Restoration and Conservation Works on the Cathedral of Saints Maximus and George in L’Aquila

The restoration of the Cathedral of Saints Maximus and George in L’Aquila represents one of the most complex interventions carried out in recent years on Italy’s monumental heritage damaged by the earthquake of 6 April 2009. The extensive structural damage, the collapse of significant portions of the building, and the need to reconcile heritage conservation, seismic strengthening, and the preservation of the cathedral’s historic and artistic features required a multidisciplinary design approach, in which structural engineering played a fundamental role.

The intervention forms part of a broader programme for the structural rehabilitation and restoration of the entire monumental complex, encompassing not only the cathedral itself but also its ancillary buildings, including the sacristy, museum, chapter house and clergy residence. The subdivision of the works into two functional phases was introduced solely for organisational and administrative purposes, while maintaining a unified design approach in which structural, architectural, building services and conservation works were conceived as closely interconnected components. The first phase focused primarily on the reconstruction of the most severely damaged portion of the building, allowing the roof structure to be reinstated and the transept and apse areas to be stabilised. The second phase is dedicated to completing the structural strengthening and restoration of the entire cathedral.

As is frequently the case with interventions on historic monuments, the level of knowledge available during the design stage proved insufficient to provide a comprehensive understanding of the actual condition of the structure. Following the earthquake, the inaccessibility of large portions of the building and the safety constraints prevented direct inspection of many structural elements. Only after the construction site had been established and the scaffolding erected was it possible to carry out close-range surveys, detailed diagnostic investigations and targeted inspections, which revealed deterioration mechanisms and structural criticalities that could not have been fully anticipated during the design phase.

Within this framework, LACELAB provided specialist structural engineering consultancy, supporting the main contractor in the development of several design refinements, the definition of construction details for some of the most complex operations on site, and the supervision and monitoring of the execution through internal AUDIT activities. The objective was to translate the design intent into fully constructible engineering solutions by introducing, where required, technical specifications, design optimisations and improved construction alternatives capable of enhancing structural performance while ensuring the highest level of compatibility with the historic fabric.

Reconstruction of the Pendentives of the Crossing

Among the most significant interventions carried out during the construction phase was the reconstruction of two of the four pendentives supporting the central crossing dome, namely those located on the side adjacent to the Archbishop’s Palace. The design amendment report highlighted that these elements had not been addressed by a specific engineering solution within the approved design and therefore required the development of a dedicated intervention during the execution of the works.

The adopted solution involved the construction of a lightweight compartmentalised masonry structure, designed to reproduce the original geometry while simultaneously reducing the permanent structural loads. From a construction perspective, the system incorporates reinforced masonry, connecting slabs, stitch-and-patch masonry repairs (cuci-scuci) at the interfaces with the existing walls, the insertion of stone bond stones (diatons) to ensure effective interlocking with the original masonry, and high-bond stainless steel reinforcing bars. To complete the intervention, widespread strengthening was provided through galvanised steel fibre mesh embedded in a lime-based structural mortar (geomortar), thereby enhancing the structural interaction between the newly reconstructed portions and the preserved historic fabric while ensuring full compatibility with the original construction materials.

Strengthening of the Crossing Piers

One of the most significant design developments concerned the strengthening of the four crossing piers, which are fundamental load-bearing elements governing the structural behaviour of the entire cathedral. During the construction phase, following detailed technical investigations and a series of institutional review meetings, a revised construction solution was developed. While preserving the performance objectives of the approved structural design, the revision introduced an alternative execution methodology.

The revised solution replaced the originally specified large-diameter steel tube reinforced drillings with smaller-diameter boreholes reinforced using high-strength Dywidag bars, strategically distributed to achieve equivalent structural performance. In addition, sub-horizontal reinforced stitching was introduced through stainless steel bars inserted into drilled holes and grouted using low-pressure hydraulic mortar injections. This solution significantly improved the constructability of the intervention, enhanced the structural connection between the different masonry sections, and fully preserved the safety levels required by the approved structural design.

Optimisation of the Roof Reconstruction

The reconstruction of the roof was also the subject of a significant design refinement. While preserving the overall concept established in the detailed design, several improvements were introduced during the construction phase, affecting both the timber structural system and the roof assembly.

Among the most significant modifications was the adoption of a double-layer cross-laminated timber decking, replacing the originally specified configuration, together with the complete redesign of the waterproofing system through the use of bituminous membranes also serving as a vapour barrier. The intervention was further enhanced by the installation of complementary components, including bird protection grilles and burnished stainless steel roof tile retaining systems, aimed at improving the durability, reliability and ease of maintenance of the new roof.

At the same time, the detailed design of the new timber trusses, structural joints and steel-to-timber connections was developed, defining the materials, connection details and construction methods for the entire roofing system.

Structural Monitoring During Construction and Service Life

The conservation of a historic monument does not end with the completion of the restoration works. In the rehabilitation of heritage buildings, structural monitoring has become an essential tool for assessing the behaviour of the structure throughout its service life. In particular, for monumental buildings that have undergone structural strengthening, monitoring provides continuous information on the condition of the structure, enabling the effectiveness of the interventions to be verified while supporting long-term inspection and maintenance activities.

Within the framework of the extensive strengthening and restoration programme undertaken following the seismic events that affected the L’Aquila area, the monitoring system developed for the Cathedral of L’Aquila was conceived precisely to achieve these objectives.

Unlike the discrete inspections performed during commissioning and acceptance testing, a permanent structural monitoring system makes it possible to observe the evolution of the building’s structural behaviour over time through objective and repeatable measurements.

For the Cathedral of L’Aquila, the monitoring system was designed to:

– monitor the evolution of the structure’s deformation state;

– measure the displacements and rotations of the most significant structural elements;

– characterise the dynamic behaviour of the building through the identification of its natural frequencies and mode shapes;

– correlate the structural response with environmental variations, particularly temperature;

– detect any anomalies with respect to the established reference conditions.

The purpose of the system is not to replace conventional structural assessments, but to provide engineers with a continuous observation tool capable of supporting decision-making both during the construction phase and throughout the subsequent management of the building, including the final acceptance process.

The investigation programme and the structural monitoring system were designed in accordance with the principles of compatibility with the historic fabric, minimising the invasiveness of the installations and giving preference to fully reversible solutions.

The monitoring system integrates two complementary measurement technologies.

The first is based on dynamic structural monitoring using high-sensitivity triaxial MEMS accelerometers installed at the most representative structural locations. These sensors continuously acquire the structure’s acceleration response and monitor its evolution over time, enabling the assessment of the building’s dynamic characteristics. The selected devices are equipped with 24-bit analog-to-digital converters and provide a noise density of 0.7 μg/√Hz, a performance level essential for accurately detecting the low-amplitude ambient vibrations typical of monumental structures.

The second component of the system consists of four laser displacement sensors with integrated inclinometers, installed transversely across the main nave. This configuration enables continuous monitoring of the relative displacements between opposing structural elements while simultaneously measuring variations in inclination and local rotations with millimetric accuracy.

The entire monitoring system has been designed to ensure continuous operation and high measurement reliability.

The accelerometers are interconnected through an EtherCAT network configured in a daisy-chain topology, in which power supply and data transmission share the same Ethernet connection, thereby reducing cabling requirements while ensuring precise time synchronisation of all measurements. The laser sensors, on the other hand, are connected to dedicated coordination nodes that manage local data acquisition and transmit the collected information to the central monitoring system.

The data are stored within a dedicated platform that enables real-time access to the measurements, visualization of temporal trends, and continuous monitoring of the operational status of the entire system. The acquired information is interpreted through the combined analysis of the monitored parameters and environmental conditions, making it possible to distinguish variations attributable to thermo-mechanical effects from those associated with potential changes in the structural behaviour of the building.

In modern structural strengthening and rehabilitation projects involving historic buildings, structural monitoring is no longer regarded as a supplementary activity but has become an integral part of the structural design strategy.

The availability of continuous monitoring data makes it possible to verify the long-term effectiveness of the interventions, support planned maintenance activities, and provide objective information on the structural performance of the building throughout its service life. In the case of the Cathedral of L’Aquila, the monitoring system developed constitutes a permanent structural health monitoring infrastructure, designed to accompany the building well beyond the completion of the restoration works, thereby contributing to the protection and long-term preservation of one of Italy’s most significant monuments of historic and cultural heritage.