A year-long monitoring of preloaded free-maintenance bolts - Estimation of preload loss on BobTail bolts

This paper illustrates the monitoring of such a bolted connection prototype with focus on the behaviour of free maintenance BobTail® bolts, which allows the tightening from the inner side of the tower. The monitoring and identification procedure for the 1-year monitoring of the force and the temperature in a prototype is presented. A calibrated nonlinear Hammerstein-Wiener model is used to withdraw the temperature effect from the analysis allowing for a long-term estimation of the preload losses. This model is able to simulate the force with less than 2.5% of error. The significant part of the preload loss was foreseen within the first two years. Furthermore, the influence of the tightening temperature on the bolts loss was discussed and, as expected the lower the temperature during tightening, the lower the long term preload losses.