Standardised friction damper bolt assemblies time-related relaxation and installed tension variability

The sliding hinge joint is a type of low-damage Seismic resistant connection equipped with a bolted friction damper at the bottom beam flange. To accurately control its flexural resistance, it is critical to govern the bolts' preload which depends on complex issues related to the installation procedure, and the short- and long-term phenomena. Despite the influence of these factors on the,initial and life-time behaviour of bolts, currently, little information exists. Nevertheless, a statistical characterisation of the variability of the preloading force (initial and during the life-time) would be needed, in order to develop reliable design guidelines for these connections. Within this framework, this paper examines experimentally, the variability of the preloading force of European bolt assemblies applied in friction dampers, through continuously monitoring the preloading at installation over a period of time. This was done to analyse the accuracy of the standardised installation procedures and the rate of loss of the initial tension over time. The tests have evidenced a higher accuracy of the torque method, highlighting some criticisms of the combined method which, conversely, proved to be inaccurate as currently codified. The short- and mid-term tests have shown that the estimated loss after 50 years, in case of assemblies with normal washers or with European standardised disc springs is, on average, equal to 10% and 27%, respectively. Additionally, in all the cases, the greatest part of the total loss (congruent to 70%) occurred in just 30 days, highlighting that time-dependent phenomena are mainly concentrated in the first days after tightening. (C) 2017 Elsevier Ltd. All rights reserved.