Establishment of a modified imperfection factor for cold-formed hollow sections

Abstract

This master's thesis evaluates the possibilities for improving the current European standards (FprEN1993-1-1:2022) for the buckling design of cold-formed steel tubular sections, particularly for high-strength steels (fy > 460 MPa). High-strength steels (HSS) offer a high strength-to-weight ratio, allowing for the design of more slender members for a same load-bearing capacity. Their use thus leads to lighter structures and reduced consumption of raw materials, offering numerous economic and environmental benefits. Moreover, many studies have shown that HSS members exhibit a higher normalized buckling resistance compared to normal-strength steels (NSS). These findings have led to partial modifications of the Eurocodes for buckling design, which until then showed increasing conservatism with rising yield strength. These changes have allowed the use of a more favorable buckling curve for hot-formed steels from grade S460 up to grade S700. However, this modification has not been introduced for cold-formed (CF) steels, even though HSS coils are already commercially available for cold-forming, whereas hot-formed members are not produced beyond grade S460. The objective of this work is therefore to improve the design standards for CF steels by accounting for the standardized increase in buckling resistance associated with higher steel grades. This study focuses particularly on cold-formed hollow sections. The methodology follows a classical scientific approach, based on experimental tests from the literature, which are used to develop and validate a numerical model capable of reproducing the experimental results. This model is then used to conduct a parametric study on various steel grades and column lengths in order to assess the performance of the European design standards as well as modification proposals from the literature based on them. At the conclusion of this analysis, a proposal for improving the standards for cold-formed hollow sections is presented.