01.18: Optimization of the pre‐loading procedure for high‐strength bolts of FREEDAM connections

• Published in: ce/papers Vol. 1; no. 2-3; pp. 316 - 331
• Main Authors: Cavallaro, Giovanni Ferrante, Francavilla, Antonella Bianca, Latour, Massimo, Piluso, Vincenzo, Rizzano, Gianvittorio
• Format: Journal Article
• Language: English
• Published: 01-09-2017
• Subjects: Bolts' loosening; Energy Dissipation; FREEDAM joints; Tightening procedures
• ISSN: 2509-7075; 2509-7075
• DOI: 10.1002/cepa.63

ABSTRACT
A FREE from DAMage (FREEDAM) friction connection is a particular type of sliding hinge joint where in correspondence of the lower beam flange is realized a friction damper with steel angles and coated plates, pre‐stressed with high strength bolts. In order to master the resistance of a FREEDAM connection it is crucial to know accurately the value of the friction coefficient of the materials employed to realize the damper and to control the preload applied to the friction interface by means of high strength bolts.
Dealing with this last topic, according to Eurocode 3 and EN 1090‐2, High Strength bolts (both of HR or HV type), when installed in friction bearing connections, must be installed based on one of four proposed procedures. The methods currently recommended are torque, combined, HRC and DTI and each of them, as already partially reported in technical literature, can lead to a different level of accuracy. Additionally, once the bolt is tightened it is well known that it undergoes to an “instantaneous” (short‐term relaxation) and to a “long‐term” loosening that, obviously, may affect the response of the FREEDAM connection which has to be designed not to slip under service loading conditions. Within this framework, the goals of the paper are multiple. First of all, by means of experimental analysis, it is wanted to assess the accuracy of some of the codified tightening methods, namely torque and combined, when applied to FREEDAM connections. Additionally, the possible bolts’ loosening occurring in short/medium periods of time (up to one month) are evaluated performing specific tightening tests. Finally, the effectiveness of Belleville spring washers in keeping the pre‐loading constant during the life‐time of the FREEDAM connection is assessed. In order to analyse all these aspects, an experimental analysis carried out at the STRENGTH laboratory of the University of Salerno, regarding 84 tightening tests performed on specimens of friction dampers is presented. All the specimens have been tested measuring with annular load cells the initial force applied to the bolts in order to evaluate the achievement of the target preload. Additionally, in some cases, the loss of pre‐load over a period of time has been monitored.