Vulnerability Assessment of Art Collections: The National Archaeological Museum “Gaio Cilnio Mecenate” in Arezzo (Italy)

Abstract

Artworks play a fundamental role in the cultural and economic assets of communities, enhancing their identity and helping with social integration. Despite their importance, they are not always adequately protected against degradation, which can be induced by aging, atmospheric and human-induced occurrences, and catastrophic events. Earthquakes certainly represent one of the main risks for art objects; however, traffic, construction works, and shipment can also represent a threat to art goods. Therefore, the assessment of the vulnerability of art collections to dynamic excitations plays a crucial role in their conservation, and it has been collecting increasing attention from researchers, academics, and museum managers. This work focuses on the vulnerability of the art collections exhibited at the “Gaio Cilnio Mecenate” museum in Arezzo. Namely, it aims to assess the effective dynamic loading experienced by the artworks, which is a function of the dynamic propagation played by the foundation soil, the building, and the displayers used for the exhibition. In this study, the dynamic properties of some of the displayers used for exhibiting the art collections are investigated by performing an experimental survey. The analysis of the experimental data led to the assessment of the proper frequencies of the displayers, which were compared to those of the building and the foundation soil of the museum.

1. Introduction

The conservation of artistic goods within museums and archives is an important issue that is collecting many contributions from the academic, administrative, and scientific worlds [1,2,3]. Indeed, artworks represent a priceless element of the cultural and economic asset of communities, especially in countries whose economy is based on cultural and artistic tourism.

Many different threats can affect art goods, both destructive events, such as earthquakes, floods, and blasts [4,5,6,7,8], and more ordinary conditions, such as humidity, temperature drop, and human-induced vibrations [9,10,11,12].

Art collections exhibited within museums are supposed to be easier to protect compared to the ones placed outside, in public areas, for both the provided security and for better protection from external occurrences, including natural events. Unfortunately, many museums are far from having adequate structural safety, possibly becoming a further threat to the exhibited items [13], especially when they consist of monumental buildings, which hardly respect the current safety standards, despite adding considerable value to the artistic impact of the exhibition.

For art collections exhibited in museums, the dynamic excitations represent a main threat. They can be induced by natural events, such as earthquakes, or by human actions, such as traffic, shipment, concerts, or construction works [14,15]. The vulnerability of artworks to dynamic excitations largely varies depending on their type [4,16,17,18], and their properties [19,20], such as material, shape, dimension, age, and state of conservation.

In these years, several contributions have been devoted to checking the safety of art collections to dynamic excitations [21,22,23,24,25,26] and to finding allowable vibration limits [27,28,29] for assessing their safety. Right now, however, affordable and unanimous values for vibration limits have not been found. Even the technical codes do not provide such information; the effects of vibrations have been checked mostly for ordinary buildings, even if they can be extended to historical construction.

At the current time, therefore, the safety of art goods can be assessed only by (i) determining the amount of dynamic load induced to the expected occurrences and (ii) performing proper analyses focused on the single items. This latter step can be faced with various levels of computational effort, i.e., by applying simplified methods based on form-filled analyses [19,30], or more analytical approaches [31,32,33,34,35].

In all cases, however, the amount of dynamic loading experienced by each item must be determined on the basis of the loading source and the propagation process played by the building and the displayers. Any excitation, indeed, propagates from its source to the items’ location, potentially increasing its intensity, or changing its frequency content in any way.

In Figure 1, the propagation of the dynamic loading from its source to the specific location of the items exhibited within museums has been illustrated. Due to such propagation, the dynamic acceleration experienced by single items is different on each floor. Furthermore, items placed on the same floor can be exposed to different accelerations depending on their positions, directly on the floor or on pedestals/windows. The role of the building in the propagation of dynamic excitations has been studied by several researchers [36], and it has been included in the most recent technical codes, whilst few studies have been devoted to the role of the displayers [29,37].

In this work, the role of propagation in determining the effective dynamic loading affecting art collections has been studied with reference to the National Archaeological Museum “Gaio Cilnio Mecenate” [38] in Arezzo, which is one of the most important permanent exhibitions of archeological artifacts in Tuscany. Special attention has been given to the role of the displayers in propagating dynamic loads, which has been studied through dynamic monitoring using seismic stations placed on the floors where the displayers are located and on the top of the displayers themselves. Furthermore, the paper presents the results of former surveys performed on the dynamic behavior of the building and its foundation soil.

In Section 2, the National Archaeological Museum “Gaio Cilnio Mecenate” is briefly described, with special attention given to the types of displayers adopted for exhibiting the art collections and to the potential dynamic loads they may experience. Section 3 briefly describes the results of former analyses, performed by some of the authors and obtained from other researchers, focusing on the dynamic response of the foundation soil and the building hosting the National Archaeological Museum “Gaio Cilnio Mecenate”. Finally, Section 4 describes the survey conducted on the museum displayers and presents the obtained results.

2. Case Study: The National Museum “Gaio Cilnio Mecenate” in Arezzo

2.1. The Museum and the Collections

The National Archeological Museum “Gaio Cilnio Mecenate” is located in the historical center of Arezzo and occupies part of the old San Bernardo Monastery next to the ancient Roman amphitheater, which dates to the beginning of the II century BC. The Monastery was commissioned by Bernardo de’ Tolomei in 1333 and includes the remains of the amphitheater, which induced the curvy trend to the Monastery facade. Figure 2 shows a photo of the building within the surrounding area, together with the corresponding points cloud provided by a proper laser scanner survey using a Cam/2 Faro Photon unit.

The current layout of the museum dates to 1951 when it re-opened after the restoration (following the destruction during WWII) under the direction of superintendent Guglielmo Maetzke. It became a National Museum in 1973, and it contains about 13,000 items, including several masterpieces (see Figure 3) such as the Cratere di Euphronios (510 BC), a big red figure Athenian vase coming from a rich anonymous grave, or a medallion made through chrysography, made of gold and silver.

Most of the exhibited items refer to the Arezzo area, tracing their origins back to the Etruscan period and encompassing thematic collections such as bronze and jewels.

The displayers used by the museum, shown in Figure 4, can be distinguished in windows, pedestals, and a combination of these two types, i.e., consisting of a window put on a pedestal or another type of support. In this last case, the dynamic behavior of the displayer must be studied as a combination of two different systems (pedestal and window), each with distinct geometry and dynamic properties.

Another important distinction regards the position of the displayer within the room. Indeed, the position of the displayers just up against the wall reduces their overturning attitude. Furthermore, in this latter case, the artifact or the window can be fixed to the wall, with a consequent advantage in its seismic safety.

The glass windows present various geometrical features: most of them consist of glass windows with a steel structure, but there are even displayers with masonry bases and wooden structures.

2.2. The Dynamic Sources of Vibrations

The exhibited collections are exposed to various possible vibrations due to the ordinary working conditions of the museum and to “extraordinary” occurrences.

The first case includes vibrations occurring in the current use of the collections, such as the potential impacts from visitors, the change in location of each item due to cleaning procedures, and changes in the exhibition layout or transfer to other museums. Even the vibrations produced by regular vehicle traffic and human activities in the nearby area, such as construction/demolition works, belong to this group. The second case includes earthquakes, landslides, and explosions.

Regarding the assumed case study, the museum is located in the center of Arezzo, therefore it is very close to all the above-mentioned vibrations sources (see Figure 5). There are vehicular paths next to it, as well as buildings where construction works could potentially be made. The train rail passes through at about 250 m from the museum. It should be mentioned that, occasionally, the amphitheater next to the museum hosts musical events, which are usually very crowded and loud.

As regards the extraordinary loading, explosions do not seem likely to occur, since the museum is equipped with a security service, and the exhibited collections are not easily accessible. Even the landslides do not represent an effective risk, due to the flat position of the area and the consolidated soil. The only source of high-intensity vibration, therefore, consists of seismic events. Arezzo is classified as subject to “low seismic hazard” according to the current Italian Technical Code NTC 2018 [39], with a Peak Ground Acceleration (PGA) equal to 0.160 g for a Return Period (RP) equal to 475 years.

Despite such low seismic hazard, Arezzo experienced various earthquakes having relevant Mercalli intensities. The Mercalli scale measures seismic intensity in terms of damage suffered by the buildings, and the constructions are not designed to resist seismic actions. Therefore, seismic activity remains the most dangerous dynamic source of the museum.

3. Former Analyses Performed on the Dynamic Properties of the Museum

3.1. The Foundation Soil

The dynamic properties of the soil were checked by Moscatelli et al. in 2014 [40], based on a former investigation on the foundation soil. In 1989, a Multichannel Analysis of Surface Waves (MASW) was performed, which provided the soil stratigraphy shown in Figure 6. The shear wave velocity of the uppermost 27 m (v_s,27), i.e., the layer over the bedrock, resulted to be equal to 335 m/s. Therefore, according to the NTC 2018 classification, the soil has been classified as C-class, whose shear wave velocity varies between 180 m/s and 360 m/s.

More recently, in 2010, further investigations have been performed [41] on the most superficial layers of the soil, which confirmed such classification.

Given the soil stratigraphy, the elastic spectra at the soil surface for various RPs have been found by performing a Soil Response Analysis on a set of 7 spectrum-compatible ground motions of the bedrock of the area. Figure 7 shows the obtained mean spectra, which are compared to those at the bedrock and the elastic spectrum provided by NTC 2018; further information can be found in [40]. As can be observed, the peak of acceleration occurs for a fundamental period of around 0.32 s, i.e., a frequency of around 3 Hz. Similar results have been provided by the survey made on the same site [42] based on the lecture on single-station recordings of the horizontal-to-vertical spectral ratio (HVSR) for ambient vibrations.

3.2. The Building

The dynamic properties of the building were checked by performing dynamic monitoring, i.e., by placing four seismic stations at the different levels of the structure. The experimental measurements were performed between 19 December 2019 and 29 September 2020.

The dynamic loading consisted of human activity and micro-seismic accelerations; it is worth noting that the dynamic monitoring considered both a “regular” period with normal vehicular traffic and the regular service of the museum, and a “quite-forced” period when all the activities, including museum opening, stopped due to the epidemic emergency in Italy. In this last period, the dynamic loading consisted of micro-seismic activity only.

The seismic stations consisted of three-axial seismometers (velocity transducers, with an eigenfrequency of 2 Hz and a useful band between 0.1 and 40 Hz) coupled to Digital Acquisition Systems (DAS) SL06 (A/D converter 24 bit), produced by SARA Electronic Instruments. The horizontal components of the seismic stations were oriented, respectively, along the North–South and East–West axes, corresponding to the transverse and longitudinal axes of the building. They were connected using a GPS for time synchronization. Figure 8 shows the position of each seismic station; the reference seismic station is placed in the hypogeum ambulacrum of the museum. The position of the higher seismic station, placed just under the roof, has been shown in the plan of level 2. The stations are indicated by a triangular symbol, whose colors correspond to a different level.

The recorded signals were arranged in order to obtain the average hourly spectra. Figure 9 shows the average spectra along the three monitored directions. For the sake of legibility, horizontal and vertical spectra are represented by different scales. As can be observed, along the two horizontal directions, the dynamic response of the building shows two peaks very close to each other, respectively equal to 3.75 Hz along the longitudinal (X) direction and 3.92 Hz along the transverse (Y) direction. It is worth noting that the main vibrational axes cannot be clearly identified, since the building has a very complex geometry, and the seismic stations are not vertically aligned.

The dynamic response of the building has been an object of further investigation: five seismic stations were placed at each of the main levels (L1, L2) to check the torsional effects related to its dynamic response. The free vibrations of each story were recorded for 40 min, which is a suitable time. On the basis of the lectures, the dynamic response of the structure has been assessed and compared to the former results [43]. The test was performed using seismic stations of the same type as those in the previous survey. Figure 10 shows the position of the seismic stations used in this investigation, while Figure 11 shows the obtained average spectra. It should be noted that, for the sake of legibility, different representation scales were used for the two levels.

The obtained average spectra are consistent with those previously found in the former analysis. All the peaks along the Y-direction occur in the range of frequencies between 3.65 and 3.85 Hz, while those along the X-direction are around 4.05 Hz. For each level and direction, the scatter between the five spectra around the peak of frequency expresses the sensitivity of the building to torsional effects. As can be seen, the differences between the lectures of the same seismic stations are greater at the second level.

4. The Dynamic Experimental Campaign

4.1. The Performed Experimental Campaign

The experimental campaign consists of monitoring the windows through three-axial seismometers respectively placed at the base and the top of the windows. The seismometers had a proper frequency of 4.5 Hz and were coupled with 24-bit Digital Acquisition Systems (DAS) SL06. The two seismometers were synchronized using a GPS connection. In all cases, the Y-direction was assumed to be along the shorter side of the window, as shown in Figure 12.

The data were recorded at 100 sps and arranged before processing to correct the sensor’s instrumental curve. The recordings were performed during the operational time of the museum, i.e., with visitors potentially walking around the windows.

The results are expressed in terms of peak particle velocity (PPV) and peak component particle velocity (PCPV), which are the quantities adopted in the main international technical codes [44,45,46,47,48] for checking the vibrational behavior of buildings. Unfortunately, such codes provide reference thresholds only for constructions, whilst the acceptable conditions for artifacts are not codified yet. PPV and PCPV are found according to the expressions reported below.

Peak particle velocity (PPV), defined as the maximum value of the modulus of the velocity vector in a three-dimensional space:

$$PPV=max\left(\sqrt{X^2+Y^2+Z^2}\right)$$

(1)

Peak component particle velocity (PCPV), defined as the maximum value of the modulus of one of the orthogonal components measured simultaneously:

$$PCPV=max(\left|Z\right|,\left|X\right|,|Y\left|\right)$$

(2)

where X, Y, and Z are the three spatial components of the velocity vector.

Before proceedings to determine the response parameters, the proper frequencies of the displayers were found. To this purpose, the FFT of the signal were determined according to the procedure described in Azzara et al. [49]. The mean FFT was found by averaging the single values found for time intervals of 120 s; in this way, each result is described through mean, standard deviation, and coefficient of variation.

4.2. The Checked Displayers

The dynamic monitoring involves 13 displayers, shown in Figure 13, that differ from each other in geometry (and slenderness), dimensions, material, and position within the building.

To better understand their dynamic properties, they have been organized into four groups, with relative homogeneity of features.

The first considered group includes four windows made of glass and metal, with a rectangular plan. They have one plan dimension much larger than the other one, and they are positioned in various locations with respect to the building’s walls. The second group consists of four glass displayers with a squared plan and rigid pedestals of various heights. The third group consists of three glass displayers, whose pedestals are made of masonry and connected to the structure. Finally, the fourth group includes two glass displayers supported by metal supports. In Figure 13 the inspected displayers are shown together with their position within the building and the assumed classification.

4.3. Results

The first response quantities checked in the analysis are the mean spectra. Figure 14, Figure 15, Figure 16 and Figure 17 show the mean spectra obtained for each monitored displayer along the three directions.

In each figure, two spectra are shown, respectively representing the signal at the base of the displayer (blue line) and the top (black line). The diagrams have been grouped by type to better evidence the similitudes between the samples. For the sake of legibility, the representation scale was not unified, since the values of the spectral amplitude vary greatly. For each displayer, however, the same representation scale was adopted for both the horizontal directions. The amplitude along the vertical direction is about one order of magnitude lower than the horizontal ones.

The type 1 displayers (see Figure 14) evidence different dynamic responses along the two horizontal directions, since the two lengths of the plan are very different from each other and, consequently, result in different slenderness. As regards the dynamic response along the main direction (Y), all the observed displayers evidence a proper frequency close to 5 Hz. The amplitude peaks experienced by the four displayers along the Y direction range between 0.18 mm/s/Hz (T1-A) and 0.06 (T1-C). It should be noted that the sample T1-A has no constraints along the Y-direction, and it is located on level 2, whilst the displayer T1-C is placed on level 1.

Figure 15 shows the mean spectra of the type 2 displayers, having a squared plan. In this case, the amplitude peaks are quite similar along the two horizontal directions. The properties that most affect the dynamic response of these samples are the behavior of the support floor and the technology of the displayer. The dynamic behavior of the floor is related to its level within the structure. Moreover, the room where the displayer T2-B is located has a floating floor, i.e., an additional wooden floor placed over the structural one. For this reason, such floors are much more flexible than the other ones, and the displayer placed in such a room evidences a higher dynamic response than all the others. As regards the technology of the displayers, as can be noticed in Figure 13, the windows of the samples T2-A and T2-C are made of glass only, whilst in the others (T2-B, T2-D) the window glasses are framed by a metal structure, which stiffens the displayers, reducing their spectral amplitude.

Figure 16 shows the mean spectra of the displayers belonging to type 3, which are the stiffer ones considered. Indeed, the pedestals are made of masonry, and they are fixed to the walls. The three displayers are on the same level (level 2) and in the same room, despite having different orientations. As can be noticed, they evidence spectra very similar to the ones of the building, especially along the longitudinal and vertical directions. Some differences can be found in the transversal response of displayers, i.e., the X-direction of the T3-A and the Y-direction of the T3-B and T3-C due to the imperfect fixing to the adjacent wall.

Finally, Figure 17 shows the mean spectra of type 4 displayers. They are the most flexible ones, due to the slender legs supporting the windows, and they are not fixed to the walls; their amplitudes exceed 15 mm/s/Hz even though both of them are placed at the 1st level. As should be expected, they have a similar amplitude in the two horizontal directions; the frequency of T4-A is different along the two directions since the support has a stiffener along one direction only.

The response quantities PPV and PCPV have been found for each monitored displayer by considering the maximum value of the velocity vector at each 5 s time interval, according to the functions (1) and (2) reported in Section 4.1. Figure 18 shows the peaks of PPV (Figure 18a) and PCPV (Figure 18b), respectively, for the checked displayers. In the figures, the values provided by the analysis are shown together with a threshold provided by international codes. As mentioned in Section 4.1, there are no limit values for artifacts; the reference thresholds, therefore, refer to historical buildings, in case of persistent/permanent excitations. Such values should be considered as a general order of magnitude reference, without any purpose of acceptance assessment.

The PPV peak values experienced by the displayers vary in a range between 0.05 (T3-A) and 9.84 (T2-B). It should be noticed that the displayer T2-B, which evidences the larger values of PPV and PCPV, was placed over the floating wooden floor, which clearly amplifies its dynamic response. Even the displayer T2-A achieves similar values of the response quantities. It proves to be the most sensitive to vibrations among those considered, due to its position (second level) and its technology: the upper glass window was not supported by any metal structure.

The velocity peaks experienced by most of the displayers are lower than the limits provided by the codes for the building’s roofs. However, three of them exceed the thresholds, resulting in values more than three times larger than the building’s acceptance limit. It is worth noting that the monitoring was made during the normal working time of the museum; it is possible, therefore, that the perturbance constituting the dynamic loading was not the same for all the checked displayers, and that the museum could be more crowded, potentially resulting in an increase in the dynamic excitation.

4.4. Assessment of the Dynamic Property of the Exposition System

As a result of the conducted survey, the amount of amplification performed by the displayers has been assessed. Figure 19 shows, as a function of the system’s frequency, the spectral ratio between the amplification at the top of the displayers and the one at their base. Only the two horizontal directions have been shown, as they are the most meaningful in the current analysis. The range of the building’s proper frequencies in the two directions has been highlighted in grey to better understand the relationship between the dynamic properties of the building and displayers. The building’s range indicated in the figure refers to both the X- and Y-directions, since the axes considered for each displayer could not coincide with the ones of the building.

The larger spectral amplifications occur at the proper frequency of the displayers. As can be noticed, the maximum amplifications of the displayers do not occur at the same frequency as the building’s response, except for the displayers whose masonry pedestal is fixed to the walls (Type 3). In this case, the displayers exhibit amplifications at different frequencies: the first peak occurs at the proper frequency of the pedestal (which is the same as the building), whilst the others correspond to the frequency of the upper window.

The values of amplification vary significantly depending on the displayer type. The larger amplifications (almost 350) affect the T2 displayers, whilst the T3 displayers evidence lower amplification (below 20).

Figure 20 shows the propagation flow, already proposed in Figure 1b, together with the values found for the proper frequencies along the horizontal directions of each system involved in the propagation. The ranges of values of the displayers include both the X- and Y-directions, and they refer to a standard deviation equal to 90% of the frequency distribution. Readings from foundation soil, the building and the displayers result in different frequencies, with the consequent absence of resonance phenomena.

5. Conclusions

In this work, an experimental survey was conducted on some displayers used for exhibiting archeological items at the National Museum “Gaio Cilnio Mecenate” in Arezzo.

Four types of displayers were selected, differing from each other in geometry, slenderness, technology, and placement within the museum. The survey consisted of monitoring the dynamic response of the displayers through the proper placement of 3-D seismic stations. The arrangement of the recorded data led to determining the mean spectra of the displayers along the three monitored directions, together with their proper frequencies and amplitudes.

The investigation provided significantly different mean spectra for the examined systems, with spectral amplitudes ranging from 0.005 mm/s/Hz for the stiffer displayer (T3-A) to almost 0.20 for the slender ones (T1-A, T2-A, T4-A, T4-B). By comparing the dynamic spectra at the base and the top of each displayer, the propagation of each of them along the three directions was determined for each frequency. Even this response parameter was largely affected by the properties of the displayers and their location.

The frequencies found for each displayer were compared to the ones of the building and the foundation soil, provided by previous analyses. Such comparison revealed that there is not any resonance threat, since the three propagation systems have different frequency content.

Finally, the PPV and PCPV have been determined for the monitored displayers and compared to the thresholds provided by the international codes for historical buildings. Some of the displayers presented high values of PPV and PCPV, suggesting a potential vulnerability to dynamic loading.

Since the monitoring was made during the normal working time of the museum, it is possible that the perturbance constituting the dynamic loading could be larger at the occurrence of special events hosted by the museum, as well as the high touristic season. Therefore, further investigations should be carried out to check the displayers which evidenced higher propagations in a wider range of operational situations of the museum.