Robustness and seismic behaviour of structures equipped with traditional and innovative beam-to-column connections

Abstract

Moment Resisting Frames are structures that withstand seismic actions by the bending of girders, columns and connections. Their main source of stiffness and lateral strength is given by the flexural resistance of members and connections, and the seismic energy dissipation capacity and ductility, is provided by the formation of a high number of dissipative zones which can be located in beams, columns or joints depending on the applied design philosophy. Classically, framed structures are designed to possess strong columns, weak beams and full strength rigid connections, so that the earthquake input energy is dissipated through the plastic engagement of the end of beams and of the end of columns of the first storey. The aim of the PhD thesis is to investigate the possibility of using innovative beam-to-column joints characterised by an appropriate stiffness under service loads but which are able to provide a high energy dissipation under seismic event and to confer a suitable robustness in case of a column loss due to exceptional events. According to the traditional strategy for the seismic design of building structures, in case of frequent and occasional seismic events whose return period is comparable with the life cycIe of structures, the earthquake input energy has to be completely dissipated by means of viscous damping. Therefore, the hysteretic energy is equal to zero because, for such seismic events, the structure has to be designed to remain in elastic range. Conversely, in case of rare and very rare seismic events whose return period is about 500 years and even more, most of the earthquake input energy is dissipated by hysteresis, but leading to severe plastic excursions and related structural damage. Such structural damage has to be compatible with the ductility and the energy dissipation capacity of structures, because, even though structural damage is accepted, collapse prevention has to be assured and the safeguard of human lives has to be guaranteed. To this scope a reliable prediction of nonlinear structural behaviour during severe seismic events is required, which represents an extremely difficult task. Although many nonlinear analysis programs exist, the accuracy of their resuIts depends on the assumptions made in the characterization of member stiffness. Therefore, experimental research remains the most reliable means of assessing seismic performance and is crucial to the development of new analytical models and design methods. Quasi-static testing can provide information on the nonlinear behaviour of members or subassemblies, but it is often difficult to relate the imposed force or displacement histories to those that might occur during an earthquake. These static methods are therefore, primarily used to calibrate analytical models or to compare the relative performance of a variety of similar structural details. Starting from the above considerations, in this work, the possibility of using steel frames with innovative bolted connections has been analysed with the aim of providing the structure of supplemental damping devices by means of properly detailed beam-to-column joints. In particular, in order to overcome the drawbacks of the traditional and passive control design strategies, the aim of the work is the development of a new design strategy whose goal is the design of connections able to withstand frequent and occasionaI seismic events but also destructive earthquakes such as those corresponding to rare and very rare events without any damage. In addition, with reference to structural robustness, has been underlined that, because of the specific behaviour of beam-tocolumn connections equipped with friction pads, significant benefits are in the catenary action resuIting, as example, in case of a column loss due to blast loading or impact loading. The development of this design strategy is also the subject of the FREEDAM project, which is an RFCS project, granted by the European Community. [edited by author]