Dr. Reto Cantieni, CEO of RCI Dynamics, Structural Dynamics Consultants, Switzerland

Title: One-year Monitoring of a Historic Masonry Bell Tower

Abstract: The San Luzi Bell Tower, located in Zuoz, Engadin, Switzerland, the base parts of stem from the 14th century, was reported to exhibiting excessive vibrations when ringing the bells. These reports were made in 2003 by technicians working on the clock control system. Experimental modal analysis under ambient excitation was subsequently performed in 2004. Measurements with the bells ringing performed at the same time yielded that the tower velocity amplitude was some 16 mm/s which is well above the 2.5…3 mm/s allowed by Swiss and German codes. And: the two largest bells, No. 1 and No. 2, were found to operate close to the tower fundamental mode. Subsequently, these two bells were worked on through introducing special yokes (I will find the correct expression for this: "cranked" ok?) and also through slightly changing their pendulum frequency. Measurements performed in 2009 showed that these measures were fully efficient: The tower velocity amplitude produced while ringing bell Nos. 1 and 2 was below 1 mm/s. But: Bell No. 3 now showed to be critical (10 mm/s). Measurements performed in 2011 showed that the measures taken in the meantime by the bell caster, Ruetschi, Aarau, were not efficient. There was not enough space to introduce a cranked yoke and the measures based on adding mass on the system yoke side were not efficient. Comparison of the 2004/2009/2011 meas-urements then showed that the tower fundamental frequency varied between 1.43 Hz and 1.49 Hz. As the German Code requires a 10% distance between the bell and tower frequencies, problems arose with where to go with the bell frequency f = 1.35 Hz (changing of a tower frequen-cy is not that easy). It was therefore decided to initiate a one-year's monitoring campaign to get an overview of the tower natural behavior as a function of wind, temperature and hu-midity in summer 2012. The paper presents the history of and the results of this campaign.

Keywords: bell tower, structural dynamics, resonance problems, experimental modal analysis, natural frequency, long-term monitoring, temperature effects, wind effects