Stability of Scaffolding Steel Pipes Under Axial Compression with Initial Defects - Study Note

Literature Overview

This research by Wang Quan (China Railway Construction Bridge Engineering Group Co., Ltd.), published in Building Structure (2025, Vol. 55, No. 17), investigates the overall buckling behavior of scaffolding steel pipes under axial compression, with particular attention to the influence of initial geometric defects. The study was funded by the Liaoning Provincial Applied Basic Research Program (2022JH2/101300130). Scaffolding steel pipes, typically conforming to GB/T 13793 or EN 10219 standards, are subjected to repeated use, transportation, and field handling, which inevitably introduces initial out-of-straightness, local dents, and other geometric imperfections. These defects can significantly reduce the buckling load compared to the theoretical Euler buckling capacity, making this a critical safety concern for temporary works.

Experimental Program and Key Findings

Test Specimen Configuration

The experimental program comprised 4 groups with 12 specimens in total. The test matrix was designed with two primary parameters: diameter-to-thickness ratio (D/t) and initial defect amplitude. The specimens were loaded in pure axial compression until failure, with load-displacement and load-strain data recorded throughout.

Failure Modes Observed

The specimens exhibited elastic-plastic overall buckling failure, characterized by lateral deflection leading to yielding at the concave side of the buckled section. As the initial defect amplitude increased, the failure load decreased markedly, and the post-buckling behavior became more pronounced with greater plastic deformation. The load-displacement curves showed an initial linear elastic region followed by a progressive reduction in stiffness as the defect amplified under loading.

Quantitative Results

The most striking finding is the sensitivity of the buckling coefficient to initial defects. When the initial defect-to-length ratio increased from 1/1000 to 1/200, the stability coefficient decreased by a maximum of 45.73%. This represents a dramatic reduction in load-bearing capacity attributable solely to geometric imperfections introduced during service life.

The corrected stability coefficient formula proposed by the author yielded an average ratio of 0.883 when compared with test results, indicating good applicability across the range of defect conditions studied.

Finite Element Analysis and Parametric Study

A finite element model was developed in ABAQUS incorporating material nonlinearity (bilinear or multilinear isotropic hardening) and initial geometric imperfections. The model was validated against experimental results, showing good agreement in terms of failure load and buckling mode shape.

Parametric Analysis Results

The validated FEA model was used to systematically investigate:

• Initial defect amplitude: Directly proportional relationship with buckling load reduction; larger defects lead to earlier yielding and lower ultimate capacity.
• Diameter-to-thickness ratio: Higher D/t ratios increase the susceptibility to local buckling and overall instability interaction.
• Steel grade: Higher yield strength provides a moderate improvement in buckling capacity but does not eliminate the imperfection sensitivity.

Standards Evaluation and Design Implications

The author evaluated the applicability of existing design formulas from both Chinese codes (GB 50007, JGJ 130) and international codes (Eurocode 3, EN 1993-1-1) to scaffolding pipes with significant initial defects. The key findings include:

• Current code buckling curves (a, b, c, d curves in Eurocode 3) are calibrated for new or minimally deformed members and do not adequately account for the severe imperfections typical of used scaffolding pipes.
• The proposed correction formula introduces a defect-dependent modification factor that reduces the buckling resistance as imperfection amplitude increases.
• The average ratio of 0.883 (formula-to-test) suggests the proposed formula is slightly conservative, which is appropriate for temporary works design.

From a quality control perspective, this study reinforces the need for:

1. Regular inspection of scaffolding pipes for geometric defects before each reuse cycle.
2. Classification of pipes by defect severity for appropriate load rating.
3. Retirement criteria based on accumulated damage rather than simple cycle counting.

Key Technical Insights

The imperfection sensitivity of thin-walled cylindrical shells under axial compression is a well-known phenomenon in shell buckling theory, but its practical magnitude in scaffolding applications is often underestimated. The 45.73% reduction in buckling capacity for a defect ratio of 1/200 is particularly alarming. For context, a 48 mm diameter, 3.5 mm wall thickness scaffolding pipe with a 2 m length and a 1 mm initial out-of-straightness (d₀/L = 1/2000) would have a significantly lower capacity than the same pipe with a 10 mm initial defect (d₀/L = 1/200). This has direct implications for scaffolding design safety factors and inspection protocols.

The proposed correction formula provides a practical tool for engineers to rate used scaffolding pipes based on measured defect amplitudes, moving beyond the simplistic "replace if dented" approach toward a more rational, quantified assessment method.