Instability behaviour of steel beams with web openings under extreme fire conditions

Abstract

Recently, there has been an upswing in building construction that utilise sustainable designs and the effective use of raw materials. As a result, there has been a surge of research with the goal of optimising the geometrical configuration of web-opening steel sections to meet cost-effectiveness in structural design. There is still a need for research in improving the design method for perforated unrestrained steel beams to evaluate their behaviour in terms of lateral torsional buckling (LTB). This research will delve into the behaviour of cellular beams that are at risk of instability-induced failure. Analysis of the beams using Eurocode will be conducted, along with numerical modelling using ANSYS software, to explore how they react to both room temperature and elevated temperatures. The study will also encompass the effect of the numerical coupling and the thickness of the endplates on the elastic and buckling simulations of the cellular beams, considering the initial geometric imperfections and material nonlinearities. A parametric study was conducted to examine the effects of variations in temperature, cross-section geometry, and web aperture configurations for simply supported beams exposed to a uniform bending load and a distributed load. Buckling curves for cellular beams were determined by comparing FE reduction factors with those prescribed by Eurocode 3 parts 1 and 2 for the equivalent solid steel beams and Eurocode 3 parts 1–13 for cellular beams. The study demonstrates that using numerical coupling in the elastic and buckling simulations cancels out local cross-section buckling, enabling the detection of the LTB mode without it being superimposed or combined with other buckling modes. On the other hand, in the parametric study, the analyses depicted the following failure modes at ambient and elevated temperatures: LTB and LTB+ plastification of the two T-section for end moments and yielding of the top tee section’s flange, web post-buckling (WPB), Vierendeel mechanism (VM) and LTB for a distributed load. Other combined failure modes have also been observed.

For intermediate and high non-dimensional slenderness, the Eurocode 3 curve, using an equivalent solid beam, gives better predictions. Considering the numerical results and Eurocode formulae, a new proposed formula for the plateau length of the LTB curves was obtained. The comparison of the predicted results using both numerical and simplified design methods reveals that the proposed formulae have reduced the discrepancy and enhanced the LTB curve to better assess the cellular beams’ behaviour.