Non-linear analysis of CFS hybrid structural building systems in presence of local and global damage phenomena

The use of cold-formed steel (CFS) members enabled, in recent years, the development of a new light constructional solution for residential buildings: the light gauge steel buildings. These structures combine rapidity and simplicity of erection with a good structural performance. In addition, their lightness makes them very attractive for seismic areas. From a structural point of view, the building is a system composed of shear-walls and floor-systems that interact between them in the mechanism of force transfer. In the seismic context, the forces that arise on floors are transferred to the shear-walls that act as a bracing system, transferring the horizontal forces to the foundation. Generally, the steel-frame, made of CFS members, has walls and floor systems sheathed with panels of various materials, such as steel sheets, OSB panels or gypsum fibreboards: i.e. the structure is hybrid. The performance is hence affected by the response features of the various materials, of the connections and the individual components. This makes the prediction of the structural response very difficult and complex. To further complicate the subject, the presence of cyclic actions causes other nonlinear phenomena such as the pinching effect that is very common in these structures. Such a complex behaviour of CFS profiles, sub-assemblages and the whole system, makes it difficult to adopt a purely theoretical approach for their design. Therefore, a mixed approach that combines experimental tests and numerical simulations is usually adopted. As to the key structural components, many studies about the shear-walls are available in the literature, while little attention was paid to the behaviour of the floor diaphragms. Moreover, the attempts to get an insight into the overall building response were even fewer. In any case, the complexity of these sub-systems and of their interaction in the whole building system makes it difficult to predict the responses of specific configurations. In this framework, the University of Trento was involved in a research project aimed to develop an innovative industrialized housing system made of CFS members. With the aim to have a holistic comprehension of the performance of these building systems, a suitable number of experimental tests and numerical simulations were performed ranging from the single component to the entire building. In detail, the CFS members may be considered the main components of these systems, therefore their axial and bending behaviours were first tested and then investigated through detailed numerical simulations. Their characterization was also needed for developing reliable models of sub-assemblies. The steel-deck and gypsum fibreboards are essential parts of the floor diaphragms and shear walls studied, therefore experimental tests for evaluating their shear behaviour were carried out considering both monotonic and cyclic loading protocols. With the aim to characterize the in-plane behaviour of shear walls, the results of an experimental study carried out in Trento were considered. These results represent the background for the development of numerical analyses enabling the calibration and validation of the models. Like the walls, the floor-systems were deeply studied both from the experimental and the numerical point of view. The experimental tests enable appreciating the in-plane shear responses of different configurations, also providing the background to the calibration of FE models. The complexity of these systems required a particular modelling approach based on layers and on the numerical characterization of the floor components. Eventually, since the evaluation of the seismic performance of this new solution is the main goal of the project, three different building models were developed and analysed. The models refer to three different building archetypes representative of three solutions of buildings that differs from the type and number of shear-walls used as the bracing system. The eigen analysis, push-over analysis and the time-history analysis (THA) are the main tools used in the study enabling the evaluation of the vibration properties, the deformation capacity and seismic response of the structure. Besides, multi-stripe analyses (MSAs) were carried out with the aim to evaluated fragility functions providing the probability of failure given an intensity measure that describes the seismic event. The developed models are too complex to be used in everyday design analysis. However, they allowed set up simplified design methods that can be of great help to designers. In conclusion, the thesis aims at providing a contribution to the knowledge of the response of cold formed steel structures in light residential buildings. This contribution expands the present state-of-the-art, pointing out the role of the various components as well as the critical issues.