Available online at www.CivileJournal.org Civil Engineering Journal (E-ISSN: 2476-3055; ISSN: 2676-6957) Vol. 11, No. 06, June, 2025 2170 Influence of Resistance Spot Welding Parameters on Cold-Formed Steel Properties and Failure Modes Iosif Hulka 1 , Viorel Ungureanu 2, 3* , Ioan Both 2 , Edward Petzek 2 , Bogdan Radu 4 1 Research Institute for Renewable Energies, Politehnica University Timișoara, G. Muzicescu 138, 300501 Timişoara, Romania. 2 Department of Steel Structures and Structural Mechanics, Politehnica University Timişoara, Ioan Curea 1, 300224 Timişoara, Romania. 3 Romanian Academy, Timişoara Branch, Mihai Viteazu 24, 300223 Timişoara, Romania. 4 Department of Materials and Manufacturing Engineering, Politehnica University Timisoara, Timișoara 300222, Romania. Received 22 October 2024; Revised 14 May 2025; Accepted 21 May 2025; Published 01 June 2025 Abstract Lightweight steel structural systems such as built-up beams and trusses are efficient and easy to handle, but the joining technique between thin-walled cold-formed steel elements requires improved solutions. Conventional welding technologies are not suitable for connecting thin sheets due to several inconveniences. The study presents a novel technological approach to connect lightweight steel beams made of corrugated galvanised sheets for webs and back-to-back lipped channel profiles for flanges connected by spot welding, as resistance spot welding (RSW) is widely used in various industrial sectors, such as automotive. This study investigates the influence of RSW parameters on the microstructural properties of spot-welded low-carbon galvanised steel sheets, as well as on their mechanical properties. Two grades of base material were used with thicknesses in the range of 0.8 - 2 mm. RSW joints were manufactured using an automated welding source, and their microstructural characteristics were evaluated by optical and electron microscopy to emphasise the importance of using optimal welding regimes to reduce weld failure. Mechanical properties were evaluated using Vickers microhardness measurements and nanoindentation. Tensile tests were carried out to assess the force-displacement curves and identify the failure mode. The results of the study show that RSW is a promising method for fabricating lightweight steel structural systems when the current, time, and interelectrode forces of RSW are carefully selected. Keywords: Lightweight Steel Structures; Built-Up Beams and Trusses; Resistance Spot Welding; Microstructure; Hardness. 1. Introduction Thin-walled cold-formed steel structures have emerged as a key innovation in modern constructions, offering a combination of economic efficiency, structural integrity, and sustainability. These lightweight structures are especially suited for a range of applications, from residential buildings to commercial or industrial ones. In the construction industry, thin-walled cold-formed steel structures are widely used, either in conjunction with hot-rolled or fabricated steel profiles or as standalone structures for low- to mid-rise constructions. Using lightweight elements, this can be achieved with good strength-to-weight ratios. It is necessary to reduce weight without compromising human safety. The economic advantages of thin-walled cold-formed steel structures are substantial. Their lightweight nature allows for reduced transportation costs and simplified foundation requirements, leading to faster construction timelines. Additionally, as a sustainable building material, cold-formed steel is recyclable and can contribute to greener building standards. As the industry focuses on efficient and environmentally responsible practices, thin-walled cold-formed steel structures stand out as a viable solution to the challenges of modern architecture. * Corresponding author: viorel.ungureanu@upt.ro http://dx.doi.org/10.28991/CEJ-2025-011-06-01 © 2025 by the authors. Licensee C.E.J, Tehran, Iran. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).