01.19: Cyclic behaviour of friction materials for FREEDAM connections

Abstract

ABSTRACTA FREE from DAMage friction connection is a connection where at the level of the lower beam flange is realized a friction damper by means of angles and plates pre‐stressed with high strength bolts. In these connections, the bending moment transferred to the column through the connection can be easily calibrated controlling the friction coefficient of the material employed to realize the friction damper and the value of the preload applied by means of high strength bolts. Therefore, in order to guarantee a correct behaviour of the friction connection both under Serviceability and Ultimate limit states (SLS, ULS), it is necessary to know with a sufficient level of accuracy the value of the static and dynamic friction coefficients and the value of the bolts' preloading.The aim of this paper is to provide a characterization of the friction coefficient of some materials appropriate for the application in the FREEDAM connections. To this scope, the results of an experimental programme consisting of 51 specimens, developed following the guidelines provided by EN 1090‐2 and EN 15129 (anti‐seismic devices), are presented. The experimental work is mainly devoted to the evaluation of the response of three different materials, applied as a coating on steel plates by means of thermal spray, investigating the influence of the bolt preloading level and of the washer's typology on their response. In particular, the possible influence of the pressure applied to the analysed interfaces is considered carrying out tests at variable values of the pre‐loading (between 40% and 100% of the standard pre‐loading value), and the influence of the washers' configuration is evaluated performing tests with flat washers and different configurations of series and parallel of Belleville disc springs. The influence of the washer's typology is considered in order to assess the effectiveness of the Belleville disc springs in the reduction of the bolts' loosening during a cyclic test.The main results of the experimental activity are represented in terms of static and kinetic friction coefficients evaluating also the bolt's force degradation, the development of heat during the test and the degradation of the effective damping. Some of the tests are realized with the aim to determine the random variability of the friction coefficient of the analysed interfaces and to provide values of the friction coefficient to be used in design for ULS and SLS.