Keywords: modal testing, structural identification, belltower, bell forces, numerical model.

This paper presents initial work on the bell-tower of Santa Justa y Rufina church in Orihuela (Alicante-Spain). This structure is a gothic belfry constructed in the XV century that possesses five bells. Some restoration work is planned on these bells and it is necessary to know the dynamic characteristics of the tower to avoid future problems originated by a possible dynamic interaction between the natural frequencies of the tower and the horizontal forces in-troduced by the bells on the tower.

Several dynamic tests have been performed on the belltower to know the mechanical parameters, vibration modes (bending and torsion) and structural damping. All of them are based on the registration of ambient vibrations at different heights and directions. Only bending and torsional vibrations are registered due to the high longitudinal stiffness. The work of Bachmann [1] and Casolo [2] fix the main torsional and bending frequencies between 0.9 and 2 Hz for slender towers, and analogous results are obtained by Gentile [3]. The work of Ivorra [4] in a similar belltower shows the evaluation of frequencies and the experimental procedures.

As a first approximation, a simplified five degree of freedom model has been used to determine the belltower response to bell forces. A more refined numerical model was done using the commercial software SAP2000TM. Eight-node-hexaedral finite elements have been used to mesh the model with three degrees of freedom per node. An iterative process has been performed to fit, through a modal analysis, the fundamental frequencies of the initial model and those obtained from the real model. After completing the iterative process, an average elastic modulus for the structure of 1.500 N/mm² was obtained. An average elastic modulus of 200 N/mm² was obtained for the basement. Once the numerical model has been calibrated using the experimental results, the horizontal forces caused by the swinging bells are introduced with the model. From this analysis the dynamic amplification factor is shown to be lower than 1 for all the bells, so no dynamic interaction is found between belltower and bells.

The conclusions of this paper are:

1. A simplified and low-cost method is described to evaluate the dynamic effect generated by the swing of bells on the bell tower.
2. With this non-destructive and low-cost method five vibration modes were clearly identified within the frequency range 0-10 Hz.
3. This gothic belltower has its first natural frequency higher than others presented in the scientific literature. This can be a singular aspect for this type of tower, because the ratio between wall thickness and slenderness can suggest a more rigid belltower than other architectural styles.
4. The swinging of bells swinging in the Spanish system causes no special or singular structural problems for this belfry.

1

H. Bachmann, W. Ammann, F. Deischl, "Vibration Problems in Structures: Practical Guidelines", Springer Verlag, Berlin, 50-55, 1997.

2

S. Casolo, "A three-dimensional model for vulnerability analysis of slender medieval masonry tower", Journal of Earthquake Engineering, 2(4), 487-512, 1998. doi:10.1142/S1363246998000228

3

C. Gentile, A. Saisi, "Ambient vibration testing of historic masonry towers for structural identification and damage assessment", Construction and Building Materials, 21, 1311-1321, 2007. doi:10.1016/j.conbuildmat.2006.01.007

4

S. Ivorra, F. Pallarés, "Dynamic investigations on a masonry bell tower", Engineering Structures, 28(5), 660-667, 2006. doi:10.1016/j.engstruct.2005.09.019

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