Structural behaviour of steel flanged cruciform sections under uniform biaxial bending

Abstract

Generally, uniform sections are the most commonly used structural steel sections in the construction of frame structures. However, due to several advantages that the steel flanged cruciform sections (FCS) have over conventional sections, they are becoming increasingly popular. The ability to provide moment connections to all four flanges enables these sections to provide two-way frame action, as well as enhanced axial load resistance. Literature on both cruciform and flanged cruciform sections is currently limited, and there are still several gaps yet to be addressed, particularly regarding the buckling behaviour due to bending of such sections. The primary aim of this study was to examine the behaviour of flanged cruciform sections subjected to either uniaxial or biaxial bending. This dissertation made use of current theoretical equations and design guidelines to assess their relevance when designing flanged cruciform sections. Finite Element Analysis showed that such sections do buckle at the critical moment, Mcrit. The analysis was carried out on LUSAS (2022) and performed on the IPEA 200 and IPEA 500. The elastic critical buckling moment values obtained for the different uniaxial and biaxial moments applied about both the major and minor axes were compared. This resulted in the FCS buckling at higher values of Mcrit during biaxial bending; hence become more resistant to buckling when contrasted to uniaxial bending. The results from FEA were applied to Eurocode 3 EN 1993-1-1 (2005) to determine the reduction factor χLt. The main buckling modes of the flanged cruciform section consist of either pure distortional or torsional deformation or a combination of both during the transition period. Recently, equations derived from previous research have demonstrated accurate predictions of the buckling behaviour of such sections when subjected to equal and opposite end moments. In this study, the formula was modified to assess its applicability in scenarios involving biaxial bending of the section which provided an excellent estimate of the critical moment for FCS. The objective of this dissertation was to provide structural engineers with methods to determine Mcrit for flanged cruciform sections during both uniaxial and biaxial bending.