Análise numérica de perfis alveolares de aço

Abstract

Castellated and cellular beams are steel structural elements with large identical openings distributed sequentially along the web of an I-section. The presence of large openings generates new failure modes, such as the Vierendeel mechanism, web-post buckling, longitudinal shear of the web-post and rupture of the welded joint. Castellated and cellular beams might also be subjected to collapse by lateral torsional buckling, compression buckling, formation of plastic hinges, and other failure modes typical of plain-webbed I sections. The correct understanding of the modes of failure is fundamental to the design of castellated beams and columns. In this work a broad investigation of the structural behavior of castellated (hexagonal openings) and cellular (circular openings) beams was performed. The study contemplated structures that fail by local modes (short and laterally braced beams), structures that fail by global modes (slender beams) and structures with intermediary geometries, in which the simultaneous occurrence of multiple failure modes might happen. A parametric numeric model was developed in the finite element software ANSYS 14.0 (ANSYS, 2011) in order to simulate a large number of experiments extracted from the literature (reference models). Such model, duly calibrated through comparison against experimental results from the reference models, was used to study aspects that were suspected to be the most relevant for the structural behavior of castellated and cellular beams: the elastic modulus, residual stresses, and the geometric imperfections. While the first two aspects turned out to be simple to study (the reduction of the elastic modulus affects the models in a predictable and controllable way, and the residual stress patterns proposed by Sonck, 2013, caused a small impact), the matter of geometric imperfections proved to be more complex. Five distinct types of geometric imperfection were studied: weak-axis bending (global); strong-axis bending (global); web bending (local); web-post misalignment (local); and the buckling mode (shape of the collapse by web-post buckling by shear). The magnitude of each imperfection was fixed (at dm/200 for the local ones and L/1000 for the global ones) and at first the isolated effect of each of them over the reference models extracted from the literature was evaluated. The effects of the combination of many types of imperfection were then studied and a set of geometric imperfections capable of generating numeric models that better represented the behavior observed experimentally was proposed. It was demonstrated that the proposed set of geometric imperfections is capable of allowing the occurrence of the global and local modes of failure, and even the simultaneous occurrence of both of them. New geometries and boundary conditions of castellated and cellular beams were carefully selected and solved in ANSYS for the study of the interaction of the internal reactions. The interaction between bending moment (about the strong axis of inertia) and compression proved to be well-behaved, given that it was possible to adjust to the data points two linear trend lines that satisfyingly capture the correlation. The interaction between bending moment and shear force, however, proved to be more elusive. Four interaction curves were proposed, none of which could be demonstrated to have any predictive capacity. In this study the particular importance of the web-post buckling by compression, a failure mode that has not received much attention in the literature through the years, became apparent. The interaction between this local instability and the global instability (LTB) governs the collapse of beams of medium slenderness subjected to a uniformly distributed load.