The importance of surveying in seismic vulnerability assessments for masonry structures: the case of the State Library of Montevergine National Monument in the Municipality of Mercogliano (AV)

The accurate survey of masonry structures is essential in seismic vulnerability assessments, as it provides the necessary data for an accurate diagnosis of the building’s structural condition. In fact, masonry has very different characteristics of strength and seismic behavior than reinforced concrete or steel structures, and a detailed survey allows the identification of any critical issues that could compromise safety during a seismic event. Through the analysis of materials, geometries, discontinuities, and possible existing injuries, the survey allows the identification of weak points and the development of numerical models that will appropriately simulate the building’s response to seismic stresses. In addition, thanks to advanced technologies such as laser scanners and drones, the survey can be carried out with high accuracy and in a short time without compromising the integrity of the structure, thus facilitating timely reinforcement and restoration interventions. In this context, surveying is an indispensable tool to optimize the design of targeted interventions and to ensure the safety of buildings in case of earthquakes.

 

SEISMIC VULNERABILITY CHECK

Vulnerability checks of a masonry building are essential to assess the structure’s resistance to seismic events, identifying critical points that could compromise its safety. Masonry, while strong, is less homogeneous and more susceptible to variables such as the quality of materials, the arrangement of stones or bricks, and the construction techniques used, which can vary considerably over time. Structural verifications, in accordance with current Standards, must consider the complex behavior of masonry under vertical, horizontal and seismic loads. In particular, shear strength and the ability to absorb deformations without damage are often critical points.

In this regard, the following is the process carried out for the seismic vulnerability verification of the State Library of the Montevergine National Monument, located inside the Loreto di Montevergine Abbey Palace in Mercogliano, whose main purpose was to assess the structural conditions of the building and identify possible criticalities in order to define appropriate intervention criteria.Una verifica di vulnerabilità sismica, in accordo alle NTC-2018 (Norme Tecniche per le Costruzioni), prevede una serie di passaggi metodologici che permettono di valutare la risposta dell’edificio a un’azione sismica:

1. Detailed survey of the structure and literature research in order to acquire accurate information on geometries, materials, construction details and any defects to then classify the building according to the structural type and seismic category of the area in which it is located, according to the provisions of NTC-2018;

2. Analysis of the mechanical properties of materials, which must be performed through direct tests or indirect estimates, according to the regulations;

3. Structural modeling of the building and subsequent dynamic analysis, using linear or nonlinear models, to simulate the structural behavior under seismic loads, evaluating the building’s ability to resist the effects of the earthquake;

4. Evaluation of the safety of the structure, based on the results obtained, comparing the strength obtained with that required by the regulations for the building in question;

5. Design of improvement and/or retrofitting interventions.

 

HISTORICAL-CRITICAL ANALYSIS AND SURVEY OF THE STRUCTURE

The building of the State Library of the Montevergine National Monument, located inside the Loreto di Montevergine Abbey Palace in Mercogliano, in its current state consists of three levels above ground plus the roof, and is made of load-bearing masonry consisting mainly of regular tufa stones and, only locally of solid bricks (columns). The building built on an older monastic complex, destroyed by the earthquake of November 29, 1732, dates back to the first decades of the 12th century; in particular, the subject of verification is only one of the four building blocks of the structural complex, for the entire development in elevation, that is, two above-ground horizons in addition to the attic, which houses on the ground floor and partly on the first the premises of the State Library of the Montevergine National Monument.

The first phase of the investigation aimed to gain an understanding of the building conducted through historical and bibliographical analyses of documents found in public archives and a thorough field survey.

The second phase involved the laser scanner survey, which was essential for creating the 3D model of the portion of the structure of interest, and the drone survey of the entire monument complex.

These tools made it possible to acquire highly accurate and detailed data, even in the most difficult-to-reach environments. The laser scanner, in fact, provides a three-dimensional point cloud that faithfully reproduces the geometry of the surveyed surface, while the drone provides a bird’s-eye view, which is useful for exploring large areas and obtaining perspectives that would otherwise be impossible to achieve with traditional methods. The combined use of these technologies makes it possible to create highly accurate digital models that are essential for restoration work, structural analysis, design and monitoring over time, reducing time and costs compared to traditional surveying methods.

Watch: Drone survey of the Monumental Complex of the Loreto Abbey Palace of Montevergine.

 

STRUCTURE MODELING

Knowledge of the building made it possible to draw up the graphical drawings and to define the level of knowledge and characteristics of the materials, fundamental points for a reliable seismic safety assessment of historic masonry buildings. In this specific case, considering that the building volumes of the library are minimal compared to the entire complex, given the particular structural typology, the vulnerability analysis is conducted by analyzing the most fragile failure mechanisms, i.e., the mechanism of overturning of the walls out of plane.

The static and seismic behavior of the structure was described with a finite element model using Edilus software (ACCA software). The verifications were conducted in accordance with NTC2018 and, specifically, a linear static analysis was performed, subjecting the structure to a system of horizontal forces parallel to the assumed earthquake directions, distributed (both planimetrically and altimetrically) so as to simulate the dynamic effects of the earthquake. 

Considering the objective of the verifications, i.e., out-of-plane tilting verification of the main facade walls, the interaction between the analyzed portion of the structural block and adjacent structures was neglected. In accordance with NTC2018, the effect of earthquake was added to the combinations resulting from static loads.

In existing masonry buildings, where there is a systematic lack of connecting elements between the walls at the level of the horizons, it is possible to require vulnerability assessment with respect to local mechanisms, which may involve not only out-of-plane collapse of individual wall panels, but larger portions of the building, i.e. tilting verification.

The facades subject to out-of-plane overturning verification are shown in the following plan.

The model used for this type of evaluation is that of the limit equilibrium analysis of masonry structures, which is referred to in §C8.7.1.2 of the 2019 Circular of the 2018 D.M. For the application of the analysis method, a zero tensile strength of the masonry, the absence of creep between blocks and an infinite compressive strength of the masonry were assumed.

The verifications showed deficiencies in resistance against seismic actions, such that it resulted in minimum safety coefficients of about 0.29; this value is compatible with the structural type.

 

POSSIBLE INTERVENTION CRITERIA

The criticalities found, taking into account the monumental value of the building and the impossibility therefore of reaching a widespread intervention of improvement, allow us to outline the main intervention criteria:

• Chaining of the pushing horizons;
• Restoration/reconstruction of the integrity of transverse bracing walls;
• Reconfiguration of new openings/windows in the wall males;
• Increasing the seismic resistance of the walls against out-of-plane tilting mechanisms.

 

BIBLIOGRAPHY

[1] Legge 5 novembre 1971 n. 1086 (G.U. 21 dicembre 1971 n. 321) “Norme per la disciplina elle opere di conglomerato cementizio armato, normale e precompresso ed a struttura metallica”.

[2] Legge 2 febbraio 1974 n. 64 (G.U. 21 marzo 1974 n. 76)“Provvedimenti per le costruzioni con particolari prescrizioni per le zone sismiche”.

[3] Indicazioni progettive per le nuove costruzioni in zone sismiche a cura del Ministero per la Ricerca scientifica – Roma 1981.

[4] M. Infrastrutture Trasporti 17/01/2018 (G.U. 20/02/2018 n. 42 – Suppl. Ord. n. 8)“Aggiornamento delle Norme tecniche per le Costruzioni”.

[5] Circolare 21 gennaio 2019, n. 7 C.S.LL.PP. (G.U. Serie Generale n. 35 del 11/02/2019 – Suppl. Ord. n. 5) Istruzioni per l’applicazione dell’«Aggiornamento delle “Norme tecniche per le costruzioni”» di cui al decreto ministeriale 17 gennaio 2018.

[6] Eurocodice 6 – “Progettazione delle strutture di muratura” – EN 1996-1-1.

[7] CNR-DT 206 R1/2018 – “Istruzioni per la Progettazione, l’Esecuzione ed il Controllo delle Strutture in Legno”.

[8] Eurocodice 5 – “Progettazione delle strutture di legno” – EN 1995-1-1.

[9] CNR-DT 215/2018 “Istruzioni per la Progettazione, l’Esecuzione ed il Controllo di Interventi di Consolidamento Statico mediante l’utilizzo di Compositi Fibrorinforzati a matrice inorganica”.

[10] Linea Guida C.S.LL.PP. (Servizio Tecnico Centrale) “Linea Guida per la identificazione, la qualificazione ed il controllo di accettazione di compositi fibrorinforzati a matrice inorganica (FRCM) da utilizzarsi per il consolidamento strutturale di costruzioni esistenti”

[11] Linea Guida C.S.LL.PP. (Servizio Tecnico Centrale) “Linea Guida per la progettazione, l’esecuzione e la manutenzione di interventi di consolidamento strutturale mediante l’utilizzo di sistemi di rinforzo FRCM”.

 

For laser-scanner survey technology: https://www.faro.com/

For drone survey technology: https://www.dji-store.it/

For the software: https://www.acca.it/software-calcolo-strutturale