Fornale Engineering, in collaboration with Eng. Valeria Rizzotti, handled the structural design of the reinforcement and consolidation intervention of the wooden floors of the former nursery school in Staggia (MO), which was hit by the 2012 Emilia earthquake.
The floor intervention is part of the seismic retrofitting works of the entire building; however, due to the particular nature of the intervention methods, we have decided to dedicate a post to the floor retrofitting.
The particular nature of this intervention arises from the need to preserve the soffits of the existing floors, which featured culturally significant frescoes under protection by the Superintendence.
The proposed intervention included the following phases:
- removal of the existing flooring/planking to expose the load-bearing structure;
- at the trimming beams, the ends of the secondary joists resting on them were cut, and these were end-anchored to the existing beam with metal brackets;
- on the top of the trimming beams, HEA steel beams were placed, sized to carry the full live load of the new floor while ensuring very small mid-span deflections;
- new load-bearing wooden beams were inserted on the lower flange of the beam, onto which the loads of the new floor would be applied;
- an OSB3 planking was laid, cut and nailed so that the edges aligned with the axes of the new underlying beams;
- crossed perforated straps were laid, whose ends along the load-bearing walls and at the ends were welded to threaded rods passing through the thickness of the masonry. In this way, a considerable in-plane stiffening was achieved, also given the reduced spacing of the cross straps at 45 degrees;
- from the outside, niches were carved into the wall thickness of sufficient size to allow the placement of a steel plate and the tightening bolt of the rods, which enabled tensioning of the perforated straps;
- a second cross-laid OSB planking was laid;
- laying of a polyethylene sheet for the subsequent screed pour and simultaneous placement of the local typical brick tiles.
In this way, it was possible to safeguard the areas of the building with frescoed floors while ensuring, on the other hand, the necessary load-bearing capacity with adequate in-plane stiffness.
In the areas where the floor was destroyed due to the collapse of the roof, we built new floors from scratch using the same technique so that the cross-strapping would be uniform over the entire surface.
On both floors of the building, which spans about 235 sqm, it was decided to work with the straps and OSB planking to ensure uniform stiffness.
Assignment: Executive co-design of wooden structures
