A case study of structural monitoring as a control in the restoration process of heritage structures: The strengthening of the Vistabella Church's Tower

The Sacred Heart Church of Vistabella (La Secuita, Spain) was designed by Josep Maria Jujol and built in the early twenties of the XX century. The slenderness of its steeple makes it extremely exposed to wind action, to the extent that its stability has been threatened since the construction started. At the end of 1934, the upper spire crumbled away, being rebuilt and reinforced with steel passive bars. After some numerical approaches, it became clear that the lightness of such a masonry construction along with the feebleness of the connections between the structural members were at the basis of the problem. In order to provide additional vertical load without substantively modifying the structural scheme, a solution based on external prestressed bars, was implemented. The bars replace the existing passive bars, which have proven to be totally useless. The active system proposed allows to eliminate the masonry's tensile stresses under the action of the wind. The additional loading to be applied varies depending on the level (three levels in total), according to the gradual post-tensioning system proposed as a strengthening technique of the steeple. Therefore, it has been necessary to design a procedure able to introduce independent tensions at the different levels throughout the same tie and the corresponding monitoring system to control the tensioning process during the implementation. The procedure, the theory that supports it, and the results of the follow-up are described in detail, these being the objective of the paper.