Non-destructive testing and Finite Element Method integrated procedure for heritage diagnosis: The Seville Cathedral case study

Highlights

• Extensive non-destructive testing and FEM software combined methodology was applied.

• Complex structural features of a significant heritage example were diagnosed.

• Results identify the settlement as the main cause of multiple site damages.

• This research established key action lines for similar heritage interventions.

Abstract

One of the major problems faced by historic cities today involves the conservation of heritage buildings. Damage suffered by these buildings can be irreversible and fast-acting, leading to their disappearance over a short period. The study and analysis of the origin of the damage suffered by these buildings have proved themselves to be key to their conservation. Non-destructive testing (NDT) can detect problems indiscernible to the naked eye, thereby preventing potential losses. In this paper, a non-invasive method for the diagnosis of building structures integrated with the Finite Element Method (FEM) was applied to the Tabernacle Chapel; a building included in the northwest wing of the Cathedral of Seville complex. Despite the many interventions carried out to date, the issue of the chapel's deterioration has yet to be entirely solved. This research describes the results of a detailed constructive and structural diagnosis methodology for heritage buildings. The data provided from NDT methods, such as Digital Image Processing (DIP), Infrared Thermography (IRT), Laser Levelling (LL), Ambient Vibration Testing (AVT), and Ground-Penetrating Radar (GPR), has been verified and integrated as boundary conditions in a 3D Finite Element Method (FEM) in order to establish the critical points of the structure, including the failure mechanisms. The results led to the conclusion that the main causes of deterioration involved the settling of the grandstand built in the northwest sector of the building and the effects of the thrusts of the dome on the lateral façades. An integrated implementation methodology of NDT and FEM has enabled accurate knowledge to be ascertained of the principal damage affecting this heritage building.

Introduction

Architectural heritage is exposed to a myriad of hazards that affect its state of conservation. Countless generations over the centuries have felt themselves responsible for the preservation of their inherited legacy, and have defined valid paradigms and operational procedures [1]. Preventive conservation is of major importance today, since it is considered one of the main disciplines in the preservation of architectural heritage [2]. Conceived from a transversal and inter-disciplinary perspective, this concept strives for the development of a set of systematised and periodical activities for the promotion of the correct visibility, understanding, and maintenance of the analysed heritage building [3]. All of this materialises into a set of preventive actions, based on monitoring and follow-up maintenance tasks, aimed at minimising future deterioration [4,5].

By bearing the impact on the materials of the building in mind, the methods applied in the analysis and diagnosis of buildings structures may be classified into three types: destructive, semi-destructive, and non-destructive [6]. The most useful Non-Destructive Testing (NDT) methods, based on the information they provide and their performance, are Digital Image Processing (DIP) [7], Photogrammetry (P) and Stereo photogrammetry (SP) [8,9], Laser levelling (LL), Ambient Vibration Testing (AVT) [10], Infrared Thermography (IRT) [11], and underground mapping analysis by Ground-Penetrating Radar (GPR) [12]. The validation of the techniques and the tests run in the laboratory through computer programs have enabled a huge leap in terms of conservation of architectural heritage in recent decades [13]. These NDTs are, to a certain extent, controversial: they are considered very favourable techniques by many researchers, but unreliable by others [14], and have successfully been applied in ancient religious and heritage buildings with proven results [15]. Previous research shows how a detailed study of cultural heritage structures generally requires the application of various detection techniques, along with historical and technical knowledge [16]. The multidisciplinary synergy of NDT in recent decades has made it possible to identify damage that architects and engineers were previously unable to analyse [17]. The use of NDT to study the damage of the church of San Juan Bautista in Spain led to the detection of the humidity that was the underlying cause of the damage [18]. Chastre et al. [19] state that it is impossible to carry out destructive tests in most historical buildings due to the damage this would imply to the heritage asset. For this reason, it is preferable to choose NDT or other similar methodologies that enable the physical and mechanical characterisation of the structures and materials of heritage buildings. Consequently, the combined application of these techniques provides significant information on the geometry, materials, constructive stages and structural descriptions of historical buildings [20,21].

Previous research, such as the methodology developed by Lourenço et al. [22] in the Monastery of San Jeronimo in Lisbon, combines non-destructive inspection techniques with structural analysis tools. The combination of these two techniques is a major factor in the improvement of the conservation of cultural heritage. Other research, such as that conducted on the dome of the Cathedral of Pisa, carries out a methodological process based on laser scanning and photogrammetric tools to achieve concise and detailed structural models [23].

The strategies for the conservation of architectural heritage require the use of tools that lead to a deeper understanding of the complex behaviour of these structures. As a result, the correct diagnosis of their state of conservation can be achieved, thereby preventing the appearance of damage and providing new solutions [24]. In addition, previous research shows how the use of current tools, such as the Geographical Information System (GIS), has allowed georeferenced data previously collected in heritage buildings to facilitate a study of the dynamic process of damage in heritage buildings [25]. Previously, NDTs have been integrated with other software, such as Building Energy Modelling (BEM), to better characterise the model, and for a greater understanding of the thermodynamics of the building envelope. This integration, however, can also be carried out for the study of damage in heritage architecture [14]. In this respect, computer analysis using the Finite Element Method (FEM) [26,27] enables some of the results obtained with NDT to be verified, and is therefore helpful in the effective identification and structural diagnosis of typologically differentiated heritage case studies [28]. The information provided by the NDTs is employed to calibrate the models when both tools are combined [27,29].

This paper focuses on the application of the aforementioned strategies on the study of a leading example of Spanish architectural heritage: the Tabernacle Chapel of Seville Cathedral. The case study has been chosen not only due to its extensive damage but also for being an example where the application of NDT has enabled full understanding of the origin of the existing pathologies. The data gathered has allowed for the evaluation of the current state of the building, and has increased the possibility of finding the origin of the existing damage.

This research aims to provide a comprehensive approach to studying and understanding the structural and constructive deterioration of heritage buildings through the application of NDT and FEM in a methodological feedback process. Once all the procedures have been applied and the data gathered, the evaluation of the identified pathologies can be performed, and a methodology is provided that can be applied in future interventions on historic buildings. The NDTs employed can complement each other towards improving the volume of data collected in order to be able to compare and have a hypothesis of the damage [17]. Hitherto, there has been very little research that uses both techniques together, except for simply using non-integrated NDT and FEM to value the damage in certain heritage buildings [30]. The outcome of this paper illustrates the potential and validity of the combined and integrated use of NDT and FEM for a better understanding and diagnosis of historical buildings of great complexity and heritage value.