San Callisto Catacombs in Rome. Pultruded profiles for a walkway in high humidity environments

Along the Appian Way, after the church of « Quo Vadis? », you can find the Catacombs of San Callisto, among the largest and most important in Rome. They belong to a very extensive burial complex (approximately 15 hectares of land) consisting of a network of galleries that are almost 20 kilometers long and spread over several levels, reaching a depth of about 20 meters. Originating in the mid-second century, they are named after the deacon San Callisto, who was appointed by Pope Zephyrinus (early 3rd century) to oversee the cemetery, which became the official burial ground of the Church of Rome.

In front of the crypt of the Popes, the small cubiculum of Orpheus was in very poor condition and concealed, thanks to an ancient masonry narrowing that was later removed, a large portion with frescoes depicting Orpheus on the central vault. In order to make the cubiculum of Orpheus accessible to the technical staff of the Pontifical Commission of Sacred Archaeology and scholars, a structure composed of lightweight, durable Fiber Reinforced Polymer (FRP) materials was created to withstand the aggressive conditions of high humidity inside the Catacombs.

The specific environmental conditions and the particular context, especially considering the geological characteristics of the site, were crucial in determining the materials to be used for the proposed structure. With these premises, the designers opted for preformed elements made of composite material, technically known as « pultruded » (from the English term « pull + extrusion, » referring to the technique used to obtain this type of material), as a replacement for products such as steel bars and welded wire mesh made of traditional metallic materials.

INTERVENTION WITH FIBRE NET SYSTEM

The walkway structure created inside the cubiculum of Orpheus consists of a framework of pultruded profiles made of Glass Fiber Reinforced Polymer (GFRP) with a polymer matrix and long glass fibers. These profiles support other composite elements on which a layer of structural glass (with a thickness of 27 mm) rests to allow pedestrian access on the walkway and minimize its impact within the narrow passage. The connecting rods and elements are made of pairs of pultruded C-shaped composite profiles measuring 200x60x10, and the base columns, which support the cantilever, rest on the lower level of the cubiculum floor through a distribution plate. The connection elements between the profiles and the foundation plates are made of galvanized steel and are connected using galvanized steel bolts. The tension rods that support the cantilever are made of stainless steel INOX Aisi 316 strands.

The intervention with pultruded materials was carried out in two successive phases, with the design and subsequent monitoring of the installation phases of:

• A steel element inserted into the walkway structure on the lower level of the cubiculum of Orpheus to support a cantilever portion of the tuff immediately below the entrance to the cubiculum.
• A support structure for the stone architrave that had a temporary safety element inserted and was at risk of cracking due to its own weight.

The great versatility of composite materials, combined with the ability to control and calibrate their performance in advance, their durability against external aggressive agents, and the benefit of their low weight, suggest interesting possibilities for the use of FRP elements in structural reinforcement, functional adaptation, and seismic resistance. Furthermore, the excellent weight-to-performance ratio and ease of on-site processing with standard cutting and drilling equipment make pultruded GFRP profiles increasingly popular.