A jewel of Renaissance architecture, the Assumption Cathedral was built starting in 1514 by the will of Alberto Pio, humanist prince and patron of the arts, and under the supervision of the Sienese Peruzzi, who made use of Bramante and Raffaello’s ideas for the construction of St. Peter's Basilica in the Vatican. Construction work continued throughout the 1600s and 1700s (the dome was built in 1768), and the Cathedral underwent refurbishing and renovation also during the nineteenth century. Currently, the oldest parts, i.e. the 16th century ones, still feature the classical style envisaged by Peruzzi, and also include the 17th century addition of the three naves.
ANALYSIS OF THE DAMAGE
Although it retained its overall integrity, the Cathedral suffered extensive damage during the 2012 earthquake that made it unfit for use and led to the closure of an entire block of Carpi’s historic town centre. The facade and the dome suffered the most serious damage. The facade was shaken by an outward motion that caused clearly visible horizontal cracks in the responds and the tympanum. The summit cross, although tied with a steel cable to the underlying wall, fell on the back of the facade, breaking through the wooden roof and the underlying brick ceiling near the main entrance. The surface wave motion also resulted in part of the masonry falling out of the two buttresses on the counter facade and the collapse of the four spherical ornaments of the facade, leading to the collapse of two other roof portions.
The dome turned out to be the most extensively damaged element, both in terms of dangerousness and extent. The dome underwent a previous structural consolidation (internal chaining to support the inner ceiling), the shaking of the earthquake caused the point of least resistance to break (near the large window located south-east: collapse of the lintel and wall below), where there are also two X-shaped shear cracks. Furthermore, the masonry placed over the internal ceiling fell out, also featuring a vertical crack. The side naves show a number of detachment cracks due to the east-west motion of the building in the arch connecting the ceiling of the central nave and the aisles.
In the face of such damage, an instrumental diagnostic survey and remodelling of the structure has made it possible to prepare a project for consolidation and seismic improvement in several stages, concerning both the dome and the masonry ceilings.
OPERATIONS ON THE CEILINGS
To mechanically improve the ceilings it was decided to use the BETONTEX®-EPOXY system, consisting of Carbon Fiber Reinforced (CFRP) reinforcements, able to absorb the tensile stresses induced by the horizontal forces that are generated in seismic conditions, assuring an improvement in the structural behaviour of the ceilings without undermining stability.
Polymeric matrix fibre-reinforced products based on continuous carbon fibres (CFRP) are anisotropic composite materials, which show a predominantly linear elastic behaviour up to collapse. Application of this type of reinforcement has assured adequate seismic improvement with the main features being lightness, high mechanical performance and anti-corrosion properties. The CFRP system has adapted perfectly to the setting, respecting the original structure of the building of historical interest and minimising invasiveness issues in the masonry, with easy installation in the limited installation space available to operators and assuring outstanding results in terms of safety.
Since the substrate was degraded, inconsistent and non-planar, it was necessary to proceed with the removal of the deteriorated part and subsequent application of mortar 25 mpa compression resistance, to assure adequate mechanical surface resistance of the substrate on which to apply the carbon tapes. Impregnating the fabric is an important stage for effectiveness of the reinforcement since the resin that the matrix consists of has the purpose of transferring the stress to the fibres. These operations, in fact, were entrusted to a specialised team, adequately trained.
The operation was carried out by installing 750 linear metres of unidirectional 300 gr/m2 carbon tape in 20 cm strips, following the layout indicated by the structural project and according to the required installation cycle. In particular, after application of the epoxy primer, a first, thin layer of impregnating thixotropic resin was laid. Subsequently, the unidirectional carbon reinforcement was laid, taking care of running the “bubble roller” over the entire surface to completely remove the air between tape and ceiling surface. Finally, a second layer of impregnating resin was laid. Connections between the tapes and the masonry are further assured by the application of carbon fiber connectors.
OPERATIONS ON THE DOME
The basic criteria for design choices in dome reinforcement was to carry out a “widespread” operation on the entire surface maintaining the principle of minimal invasiveness to respect the building’s history, i.e. not only preserving the original materials but above all without changing the existing structural behaviour, with the aim of assuring a significant improvement with regards to behaviour under shear actions.
The operation was carried out using the reinforced mortar technique, RI-STRUTTURA SYSTEM, made with Glassfiber Reinforced Polymer (GFRP) net 66x66mm mesh to be installed with GFRP corners and connectors. The system was selected taking into account the priority to assure, in addition to the necessary mechanical and resistance performances, the required durability of the installation upon changed climatic conditions as well as breathability of the areas concerned by the installation. The structural reinforcement was carried out by installing the GFRP mesh on the dome’s internal walls together with natural lime mortar.
[by Cecilia Zampa]