Seismic Vulnerability of Catalonian Romanesque Churches
The main aim of this thesis, by the author Ellen Tatiana Key, is to assess the seismic vulnerability of Catalonian Romanesque churches, specifically the churches of the monasteries of Santa Maria de Poblet and Sant Miquel de Cruïlles.
Historical buildings are essential to a region’s history and identity. Though they may have been constructed centuries ago, they are often still frequently-used structures in today’s society and symbolize the pride of a community in the artistic and structural accomplishments of their forebears. For these reasons, their conservation should be a priority.
This thesis will focus on those of the Romanesque period, which took place in the 11th to 12th centuries. Catalonia, a region in the northeast of Spain, accepted and implemented the Romanesque style of art and architecture, which is characterized by solid and sober lines. Many monasteries and churches from that period still survive today. Although Catalonia is a region of low seismicity, it is interesting to examine the seismic vulnerability of Romanesque churches of the area.
The objective of this paper is to implement and evaluate two known methods for analyzing historical masonry buildings, specifically in the area of seismic vulnerability:
- The vulnerability index method, which considers the vulnerabilities and anti-seismic measures present within a structure and determines its level of vulnerability.
- The kinematic limit analysis method, which determines the vulnerability of individual structural elements in relation to a given failure mechanism.
The vulnerability index method
Vulnerability is defined as the degree of damage to one or more elements due to a certain hazard; thus, the seismic vulnerability of a building is a measure of the damage the building will likely experience due to ground motion of a certain intensity.
The vulnerability analysis method under study is based on Lagomarsino & Podestà (2004b) and the Italian guidelines for evaluation and mitigation of seismic risk to cultural heritage (G.U. no. 47, 2011) and was developed to quickly analyze a multitude of structures with a checklist-based approach.
This thesis deals with the church typology, which the guidelines acknowledge to be a wildly diverse group of structures. The main distinguishing feature about this typology is the lack of intermediate diaphragms in halls that are typically large.
The vulnerability index method is a widely popular method for quick observation of vulnerability and damage; variations have been implemented by a multitude of authors. The method has some flexibility: not only can it be applied to countries other than Italy, but also when studying a large population of churches after a disaster, the vulnerability and damage indices can be derived for the entire population of churches, or for only the churches that experienced a certain seismic intensity, or for all examples of a certain macroelement. It has been observed that this procedure can be used to calculate the damage resulting from any earthquake intensity, therefore this procedure can be used as a predictive tool for future events rather than as a retroactive observational analysis.
The vulnerability analysis method does not aim to achieve an extremely accurate picture of structural failure. Rather, it uses simple indicators that have been observed in many structures to make predictions about the church under study. It aims to provide an idea of the level of vulnerability after just a geometric survey of the building.
There are one or more collapse mechanisms associated to each macroelement; for eleven macroelements, there are twenty-eight collapse mechanisms (Table 2.4.1).
For example, the three mechanisms for the facade (overturning, damage at the top of facade or gable and shear) are shown in Figure 2.4.1.
The kinematic limit analysis method
The kinematic limit analysis method is a way to analyze specific macroelements of a structure, which may be its most vulnerable components. This way, the structure’s susceptibility to local failures can be characterized.
After calculating the capacities of all possible failure mechanisms for a macroelement, the governing collapse mechanism is the one for which the activation acceleration is the smallest.
The method essentially involves a calculation of the seismic capacity of one macroelement according to a given failure mechanism, i.e. the capacity of the facade in relation to out-of-plane overturning.
The capacity curve is obtained in terms of the relation between the base shear of the structure and the displacement of a chosen control point. Then, this capacity curve can be evaluated against the seismic demand, represented by the appropriate response spectrum. The goal is to determine the ground acceleration necessary to activate each mechanism and assess the level of performance achieved by way of finding a performance point.
Catalonia is a region in the northeast of Spain, bordering France and Andorra. Geographically, it reaches the Pyrenees Mountains in the north and the Mediterranean Sea on its eastern and southeastern coast. Its capital is Barcelona. When the County of Barcelona rose in power in the 11th century, the region of Catalonia gained prominence.
The 11th century in Catalonia, the beginning of the Romanesque era, was an age of prosperity when noble families were organizing a feudal society. Christians were gradually conquering New Catalonia from the Muslims until the mid-12th century, and so the Romanesque style slowly spread from the north to the south. This explains the prevalence of Romanesque buildings in the northern regions.
Lasting into the 13th century in Catalonia, Romanesque architecture consisted of solid and sober lines, with churches mainly made of massive masonry walls upholding a barrel vault, semi-circular apses with arcatures and lombard bands which look like pilasters (Figure 2.6.2).
There are almost 2,000 Romanesque churches in Catalonia.
Seismicity of Catalonia
Catalonia is a land of low seismicity, reaching basic seismic acceleration values of less than 0.12g per the Spanish seismic code NCSE-02. As can be seen in Figure 2.6.3, the intensity values on the MSK scale range from V to VIII in the region, for an earthquake with a return period of 500 years.
The most damaging earthquakes recorded in Catalonia occurred in the 14th and 15th centuries. Only those earthquakes that were large enough to cause recordable damage are marked in Figure 2.6.4.
Monastery of Santa Maria de Poblet
The first case study considered in this report is the church of the well-preserved Cistercian monastery of Santa Maria de Poblet, referred to as Poblet from here on. This monastery boasts a grand complex, which had many uses throughout history for the military and nobles, as a royal palace, residence and pantheon hosting the tombs of several Aragon royals. Today, it is a UNESCO World Heritage Site.
The monastery of Santa Maria de Poblet is located in the region of Conca de Barberà, province of Tarragona, town of Vimbodí i Poblet. Figure 2.7.1 pinpoints its location east of Barcelona and north of Tarragona. It lies in a rural area at the feet of the Prades mountains, near the river Sec. The soil is considered a soft deposit 5 to 20 meters deep, in contact with hard rock.
The first documented reference to Poblet monastery is in 1150. Construction of the church began in 1170 with the apse, which was completed in 1196. In the 13th century construction continued on the rest of the buildings: sacristy, capitular roof, monk dormitories, cloister.
Geometry and structure
The church of the monastery of Poblet is part of a large, beautiful monastic complex. This report will only discuss the geometry of the church.
It is a large three-nave church of basilical plan. The church is made of limestone, probably of three-leaf construction with an internal rubble leaf. The original mortars are lime with gypsum and calcareous aggregates.
Present condition and damage state
In the church of Poblet, deformation and some cracks were observed. Cracks have been reported since the 16th century when they were attributed to a fire in the church.
There is also a crack in the center of the main nave expected to be from an earthquake in the 18th century, plus another fire that afflicted the monastery in 1822. Soil settlement could also be an issue, causing some cracking. Cracks in the bay farthest from the transept are related to the separation or overturning of the façade.
Deformation can be seen in the main nave. First, out-of-plane bulging is visible in the bay second from the transept. The entire vault is actually deflecting downwards by as much as 50 cm, as can be seen in Figure 2.7.13.
Deformation was also noticed in one of the vaults of the north aisle, in which the ribs appear to be twisting. Sabouret cracks are also visible at this vault, where the vault is separating from the wall above the window.
There are also many thin cracks in the chapels surrounding the apse. Figure 2.7.17 shows one example of cracking in a chapel wall, which seems to have even led to the spalling of a thin layer of the stone.
Monastery of Sant Miquel de Cruïlles
Not much remains today of the monastery of Sant Miquel de Cruïlles. This Benedictine monastery was much smaller and influence than Poblet, but home to pieces of beautiful Romanesque art, including wall paintings, of which fragments still remain. Today, the church can only be visited by appointment.
In the region of Baix Empordà, this monastery is located just 1 kilometer from the village of Cruïlles, on the road from Bisbal d’Empordà to Cacà de la Selva. It is in a quiet rural area near the river Daró. It is a region of high relative humidity. The soil type in the region is soft rock or stiff soil with a depth less than 100 m. Figure 2.8.1 shows a map of the monastery’s location.
The age of the monastery is not precisely known, so evidence of its establishment must be searched for in historical texts. The church of Sant Miquel de Cruïlles was consecrated in 904. The building itself dates to the end of the 11th century; it was built on top of another older temple.
Geometry and structure
The church was originally part of a monastic complex, of which almost nothing else remains today. One wall with arches has been preserved next to the sacristy; it used to belong to the capitular room. The monastery spread from the south of the church and was probably centralized around a cloister. As mentioned, the church was likely planned as a single-nave church, but was eventually built with lateral naves.
It is easy to see from Figure 2.8.2 that there were once four bays in the nave, but today there are only three because the 16th-century façade was created by filling in the arch between the first set of columns. This arch can be seen by observing the façade (Figure 2.8.3(a)).
Present condition and damage state
The vaults and many of the side walls and dividing walls are covered with plaster, which makes damage inspection difficult. In addition, the irregularity of the masonry makes it hard to distinguish cracks in some cases.
In general, cracks are seen in the apse and chapels, with some cracks in the transept and drum also.
The apse vault has severe radial cracking (Figure 2.8.10(a)). In the walls of the north side chapel can be seen diagonal cracks, appearing to indicate the formation of an overturning mechanism (Figure 2.8.10(b)).
In the south side chapel, the cracks are vertical and less apparent. In the north transept, there are some diagonal cracks that may indicate possible activation of an overturning mechanism in the transept façade (Figure 2.8.11).
A few columns inside the church show vertical cracks, which could be indicative of compressive stresses that are too high. Crack maps for the interior of the building are provided in Figure 2.8.12.
The north column embedded in the façade was built with some very granular sandstones that are now deteriorating in the moist air. Compression cracks in the column have become rather severe, and at the base, the column has lost a sizable amount of its section (Figure 2.8.15(a) and (b)).
The humidity is now monitored and regulated in the church in order to limit the amount of water absorbed by the stone. Deterioration of the stone is also visible in a few other places, including just outside the main door of the church (Figure 2.8.15(c)).
There is also some slight deformation visible in the arches of the nave (Figure 2.8.16).
This report first presented the vulnerability index analysis of the churches of Sant Miquel de Cruïlles and Santa Maria de Poblet.
- For Cruïlles, the damage index was higher, but the vulnerability index lower in comparison to Poblet.
- The acceleration required to reach the limit states was also higher for Cruïlles.
- The vulnerability indices both fell into the range found in (De Matteis, Brando, Corlito, et al., 2019), so the churches can be considered to be of comparable vulnerability to most Italian churches.
- Both Poblet and Cruïlles are expected to reach the damage limit states for the code-specified reference ground accelerations. For an earthquake of 475-year return period, Poblet would be expected to reach damage level µD = 1.9 and Cruïlles would reach level µD = 2.4.
- The kinematic limit analysis method found that damage will be experienced, of level “moderate” to “extensive”, in almost all elements for both churches.
- For Poblet, the elements with the highest capacity with respect to overturning are the gables of the façades and the apse case 4 (no infill and a transverse arch stress distribution). The weakest element is the transept façade (both cases 1 and 2), followed by the main façade and bell tower. Most of the elements have an activation acceleration higher than the reference ground acceleration.
- For Cruïlles, the elements with the highest capacity are the gable of the main façade, bell tower, main apse and lateral apse. The rest of the elements may be vulnerable to overturning given the reference ground acceleration.
So, for local failures assessed by the kinematic limit analysis method, Cruïlles seems to be more at-risk than Poblet, since most of its elements have capacities below the reference ground acceleration.
Generally, not many anti-seismic measures are present in Romanesque churches, making them vulnerable.
- In Cruïlles, only 5 out of the 20 mechanisms applicable to the church had a non-zero anti-seismic score.
- In Poblet, just 1 of the 22.
From the studies presented here, a few observations can be made.
- The vulnerability level may be influenced by elements that appear to be very vulnerable (e.g. gable belfry of Poblet). When the kinematic limit analysis is conducted, the macroelements may not prove to be as weak as expected. The vulnerability method is a more judgment-based qualitative approach, sensitive to the opinion of the analyst, whereas the kinematic limit analysis takes in the geometry to quantitatively assess the capacity of the structure. If a reliable geometrical survey cannot be obtained, this analysis could be problematic.
- For Cruïlles, the damage index was higher, but the vulnerability index lower in comparison to Poblet. The acceleration required to reach the limit states was also higher for Cruïlles. This is surprising considering the current damage state of the apse; it seems intuitive that a very small acceleration would be required to cause further damage. However, the vulnerability index method does not adequately describe local behavior like the kinematic limit analysis does. Plus, neither of the methods offers a way to reduce capacity due to existing damage because that would be too complex to quantify.
- The kinematic limit analysis method rated the level of damage expected in each element of the church rather than in the church as a whole, like the vulnerability index method. With only results from the vulnerability index method, an engineer might add some anti-seismic measures to the main nave for global stability and neglect a transept façade that is very weak. The kinematic limit analysis method gives a better understanding of where interventions may be necessary to improve the local and then global behavior.
- The vulnerability index method is certainly much faster than the kinematic limit analysis and has proven to be useful throughout years of Italian earthquake surveys. It has a set procedure and is performed the same way with every church. The kinematic method is more variable, tailored to each church’s geometry and architectonic configuration. There is no clear end to the process; at some point, the mechanisms become significantly unfeasible and the engineer must assume that she has likely addressed the most dangerous failure mechanisms of the structure.
The vulnerability index method is derived from seismic surveys designed to analyze churches on a statistical basis rather than an individual one, although it can offer information about individual churches too. It would be interesting to implement the method on more Romanesque churches in Catalonia, to see if patterns can be drawn between typologies or similarities can be found in the macroelements and their behavior.
Not many studies combine and compare the vulnerability index method with kinematic limit analysis. It would be interesting to perform kinematic limit analysis on churches that have already been studied in post-earthquake surveys with the vulnerability index method.
The recently-proposed method by (Lagomarsino et al., 2019) will presumably be tested in the next few earthquakes and it will be interesting to see how it fares. Hopefully, its results will continue to prove more accurate than the current method and representative of a wider array of churches. It should soon be applied to the vulnerability index calculation as well, and it will be interesting to note whether it makes different predictions about the vulnerability of each church.
It’s clear that the field of seismic analysis is perpetually evolving; and improving simplified methods for expedited analysis in order to safeguard as many valuable heritage structures as possible should continue to be a priority in this field.