Filtering of high frequency motion due to the gap effect on bolted anchorage

Abstract

In the context of the equipment qualification against induced vibration due to the impact on reinforced concrete structures, the question of the transmission of high frequency and high amplitude motion through bolted anchorages of the equipment is raised. To better comprehend this phenomenon, the 3rd phase of the “Improving Robustness assessment of structures Impacted by missileS” (IRIS 3) impact tests under the cover of the OECD/NEA has been carried out in the VTT laboratory in Finland. Two types of assemblage for equipment: bolted and welded, were specifically added to the mock-up in order to assess the influence of connection on induced vibration to equipment. Displacement and acceleration sensors compatible with high frequency and high amplitude motion are mounted to multiple locations, especially upstream and downstream of the anchorage locations. Analyses of measured data of these tests have shown significant differences between bolted and welded equipment while their experimentally identified modal characteristics (frequency and damping ratio) were the same. Indeed, the bolted equipment exhibits a reduced motion with respect to the one of welded equipment, especially regarding response spectra performed from time history acceleration measured on different equipment, so-called in-equipment response spectra. The filtering effect of bolted equipment occurs even without any damage around the bolted assemblage, which is used to be pointed out as the main reason for the filtering effect according to the literature. These analyses on IRIS 3 measurement raise the hypothesis that the main cause of the filtering effect is the annular space between the anchorage rod and the clearance hole, conventionally called by anchorage gaps. In effect, anchorage gaps enable the slip of different surfaces of bolted assemblage which transmit only low amplitude motion by the friction force. To check the validity of this hypothesis, a gap model is proposed to simulate the response of bolted equipment. A very good agreement of simulations with gap model and measurements confirms the important role of gaps on the filtering effect of bolted equipment in the context of induced vibration due to impact. In conclusion, the bolted anchorage should be regarded as a filter which allows to mitigate the transferred motion due to impact vibration from supporting structures to equipment. This conclusion of the gap effect on equipment in the impact field could have significant extension to seismic hazard, especially regarding to the reduction coefficient αgap=0.5 on the resistance of anchorage bolt in case of gaps. With new findings in this study, the gap could not be systematically considered as a harmful effect as proposed by the standard of concrete fasteners. Further studies on the gap effect should be carried out to identify the role of friction and impact phenomena on the gap effect.