One-Year Monitoring of a Historic Bell Tower

In 2003, people working in the San Luzi, Zuoz, bell tower reported excessive tower vibrations when ringing bells. Zuoz is a village in the Upper Engadin Valley, Switzerland. Measurements performed in 2004 showed a maximum tower vibration velocity amplitude of 16 mm/s which is more than five times the acceptable value ([1], [2], [3]). In 2008 the two large bells (out of four) were equipped with cranked yokes and their pendulum frequency was reduced to get a larger distance to the tower natural frequency. Measuring again in 2009 yielded the two large bells no longer exciting large tower vibrations. However, this was not true for bell No. 3. Subsequently, this bell's pendulum frequency was also reduced. In 2011, measurements showed that this last measure had negative instead of positive effects. And, now disposing of three values, the tower natural fundamental frequency was found to be somewhere in the f = 1.43...1.50 Hz region. It was then decided to monitor the tower dynamic behavior for one year to get reliable information on the natural frequency scatter. Monitoring started June 11, 2012, and ended October 7, 2013. The results: a) The tower fundamental frequency varied in an f = 1.42...1.59 Hz range, b) The tower is stiffer in winter than in summer, c) On a daily or weekly schedule, the tower fundamental frequency is following the temperature curve: increasing stiffness with increasing temperature and vice versa. A possible explanation for this contradiction is given. Due to space restrictions, also monitored bell ringing excited tower vibrations can not be discussed here.