BETONTEX – Consolidation and seismic strengthening for a Cathedral damaged by the earthquake – Carpi

Jewel of Renaissance architecture, the Cathedral of the Assumption was built starting in 1514 by the will of Alberto Pio, a humanist prince and patron, and under the direction of the Sienese architect Peruzzi, who drew inspiration from Bramante and Raphael for the construction of St. Peter’s Basilica in the Vatican. Built in multiple phases during the 1600s and 1700s (the dome was completed in 1768), the cathedral underwent modifications and interventions throughout the 1800s. Currently, the oldest parts, dating back to the 16th century, have preserved the classical style desired by Peruzzi, as well as the 17th-century addition of the three aisles.

DAMAGE ANALYSIS

During the earthquake in May 2012, although the cathedral maintained its overall integrity, it suffered a series of damages that rendered it unusable and led to the closure of an entire block in Carpi’s historic center. The most severe damages were observed on the facade and the dome. The facade experienced shaking with outward movement, causing visible horizontal fractures to the pilasters and the tympanum. The top cross, despite being tethered to the underlying wall with a steel cable, fell onto the back of the facade, breaking through the wooden roof and the underlying brick vault near the entrance door. The undulating movement also resulted in the expulsion of part of the masonry in the two counterforts on the opposite side of the facade and the collapse of the four spherical ornaments on the facade, causing the roof to rupture in two additional places.

The dome suffered the most significant damage in terms of both hazard and extent. Previously subject to structural consolidation (internal reinforcement at the support level of the inner vault), the shaking during the earthquake caused the rupture of the weakest point (near the southeast window: collapse of the architrave and the underlying wall) where two X-shaped fractures also occurred. Additionally, there was expulsion of the masonry above the inner vault accompanied by a vertical fracture. The lateral aisles showed detachment fractures due to the east-west movement of the structure, which occurred at the connection arches between the central nave and the side naves.

In response to these damages, an instrumental diagnostic campaign and structural remodeling allowed for the development of a comprehensive consolidation and seismic improvement project, addressing both the dome and the masonry vaults.

INTERVENTION ON THE VAULTS

Regarding the vaults, the BETONTEX®-EPOXY reinforcement system was chosen, consisting of unidirectional carbon fiber strips capable of absorbing the tensile forces induced by horizontal forces that occur under seismic conditions, ensuring an improvement in the structural behavior of the vaults without compromising stability. Fiber-reinforced polymer composite products with continuous carbon fibers (CFRP) are anisotropic materials that exhibit predominantly linear elastic behavior until failure. The application of this type of reinforcement has provided adequate seismic improvement with characteristics of lightweight, high mechanical performance, and anticorrosive properties. The CFRP system has adapted perfectly to the context, respecting the original structure of the historically significant building and minimizing invasiveness in the masonry, facilitating installation in limited spaces, and ensuring exceptional safety results. Since the support was degraded, inconsistent, and uneven, it was necessary to remove the damaged part and subsequently apply Fibre Net FB CalceCem 25 mpa mortar to ensure adequate surface mechanical strength for the application of carbon strips. The impregnation of the fabric is an important step for the effectiveness of the reinforcement since the resin that constitutes the matrix serves to transfer the force to the fibers. These procedures were entrusted to a specialized team adequately trained in specific installation courses at Fibre Net. The intervention was carried out by laying 750 linear meters of unidirectional carbon tape weighing 300 g/m² in 20 cm wide strips, following the arrangement indicated by the structural design and according to the required installation cycle. In particular, after the application of epoxy primer, a thin layer of tixotropic impregnation resin was applied. Subsequently, the unidirectional carbon reinforcement was laid, ensuring that the entire surface was passed with an « anti-bubble roller » until all air was completely removed between the tape and the support. Finally, a second layer of impregnating resin was applied to the entire surface. The connection of the strips to the masonry is further ensured by the application of aramid fiber connectors with a carbon fiber core.

INTERVENTION ON THE DOME

The basic criterion for the design choices in reinforcing the dome was to carry out a « diffusive » intervention over the entire surface while maintaining the principle of minimal intervention in respect of the building’s history, preserving not only the original materials but also the pre-existing structural behavior in order to ensure significant improvement in response to shear actions. The intervention was carried out using the reinforced plaster technique, the RI-STRUTTURA SYSTEM, which involved the use of a GFRP (glass fiber reinforced polymer) mesh with a mesh size of 66x66mm, to be installed together with GFRP angles and connectors. The choice of this system was made considering the priority of ensuring not only the necessary mechanical performance and strength but also the durability of the intervention in varying climatic conditions and the breathability of the areas involved. The structural reinforcement was carried out by installing the mesh on the internal masonry of the dome, covering a total surface area of over 200 square meters.

CONSTRUCTION SHEET PROJECT: Seismic reinforcement intervention on the dome and vaults of the Cathedral of the Assumption

LOCATION: Carpi

DESIGNER: Technical Office of the Archdiocese of Carpi

MAIN CONTRACTOR: C.M.B. Carpi

COMPLETION: 2015