The experimental analysis described in this paper has been performed within the activities of the FREEDAM project (FREE from DAMage steel connections), which is an RFCS project (Research Fund for Coal and Steel), granted by the European Community, devoted to the development of a new design strategy based on the employment of friction dampers in beam-to-column joints. In past, the application of dampers has been widely investigated within the development of supplemental energy dissipation strategies, with the aim to improve the energy dissipation capacity of the primary structure and to reduce the structural damage by concentrating the energy dissipation supply in fuses located in zones of the structure where high relative displacements are expected. Conversely, the strategy studied in the FREEDAM project is based on the application of damping devices, specifically friction dampers, under a new perspective. In fact, they are conceived as elements included in connections with the aim to completely substitute the traditional dissipative zones of MRFs, i.e. the beam ends. To this scope, beam-to-column joints are equipped with friction dampers constituted by steel plates and friction pads pre-loaded with high-strength bolts, that can be located either at the level of both flanges, or at the bottom flange level only, or on additional haunches which can be used to increase the lever arm of the damper. In particular, the connection studied in the FREEDAM project is realized as a modification of the classical detail of a Double Split Tee Joint (DST) where, in substitution of the bottom Tee element, a symmetrical friction damper is introduced which is realized pre-loading friction pads located between the webs of a couple of angles used to fasten the beam to the column and the beam flange or the lowev plate of an eventual beam hunch, which is slotted in order to allow the sliding of the damper. With this connecting system, under bending actions, the joint is forced to rotate around a rotation center located at the base of the upper T-stub web and the energy dissipation supply is provided by the alternate slippage of the lower beam flange on friction pads. In this way, provided that the steel components of the connection are properly over-strengthened, the joint resistance and the rotation capacity can be easily governed by calibrating the preload applied to the frictional interfaces and realizing slotted holes whose length provides an adequate stroke for the dissipative device. The main feature of this connection is that, even under the occurrence of destructive seismic events, the only damage is due to the consumption of the friction pads, while all the steel elements are completely preserved obtaining, in this way, a connection able to withstand destructive seismic events without any damage to the steel parts, namely a FREE from DAMage connection. Clearly, in order to accurately control the design of such a connection typology, the knowledge of the value of the friction coefficient of the material employed to realize the friction pads is of paramount importance. Therefore, among all the possible materials, it is necessary to select those able to develop high values of the friction coefficient and a stable behavior under cyclic loading conditions. In this paper, the main results of preliminary analyses devoted to select the best materials to be employed as friction pads in the FREEDAM connections are presented. To date, twelve tests on eight different materials combined with stainless steel plates have been realized. The tests performed, mainly devoted to evaluate the behavior of the friction interfaces in terms of static and kinetic friction coefficient and in terms of degradation, are inspired to the sliding test proposed by EN1090-2 adopting the loading protocol suggested by the EN12159 for the qualification of seismic devices. The initial value of the friction coefficient and its evolution during the tests, have been determined for the different materials by monitoring the values of the forces of the bolts used to apply the pre-load to the friction interface and the value of the sliding force. The obtained results are presented both in terms of force-displacement curves and in terms of friction coefficient, providing a comparison between the performances of the analyzed interfaces in terms of energy dissipation supply.

Experimental analysis of friction material for Free from Damage connections

FERRANTE CAVALLARO, GIOVANNI;FRANCAVILLA, ANTONELLA BIANCA;LATOUR, MASSIMO;PILUSO, Vincenzo;RIZZANO, Gianvittorio
2017-01-01

Abstract

The experimental analysis described in this paper has been performed within the activities of the FREEDAM project (FREE from DAMage steel connections), which is an RFCS project (Research Fund for Coal and Steel), granted by the European Community, devoted to the development of a new design strategy based on the employment of friction dampers in beam-to-column joints. In past, the application of dampers has been widely investigated within the development of supplemental energy dissipation strategies, with the aim to improve the energy dissipation capacity of the primary structure and to reduce the structural damage by concentrating the energy dissipation supply in fuses located in zones of the structure where high relative displacements are expected. Conversely, the strategy studied in the FREEDAM project is based on the application of damping devices, specifically friction dampers, under a new perspective. In fact, they are conceived as elements included in connections with the aim to completely substitute the traditional dissipative zones of MRFs, i.e. the beam ends. To this scope, beam-to-column joints are equipped with friction dampers constituted by steel plates and friction pads pre-loaded with high-strength bolts, that can be located either at the level of both flanges, or at the bottom flange level only, or on additional haunches which can be used to increase the lever arm of the damper. In particular, the connection studied in the FREEDAM project is realized as a modification of the classical detail of a Double Split Tee Joint (DST) where, in substitution of the bottom Tee element, a symmetrical friction damper is introduced which is realized pre-loading friction pads located between the webs of a couple of angles used to fasten the beam to the column and the beam flange or the lowev plate of an eventual beam hunch, which is slotted in order to allow the sliding of the damper. With this connecting system, under bending actions, the joint is forced to rotate around a rotation center located at the base of the upper T-stub web and the energy dissipation supply is provided by the alternate slippage of the lower beam flange on friction pads. In this way, provided that the steel components of the connection are properly over-strengthened, the joint resistance and the rotation capacity can be easily governed by calibrating the preload applied to the frictional interfaces and realizing slotted holes whose length provides an adequate stroke for the dissipative device. The main feature of this connection is that, even under the occurrence of destructive seismic events, the only damage is due to the consumption of the friction pads, while all the steel elements are completely preserved obtaining, in this way, a connection able to withstand destructive seismic events without any damage to the steel parts, namely a FREE from DAMage connection. Clearly, in order to accurately control the design of such a connection typology, the knowledge of the value of the friction coefficient of the material employed to realize the friction pads is of paramount importance. Therefore, among all the possible materials, it is necessary to select those able to develop high values of the friction coefficient and a stable behavior under cyclic loading conditions. In this paper, the main results of preliminary analyses devoted to select the best materials to be employed as friction pads in the FREEDAM connections are presented. To date, twelve tests on eight different materials combined with stainless steel plates have been realized. The tests performed, mainly devoted to evaluate the behavior of the friction interfaces in terms of static and kinetic friction coefficient and in terms of degradation, are inspired to the sliding test proposed by EN1090-2 adopting the loading protocol suggested by the EN12159 for the qualification of seismic devices. The initial value of the friction coefficient and its evolution during the tests, have been determined for the different materials by monitoring the values of the forces of the bolts used to apply the pre-load to the friction interface and the value of the sliding force. The obtained results are presented both in terms of force-displacement curves and in terms of friction coefficient, providing a comparison between the performances of the analyzed interfaces in terms of energy dissipation supply.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11386/4686811
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