Numerical model for framed structures with thin-walled cross-section members

The aim of this thesis is to develop an adequate tool for the analysis of beam elements with thin-walled open cross-sections. Although the topic has been exhaustively investigated through the years by different authors, for the moment there are not suitable commercial software able to predict the behavior of such elements. In the last decades, the use of thin-walled steel members is significantly increased, in particular in the field of logistic, where goods and products are stored in pallet racks. These structures are generally composed by uprights which have mono-symmetric lipped channel cross-sections. Hence, a suitable finite element software should be characterized by seven degree of freedom per node. Indeed, only the presence of the seventh degree of freedom makes possible to correctly estimate both displacements and internal stresses, including warping displacements and bimoment stresses. Furthermore, this formulation is able to correctly predict the flexural-torsional and lateral-torsional buckling, derived by the coupling between flexure and torsion. The previously mentioned effects are neglected in the routine rack design, mainly because no useful indications can be found in literature on this topic. Because of that, a research project in conjunction between the Politecnico di Milano and the University of Pavia has been established with the aim of improving the design rules for those types of structures. After an introduction to the racks in the first chapter, the second shows the matrices developed. Similar formulations have been presented in the past. One of these introduces the seventh degree of freedom, but neglects the eccentricity of the shear center from the centroid and, as a consequence, does not consider all the Wagner coefficients, limiting the formulation to bi-symmetric cross-sections. The second one is more general and it is usable for non-symmetric cross-sections, but it does not take into account the reduction of the axial stiffness, as effect of the second-order. Considering all the previous features, an academic open-source software has been modified. The new software has been called Śiva (System of Incremental and Vibration Analysis) and allows to perform several types of analyses. An exhaustive validation, dealt in the Appendix A, permits to consider adequate the software. As the first phase of research has been successfully completed, attention is herein focused on the study of the behavior of the steel storage pallet racks. Different scientific articles have been produced. At the beginning the static design and the member stability have been considered. Consequently, the use of simplified approaches for the structural analysis has been evaluated. Then the beam design of non-symmetric cross-section has been investigated and currently attention is paid on the procedures adopted for seismic design.