Numerical analysis of cellular steel beams failure modes in fire conditions

Abstract

Cellular Beams with long spans are increasingly used in the steel building constructions and usually are used with openings for technical equipment and ventilations systems cut in their web. These elements are deeper than normal rolled sections, with distributed circular openings, or other like squared or hexagonal, produced by cutting and welding hot rolled steel sections. Cellular beams in fire conditions may collapse by local buckling instability of the web-post between the openings, lateral torsional buckling or by Vierendeel bending at the openings. This behavior is analyzed and evaluated by a numerical method, using the finite element method, considering geometrically and material nonlinear simulations. A parametric analysis is performed for different cellular beams, considering the variation of: (i) different cross sections; (ii) web post widths; (iii) hole diameter to the section height ratios and (iv) distance between holes. Additionally the residual stress influence in the moment resistance is analyzed considering different distributions. The cellular beams collapse modes are related to the length of the beam rather to the geometric parameters of the apertures. Short beams, with relatively small T height, collapse by Vierendeel mechanism, for higher heights the collapse is due to web-post buckling. Longer beams collapse mainly by lateral torsional buckling. The Lateral Torsional Buckling resistance determined by the Eurocode 3 part 1.1 formulae, and considering the 2T section, is higher than the resistance obtained by the numerical method. For the studied cases, the presence or absence of residual stresses in the web does not influence the resistant capacity or collapse mode.