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

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 (≅ 70%) occurred in just 30 days, highlighting that time-dependent phenomena are mainly concentrated in the first days after tightening.