L’USO DEL METALLO NEL RESTAURO E MIGLIORAMENTO SISMICO DELLA BASILICA DI S. ANDREA A MANTOVA
Andrea Alberti1, Anna Maria Basso Bert2, Alberto Moretti3, Monica Nascig4,
1 Soprintendente architetto, Soprintendenza B.A.P. per le province di Venezia, Belluno, Padova e Treviso, Venezia, andrea.alberti@beniculturali.it
2 Architetto, Soprintendenza B.A.P. di Brescia, Cremona e Mantova, Brescia, annamaria.bassobert@beniculturali.it
3 Ingegnere, Safexpertise s.r.l., Udine, alberto.moretti@safexpertise.com
4 Architetto libero professionista, Mantova, monica@nascig.it
In the basilica of Leon Battista Alberti, an astonishing example of Renaissance architecture characterized by massive masonry walls, the metal is widely used both to sustain masonry structures in the form of tie bars, pattress plates and tie-rods in the jack arches and as a material for casements fixture and anchor systems.
The restoration of the surfaces performed in the last six years provided the occasion for the study and extended analysis of the nature and function of the metallic elements present in the building and encouraged, inter alia, the construction of new metallic structures, both for seismic improvement of the lantern and of the counter facade and for the consolidation and static reinforcement of the northern transept portal. It was chosen to use steel consistently, every time taking advantage in a different way of the mechanical properties of this material and of the numerous possibilities of surface treatment in order to achieve the intended goals.
The earthquakes of 20 and 29 May, 2012, have concentrated damage on the main counter facade, on the northern transept counter facade and, above all, on the lantern as a structure that suffered the greatest damage.
The choices of the operating methods for specific interventions managed simultaneously by different subjects, the Superintendency of Fine Arts and Landscape of Brescia, Cremona and Mantua and Diocese of Mantua, were made according to an unitary structural scheme with the complete sharing of the criteria of effectiveness, of the material consistency preservation, of integration with the existing structure in accordance with the overall structural behaviour of the single parts, of durability and of search of reversibility and finally paying the maximum attention to reducing the invasiveness, both material and perceptive, considering the delicacy and of the richness of architectural and artistic context.
For the static reinforcement of the northern transept portal it was chosen to use a visible "prosthesis" made with an tensioner on the inside of the architrave acting as a counterforce to the gravitational loads and to counteract the possible collapse. The height limit demanded of the stainless steel structure in order to mitigate the visual impact, required a highly detailed design and particular solutions in anchoring, installation and tensioning.
On the main counter facade stainless steel was used to support the entablature, significantly damaged even due to non-seismic issues. In this case were designed the brackets, also left visible, and the pursuit of the structural efficiency was accompanied by that of the shape and arrangement of plates and ribs and of the surface treatment type, in order to be able to relate properly to the characteristics of the decorative elements.
For the lantern, placed at a height of 70 m, after the creation of a provisional vertical truss structure, it was decided to construct the final structure according to mimesis with existing fixtures criteria. The intervention consisted in the installation of steel braced frames inside the holes of the lantern in order to counter the oscillation produced by future earthquakes.
Key-words: Renaissance, structures, restoration, seismic improvement, mimesis.