San Giovanni Battista church: Operational modal analysis after injected anchors strengthening

The paper reports the results of the efficacy of a strengthening intervention on the Church "San Giovanni Battista" in Sottochiesa (BG-Italy) by tying the entire building through injected anchors equipped with a special fabric sleeve to guarantee effective and controlled grout injections. The Church presented a relevant crack pattern especially on the apse masonry dome. Anchors placed along the perimeter walls through-thickness (27 m) and along the facade and the apsewalls (10-12 m)were pre-stressed and injected.The retrofit measurewas necessary to counteract wall instability, to contain the existing crack pattern, in order to prevent their widening, and to obtain a general improvement of the stability of the archivaulted walls. Preliminary in-situ tests, such as pull-out tests on sample (sacrificial) anchors installed in masonry, were performed before designing the retrofit intervention. The effectiveness of the intervention was evaluated by comparing the results of ambient modal characterization of the building before and after the retrofit works. Since the final aim was to provide new continuity to the walls, reducing and controlling the main vulnerability in the building without significantly altering the behavior of the church, the experimental results have confirmed the efficacy of the intervention, showing unchanged modal shapes and a slight stiffness increase of the entire building. The study gave also the opportunity of testing and evaluating the possibility of making long diamond core drilling works along the perimeter walls in order to tie large buildings through injected anchors equipped with fabric sleeve.