steel tube at regular intervals, creating a cellular or ribbed internal structure that provides additional lateral support to the tube walls. From a steel pipe and welding engineering perspective, the design and fabrication of these ribs involve critical welding considerations, including weld quality, residual stress management, and the interaction between the rib welds and the tube wall deformation.

Typical Column Parameters

Experimental Programme and Orthogonal Design

The experimental programme employs orthogonal experimental design (also known as Taguchi design or L₉ orthogonal array) to efficiently investigate the effects of three factors—slenderness ratio (λ), width-to-thickness ratio (b/t), and rib height-to-thickness ratio (h_r/t)—on the load-bearing capacity of the columns. Each factor is varied at three levels, resulting in a total of nine test specimens. This approach allows the identification of the most significant factor affecting load-bearing capacity with a minimal number of tests, which is particularly valuable for expensive and time-consuming full-scale structural testing.

The test specimens consist of square steel tubes filled with concrete, with internal stiffening ribs welded at regular intervals along the column length. The ribs are fabricated from flat steel plates and welded to the inner tube walls using fillet welds. The welding procedure, including preheat temperature, interpass temperature, and post-weld heat treatment, is critical to ensuring weld quality and minimising residual stresses that could adversely affect the column's structural performance.

Orthogonal Design Matrix

Key Experimental Findings

Effect of Stiffening Ribs on Column Behaviour

The most significant finding of the study is that the introduction of stiffening ribs markedly improves the structural performance of square SRC long columns. In conventional (unribbed) SRC columns, local buckling of the tube walls occurs well before the ultimate load is reached, resulting in premature failure through weld cracking. The outward bulging (drumming) of the tube walls reduces the effective confinement pressure on the concrete core, thereby diminishing the composite action between steel and concrete. The concrete core, which should be in a triaxial compressive state due to the confinement of the steel tube, instead experiences reduced confinement and fails at a lower stress level.

With the introduction of stiffening ribs, the local buckling of the tube walls is significantly delayed, allowing the column to reach a higher load level before failure. The ribs provide additional lateral restraint to the tube walls, effectively reducing the unsupported length of the tube wall between stiffeners. This is analogous to the effect of external stiffeners on plate girders in steel construction, where the stiffeners reduce the effective panel height and increase the local buckling resistance.

Influence of Width-to-Thickness Ratio (b/t)

The orthogonal design analysis reveals that the width-to-thickness ratio (b/t) is the most significant factor affecting the load-bearing capacity of ribbed SRC long columns. A higher b/t ratio results in a thinner, more flexible tube wall that is more susceptible to local buckling, even with the presence of stiffening ribs. As b/t increases from 30 to 50, the load-bearing capacity decreases markedly, with the reduction being more pronounced at higher slenderness ratios. This finding is consistent with the classical plate buckling theory, which predicts that the critical buckling stress of a plate decreases with increasing width-to-thickness ratio.

Influence of Slenderness Ratio (λ)

The slenderness ratio has a secondary but still significant effect on the load-bearing capacity. As λ increases, the stability coefficient (φ) decreases according to the column curve, resulting in a lower allowable axial stress. The study shows that the stability coefficient for ribbed SRC columns is higher than that for conventional SRC columns at the same slenderness ratio, indicating that the stiffening ribs improve the overall stability of the column in addition to delaying local buckling. This dual benefit—improved local buckling resistance and improved overall stability—makes ribbed SRC columns particularly attractive for applications where high slenderness ratios are unavoidable.

Influence of Rib Height-to-Thickness Ratio (h_r/t)

The rib height-to-thickness ratio has a moderate effect on the load-bearing capacity. A higher h_r/t ratio provides greater lateral restraint to the tube walls, but the benefit diminishes beyond a certain threshold (approximately h_r/t = 4-5). This is because the effectiveness of the ribs is limited by the interaction between the rib welds and the tube wall: if the ribs are too tall relative to the tube wall thickness, the welds may become the critical failure element rather than the tube wall itself.

Welding Considerations in Ribbed SRC Column Fabrication

From a welding engineering perspective, the fabrication of ribbed SRC columns involves several critical welding challenges that must be addressed to ensure structural integrity. The internal ribs are typically attached to the tube walls using fillet welds or T-joint welds, and the quality of these welds directly affects the load-transfer mechanism between the ribs and the tube walls.

The weld sequence is particularly important for minimising angular and longitudinal distortion of the tube. A balanced, symmetrical welding sequence—where opposite walls are welded alternately and the welds on each wall are welded from the centre outward—can reduce distortion by 50-70% compared to an unbalanced sequence. Post-weld stress relief heat treatment is recommended for columns with wall thicknesses exceeding 8 mm or where the steel grade is Q345 or higher, as these conditions increase the susceptibility to hydrogen-induced cracking in the heat-affected zone (HAZ).

Engineering Practice Implications

For structural engineers designing ribbed SRC columns, the key design parameters to optimise are the width-to-thickness ratio (b/t) and the rib spacing/height. The study suggests that b/t should be kept below 40 for optimal performance, and that rib heights of 3-5 times the tube wall thickness provide the best balance between buckling resistance and fabrication practicality. The rib spacing should be selected to ensure that the effective panel height between ribs does not exceed the critical buckling length for the given b/t ratio.

For steel pipe fabricators, the fabrication of ribbed SRC columns requires careful attention to weld quality, dimensional accuracy, and post-fabrication inspection. The internal rib welds are difficult to access and inspect, making ultrasonic testing (UT) the preferred NDT method. Additionally, the residual stresses from the rib welding can interact with the external loading to reduce the effective buckling resistance, so post-weld stress relief is recommended for critical applications.

Summary and Concluding Remarks

The research by Li et al. demonstrates that the introduction of internal stiffening ribs in square steel tube concrete long columns significantly improves their structural performance by delaying local buckling, enhancing the composite action between steel and concrete, and increasing the overall stability coefficient. The orthogonal design analysis identifies the width-to-thickness ratio as the most influential parameter, followed by the slenderness ratio and the rib height-to-thickness ratio. For engineers involved in the design and fabrication of ribbed SRC columns, the key practical recommendations are to maintain b/t below 40, select rib heights of 3-5 times the tube wall thickness, ensure high-quality rib welds with appropriate preheat and post-weld treatment, and implement rigorous NDT procedures to verify weld integrity. The findings of this study contribute to the continued development of SRC columns as an efficient and reliable structural solution for modern construction.