A case study of structural monitoring as a control tool in the restoration process of Heritage Structures: The strengthening of the Vistabella Church’s Tower

Abstract

The Sacred Heart church of Vistabella (La Secuita, Tarragona) was designed by Josep Maria Jujol, one of the masters of Catalan Modernism, and built in the early twenties of the XX century. The structure of the steeple consists of four inclined T-shaped masonry ribs, crowned by a cross. Two intermediate levels sustained in short and shallow masonry arches, act as a bracing systems. At the end of 1934, the upper spire crumbled away, being rebuilt and reinforced with a pair of steel passive bars per rib. 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, has been implemented. They replace the existing passive bars, which have proven to be totally useless. The active system proposed allows to eliminate the masonry’s tensile stresses. Hence, eight steel bars (ties) have been placed where previously the steel passive bars were. By prestressing them, a permanent vertical loading will be introduced in the structure, improving its structural behaviour in front of the horizontal loads (wind action). The additional loading to be applied varies depending on the level (three levels in total). Therefore, it has been necessary to design a procedure able to introduce independent tensions at the different levels throughout the same tie. Moreover, the prestressing process had to balance two different requirements: on one hand, the need to add load to the structure, and on the other - due to the slenderness of the masonry ribs - do not exceed the allowable compression of the material. For this reason, 24 strain gages - 8 per level - have been placed along the steel bars, to individually control the stresses at any bar and level, by means of monitoring these. The ties have been prestressed from their lower end. Whenever all bars of a certain level reached the design value, they have been attached to the bracing system situated underneath. The procedure, the theory that supports it, and the results of the follow-up are described in detail.