Structural performance of Cold-Formed Sigma steel beams subjected to web crippling action

Abstract

Cold-Formed (CF) sections have emerged in the construction industry due to their merits, including ease of fabrication for different section profiles, sustainability, and cost-effectiveness. As a result, various innovative section profiles have been introduced to the CF industry. Sigma sections are one of the innovative sections modified from the conventional section to improve its bending and shear capacity. However, web crippling performance of sigma sections remains an area of uncertainty, as no dedicated studies have yet explored their behaviour under concentrated load conditions that typically lead to web crippling in CF sections. This thesis presents a comprehensive investigation into the structural performance of CF sigma beams subjected to web crippling under concentrated loading conditions. The study focuses on sigma sections fabricated from three materials: carbon steel, stainless steel, and aluminium and evaluates their behaviour under four critical load cases: End-Two-Flange (ETF), Interior-Two-Flange (ITF), End-One-Flange (EOF), and Interior-One-Flange (IOF). The primary objective is to address the inadequacies of existing design standards and propose unified equations for predicting the web crippling capacity of sigma sections across all materials and load scenarios.
The research employs Finite Element Analysis (FEA) using ABAQUS/CAE to simulate the web crippling behaviour of sigma sections. A total of 2,916 numerical models were developed, covering a wide range of parameters including section depths (140 mm, 200 mm, 240 mm, and 300 mm), thicknesses (ranging from1 mm to 3 mm), bearing lengths (50 mm, 100 mm, and 150 mm), corner radii (3 mm, 5 mm, and 7 mm), and yield strengths (ranging from 180 MPa to 550 MPa). The models were validated against 13 sets of experimental data, showing excellent agreement in terms of ultimate load capacity, failure patterns, and load-displacement behaviour.
Key findings reveal that stainless steel sigma sections consistently exhibited the highest web crippling capacity, followed by carbon steel and aluminium. The effect of parameters on web crippling capacity was analysed. For instance, under ETF loading, increasing the thickness from 1 mm to 2.5 mm resulted in an 8-fold increase in web crippling capacity for aluminium, a 7-fold increase for stainless steel, and a 5-fold increase for carbon steel. Similarly, increasing the bearing length from 50 mm to 150 mm led to a 30–38% increase in capacity across all materials. Conversely, increasing section depth from 140 mm to 300 mm resulted in a 25–55% reduction in web crippling strength, emphasizing the vulnerability of deeper sections to concentrated loads.
The study also evaluated the performance of existing design standards, including AS/NZS 4600, AISI S100, EN 1993-1-3, AS/NZS 1664.1, and AS/NZS 4673. These standards were found to be inadequate for sigma sections, with prediction errors exceeding 30% in many cases. For example, EN 1993-1-3 underestimated the web crippling capacity of carbon steel sigma sections by up to 34% under ETF loading. To address this, the thesis proposes modified and unified design equations tailored to sigma sections. These equations incorporate new coefficients for each material and account for key parameters. The proposed equations demonstrated high accuracy, with mean prediction-to-FEA ratios close to 1.00 and coefficients of variation (COV) below 0.15 for all materials and load cases. Reliability analyses were conducted in accordance with AISI S100 guidelines, confirming that the proposed equations meet the required safety margins. Reduction factors (ϕw) were calculated to be 0.85 or higher, exceeding the minimum acceptable threshold of 0.75, thereby validating the robustness of the proposed design provisions.
In addition to numerical modelling, the thesis includes a comparative study between sigma and SupaCee sections. The results showed that sigma sections outperformed SupaCee sections in all load cases, due to their enhanced torsional rigidity and resistance to local buckling. This finding supports the recommendation to adopt sigma sections in applications where high point loads or bearing forces are expected. The thesis concludes with the recommendation of incorporation of proposed unified equations into design standards to improve the accuracy and efficiency of structural design involving sigma sections.
In summary, this thesis makes a significant contribution to structural engineering by providing a validated, unified framework for predicting the web crippling capacity of cold-formed sigma sections. The findings have direct implications for improving design safety, material efficiency, and code development, thereby advancing the application of CF steel in modern construction.

Date of Award	30 Sept 2025
Original language	English
Awarding Institution	Northumbria University
Supervisor	Keerthan Poologanathan (Supervisor), Brabha Nagaratnam (Supervisor), Muhammad Rahman (Supervisor) & Mohammadali Rezazadeh (Supervisor)