The consolidation of the wooden structures carried out with CFRP tapes – San Domenico Basilica – Siena

“The Basilica of San Domenico, one of the main churches in Siena, was built in the 13th century and expanded in the following century. Its brick architecture, typical of mendicant orders, is in Cistercian Gothic style with a gabled facade, smooth and devoid of decorations, and a large central rose window. The main entrance is lateral because the chapel’s vaults protrude from the surface of the facade. The building has an Egyptian cross plan with a single wide nave covered by exposed beams, a transept also covered by beams, and numerous vaulted cross-shaped chapels within the back wall.

The project for the central nave of the Basilica was carried out by the Superintendence for Fine Arts and Landscape for the provinces of Siena, Grosseto, and Arezzo. The aim was to consolidate the structure effectively while minimizing the visibility of reinforcement elements. The intervention needed to be compatible with the original materials and construction technique, non-invasive, and lightweight. The goal was to ensure that in the event of an earthquake, the existing roof structure, composed of embedded steel trusses and purlins, would not amplify harmful local collapse mechanisms on the central nave walls.

STRUCTURAL ANALYSIS AND PROJECT OBJECTIVES

The existing roof structure consists of “embedded” trusses (metal carpentry profiles covered with wooden panels) in the Lombard style, with secondary embedded elements (terziere) and wooden tertiary elements (small beams). On top of these overlapping orders, a wooden flooring with a thickness of 3 cm was added. Given this situation, the objective of the consolidation project was to ensure a good degree of monolithic behavior of the roof, generating a distribution, and consequently, the most uniform possible redistribution of seismic actions on the walls. The need to increase the stiffness of the roof primarily arose from the high slenderness ratio of the walls (the ratio between the height of the walls and their thickness) that make up the load-bearing masonry “box” of the central nave. By ensuring an effective connection between the trusses (the consolidation of the central nave of the Basilica is treated and schematized as the system used to stiffen the roof of a metal shed) and inserting vertical FRP (Fiber Reinforced Polymer) connections between the roof reinforcement and various secondary and tertiary orders, as well as anchoring these roof reinforcements deep into the perimeter walls, it is possible to increase the stiffness of the two roof levels and improve the overall behavior of the structure.

The project, therefore, involved creating two rigid diaphragms to evenly transmit shear forces resulting from potential seismic events to the perimeter walls of the nave.

INTERVENTION

The consolidation of the wooden structures was carried out by inserting an auxiliary bracing structure made of HT (High Tenacity) carbon fiber-based unidirectional tapes, metal plates, steel connections, aramid flakes, and steel-carbon combinations on the extrados of the roof deck. After removing the non-structural roof covering, S355J2 steel plates (10 mm thickness) were installed. The roof deck and the underlying masonry were drilled to ensure a proper connection between the metal plates and the load-bearing masonry using chemical anchoring with threaded bars. This connection to the masonry was made more effective by extending some of the threaded bars to engage with the existing reinforced concrete beam. Additional connections, made entirely of aramid fiber rope, were installed to strengthen the masonry/metal plate complex and ensure adequate transmission of shear forces from the carbon fiber tapes to the masonry. The connection system involved drilling a Ø16 hole, placing two cross-shaped CFRP tape strips impregnated with epoxy resin on the hole, inserting a Ø10 aramid fiber rope treated with surface epoxy resin into the hole, and saturating the hole with epoxy resin, paying attention to the tails that protrude from the hole onto the support. Where it was not possible to anchor to the perimeter walls, the intervention consisted of an anchorage that connects the metal plate systems for distributing the bracing/decking/beams/metal terzere and decking/beams/metal terzere. The connections were made similarly to the perimeter walls, inserting Ø10 mm aramid fiber rods into the specially prepared hole to create a rigid connection with the metal terzere. The connection was then strengthened by applying 200 mm wide carbon fiber fabric and epoxy resin for bonding/impregnation. The bonding/impregnation resin consists of a primer (a low-viscosity epoxy resin and hardener with high penetration capacity for surface preparation) and impregnating resin (a thixotropic epoxy resin and hardener for impregnating fabrics), the latter applied before and after positioning the fabric. The connection between the metal plates and the roof deck was further reinforced with scattered nailing (metal screws for wood). Once the connections were completed on the walls and the fabric reinforcement on the open-span roof, the intervention involved the application of high modulus 400 g/m² unidirectional carbon fiber fabric impregnated and bonded to the roof using epoxy resins to create the designed bracing system. The intervention exclusively focused on the extrados of the roof (after removing the non-structural components) and had no impact on the interior of the Basilica.

CONSTRUCTION RECORD OBJECT_Consolidation of the roof structures of the Basilica of San Domenico in Siena

PROJECT_Superintendence for Fine Arts and Landscape for the provinces of Siena, Grosseto, and Arezzo

CONTRACTING COMPANY_FAESULAE S.r.l. Florence

PROCEDURE MANAGER_Architectural Design by Emanuela Carpani, Second Phase by Anna di Bene

STRUCTURAL DESIGNER_Engineer Claudio Neri – Siena

PROJECT MANAGER_Architect Massimo Bucci – Superintendence for Fine Arts and Landscape for the provinces of Siena, Grosseto, and Arezzo

OPERATIONAL MANAGER_Geomatics Technician Danilo Faleri and Geomatics Technician Mauro Marchetti – Superintendence for Fine Arts and Landscape for the provinces of Siena, Grosseto, and Arezzo COMPLETION_2015