Seismic vulnerability of monumental buildings in Switzerland · 2014. 5. 20. · Most of the major...

Seismic vulnerability of monumental buildings in Switzerland

M. Devaux & P. Lestuzzi Applied Computing and Mechanics Laboratory, Department of Civil Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland

Abstract

Switzerland is a country rich in monumental buildings of national and international importance. Amongst them, there are treasures like Romanesque churches (Romainmôtier) and Gothic cathedrals (cathedral of Lausanne, Baroque edifices (abbey of Einsiedeln). Even though Switzerland is in a region with moderate seismicity, many of these buildings have experienced seismic events and have been moderately, or even seriously damaged. For instance, the city of Basel was almost completely destroyed in 1356 following the most violent earthquake (according to historical sources) that occurred in Central Europe. Although the seismic safety of common buildings is well defined through modern building codes, the seismic vulnerability of monumental edifices has been only partially dealt with in Switzerland. In order to fill this gap, a national research program was initiated few years ago, whose main purpose is to develop a methodology that allows us to assess the seismic vulnerability of monumental edifices. Besides addressing topics such as seismic hazard in Switzerland, this paper deals with the structural behaviour of an edifice through simplified calculation methods. The cathedral of Sion has been chosen since it is situated in Valais, which is the zone with the higher seismic hazard in Switzerland. Keywords: structural behaviour, seismic vulnerability, churches, Switzerland.

1 Introduction

Switzerland is not only rich in monumental buildings of great importance, but also in an interesting diversity of architectural styles and types. Every artistic

© 2005 WIT Press WIT Transactions on The Built Environment, Vol 83, www.witpress.com, ISSN 1743-3509 (on-line)

Structural Studies, Repairs and Maintenance of Heritage Architecture IX 215

current, from the Romanesque style to the Neo-gothic, is well represented through edifices of diverse importance erected throughout the whole country. As the Swiss ground sometimes shakes, the national heritage might be slightly or seriously damaged. One of the main purposes of the research carried out is to develop a methodology that allows us to assess the seismic vulnerability of Swiss cultural heritage. It was decided to base the research on the analysis of one edifice whose results will be extended to other Swiss edifices. The cathedral of Sion was chosen to be focused on, because it is a basilica, which is the major kind of configuration of Swiss churches, and also because it is situated in Valais. In the first stages of the research, the seismic vulnerability of the cathedral was assessed through simplified methods of calculation. This paper presents the results obtained up to this stage.

2 Monumental buildings in Switzerland

After being under Roman domination, Switzerland was quickly Christianised. The most ancient building which dates from the beginning of the 5th century and which is still standing is the baptistery of Riva San Vitale in the Tessin (Figure 1).

Legend:

Tessin (canton) Graubünden (canton)Valais (canton)

Figure 1: General map of Switzerland.

During the Carolingian era (8th-9th C.), religious architecture was characterized by a new growth. The church in Müstair, is one of the few buildings still remaining from this period. About the end of the first millennium, the Swiss area was mainly under the domination of the Kingdom of Burgundy in the western part, of the duchy of Souabe in the east and Milan reigned over the southern part [3]. From the beginning until the end of the Romanesque period, several cathedrals and abbatials built at the beginning of the Middle Ages were removed and new edifices were rebuilt. The cathedral of Basel, which included Italian, Burgundian and French influences, is the greatest monumental building dating from the end of the Romanesque period.


216 Structural Studies, Repairs and Maintenance of Heritage Architecture IX

Gothic architecture appeared in Switzerland at the end of the 12th C. Whereas the alpine regions were reluctant to adopt this new architectural style (the Romanesque style survived almost until the end of the Middle Ages as shown by the bell towers of churches in Valais and in Tessin), it was not the case of the cities and their surroundings. The greatest Gothic monumental edifice in Switzerland, which is also the first main gothic sacred edifice in Burgundy, is probably the cathedral of Lausanne. Erected later (1421), the collegial of Bern is likely to be the most important edifice of flamboyant Gothic architecture [3]. The Renaissance style was, in a general way and apart from Tessin and the Italian valleys of the Graubünden, not adopted in Switzerland since it was considered pagan.

Figure 2: Abbey of Einsiedeln (source: Tourism of central Switzerland).

The Baroque style appeared in Switzerland in the 17th C. Among the master edifices, there are the abbey of Einsiedeln (Figure 2) and the abbey of St-Gallen. The Neo-classicism (18th C.) is quite well represented in Switzerland; a great example is the cathedral of Solothurn (Figure 1). The Neo-classicism and the Neo-gothic influenced the architectural styles of the edifices built during the 18th and 19th centuries.

3 Seismic hazard in Switzerland

The alpine arc was created by the collision between the European and African tectonic plates. Although the seismic activity around the Alps is not as intense as in the south European countries for instance, this tectonic situation results in quite a high seismic hazard. Since the 10th C. and until 2001, 40 earthquakes of a higher intensity than or equal to VII were recorded, 12 with an intensity higher than or equal to VIII and finally, one very strong earthquake whose intensity was equal to IX [4]. Most of the major seismic events, which occurred in Switzerland, are reported in the Table 1 (for the name of the places, please see Figure 1). Every seismic event recorded in Switzerland (whose epicentre was either in Switzerland or in its surroundings) with an intensity higher than or equal to V is reported in Figure 3.



Table 1: Few seismic events amongst the most damaging that occurred in Switzerland during the last millennium (source: Swiss seismological service).

Date Region (Epicenter)

Intensity Date Region (Epicenter)

Intensity

04.09.1295 Chur VIII 10.09.1774 Altdorf VIII 18.10.1356 Basel IX 09.12.1755 Brig VIII April 1524 Ardon VIII 25.07.1855 Visp VIII 18.09.1601 Central

Switzerland VIII 25.01.1946 Sierre VIII

Figure 3: Historic earthquakes with an intensity higher than or equal to V between 1000 and 2001 (source: Swiss seismological service [5]).

As is highlighted by Figure 3, the seismically most interesting regions are: the Valais, the Basel region, the central region of Switzerland, the Rhine valley and the Graubünden (see also Figure 1).

3.1 Seismic hazard in Valais

As mentioned above, the area of Valais is included within an unfavourable tectonic area. It is crossed by important splits E-W, more or less parallel to the Rhone valley. Research carried out at the Swiss seismological service during the seventies showed that a seism with an intensity of VIII-IX on the MSK scale was likely to occur in Valais with a return period of 400 years. Moreover, the Rhone valley is filled upon a quite high depth of alluvium that could be very harmful to the buildings stock since it might be at the root of site effects, i.e. amplification of the seismic waves and/or liquefaction of soil.

4 Seismic vulnerability of the Swiss monumental buildings

The earthquake, which occurred in mid-October 1356 in Basel, caused very serious damage to every kind of buildings. Moreover, in the “Roten Buch” (red book) of the city of Basel, it is written: “One should know that this city was



destroyed and broken into pieces by an earthquake; no church, no tower and not even a masonry house remained completely intact in the city and the surroundings […]”. An illustration taken from the Kosmographia (Figure 4), released in 1544 by Sebastian Müller, shows the dramatic situation caused by seismic events of high magnitude.

Figure 4: Engraving of the city of Basel after the earthquake which occurred in 1356 in this region (Source: Swiss Reinsurance Company: Historic earthquakes in Europe).

As is quite obvious in Figure 4 and as is often reported in archives, by woodcuts, engravings, papers and correspondences, one of the most vulnerable parts of the monumental buildings is the bell tower. There are actually numerous other examples that confirm this observation in Valais.

4.1 Seismic vulnerability of churches in Valais

The three major earthquakes, which occurred in Valais since the 18th century, namely in 1755, 1855 and 1946, caused serious damage to many common buildings and also to monumental edifices. As they did not happen so far in time, information about damage observations is fortunately available. One of the most serious damage recorded is, for instance:

The spire of the St-Martin’s church tower, in Visp, crumbled due to the earthquake which occurred in 1855 and whose epicentre was situated in Sankt-Niklaus.

Before the earthquake After the earthquake

Figure 5: Damage on Saint Martin’s church resulting from the earthquake occurred in 1855 in the Visp region. (Source:[6] and the Swiss seismological service).



The church of Chippis sustained serious damage by the seismic event which occurred in 1946 and whose epicentre was close to Sion. As you can see in Figure 6, the ceiling vaults collapsed.

Figure 6: Church of Chippis after the earthquake occurred in 1946 (Source: Keystone/Photopress).

5 The Cathedral of Sion

The edifice, whose plan is shaped like a Latin cross, has the architectural features of a basilica. Its vessel encloses a central nave flanked by two aisles along three bays, a transept and a chancel with a polygonal apse. The sacristies and chapels are placed on the chancel sides (Figure 7).

Figure 7: Plan of the cathedral of Sion (source: [7]).

5.1 The structure

Each bay of the nave has quadripartite rib vaults. The buttresses, which resist the thrust coming from the vaults, follow the roof of each aisle and the thrust is brought down to the ground through massive pillars adjoining the wall. The bell tower was built during the 12th C. At first, there was only one of its sides that was connected to the rest of the church; then, as they built a small chapel along its north façade, the tower became almost completely inserted within the church (two walls are connected to the church).



Figure 8: Chancel and buttresses of the cathedral.

Figure 9: West side of the cathedral, with the Romanesque tower.

5.2 The seismic vulnerability of the cathedral of Sion

The most vulnerable structural elements are a priori the tower and the vaults. The transept walls may also be seismically vulnerable. The last important seismic event, which occurred in 1946, mainly damaged the upper part of the tower: the spire tip was broken and the masonry of the last storey was highly dislocated.

5.2.1 The tower seismic vulnerability The tower is about 45 m high and has a squared section of about 10m x 11m. The three upper storeys are opened by about 2.5 m high arched windows. The calculations are based on a simple model of a cantilever beam with 6 punctual masses. The dynamic actions are determined according to the equivalent force method and also to the Swiss building code SIA 261, 2003 [8]. The masses include the walls weight, the wooden floors weight and the bells for the mass 4 (Figure 10). For calculating the section resistance, four L parts were allowed for in the storeys 3, 4 and 5 (s3, s4 and s5 in Figure 11); both first storeys were modelled without openings.



5.4 m

5 m4.6 m5 m4.6 m4.2 m

M3M2

M6

M4M5

M1section 1

s 2s 3s 4s 5s 6

Nave x

Figure 10: Model of the tower.

y

x10.8 m

4.1 m

1.3 m

10.2 m

4.4 m

Figure 11: Model of the cross section (sections 3 and 5 in Figure 10).

Assumptions on the structural behaviour:

• The base is rigid enough for being allowed for fully restrained and the embedment is situated at 5 m from the ground (Figure 10).

• The tower is free from the rest of the structure • The rigidity distribution as well as the masses distribution allows the

application of the equivalent force method Assumptions about materials and weights:

• The masonry is not cracked and is uniformly made of limestone with cement

5.2.1.1 Results The calculated (by Rayleigh’s quotient) frequency fx is about 1.26 Hz and its period Tx = 0.79 s; fy is about 1.14 Hz and its period Ty = 0.88 s. In both directions, the main problem appears very early at the fourth level (Figure 10) and it is essentially caused by the change of shape and mass (from a spire to a square section) and also by the mass of the bells (in M4). The bending



moments resulting from horizontal actions are too high for the structure, whose walls (s3 and s4, Figure 10) are actually not resistant enough to the seism defined by the Swiss codes for the region of Valais [6]. These results, although resulting from calculations using simple models, show that this structure is seismically vulnerable. Further calculations will be performed in order to improve the result accuracy. It is worth noting that this vulnerability must have been already known few decades ago, since reinforced concrete head beams were carried out (M6 level, Figure 10) and connected to the masonry with vertical pre-stressing tie-rods.

5.2.2 The seismic vulnerability of the transepts main façade The calculation methodology is taken from [9], [10]. The proposed equations are: Without taking into account tie-rods: h

sSa == λ ; with tie-rods: mghTh

hsSa ⋅

⋅+== 0'

2λ

where: Sa Spectral ac. h Wall height To Int. force in the tie-rod s Wall width h’ Height of the tie-rod

anchorage m Mass of the wall

Assumptions on the structural behaviour:

• The windows do not have other effects on the out-plane behaviour than just a lack of material

• Tie-rods are well embedded in the perpendicular walls fabric.

8.6 m

13.6 m

15 m

Anchorage

Figure 12: Plan of the south wall of the south transept.

5.2.2.1 Results The first step does not allow for the tie-rods and the maximal ground acceleration that can be withstood by the structure (without the overturning of the façade) is amax= 0.8 m/s2. This value is actually lower than the acceleration determined in the Swiss code (amax=1.6 m/s2) [8]; consequently, the considered wall might be quite seriously damaged in case of an important



seismic event (according to the code). Taking into account the tie-rods highly improves the resistance of the wall against horizontal actions. The maximal ground acceleration that could be withstood by the wall is actually amax= 2.8 m/s2, which is higher than 1.6 m/s2. According to the model and its assumptions, the transept walls are resistant enough to the prevalent seismic event determined by the Swiss code.

5.2.3 Discussions According to these first and quite simple calculations, the bell tower is seismically vulnerable. Nevertheless, the model accuracy must be improved in order to take into account every prevalent parameter that can influence the dynamic behaviour of the tower. Consequently, for instance, the boarding conditions having to be as close as possible to the reality, the link between the tower and the other structural parts of the church must be allowed for. The impact of diverse masses that belong to the structural elements must be better defined and modelled, like the reinforced concrete beams being around one of the upper storey (5.2.1.1). Amongst the assumptions made in the calculations, the diaphragms were considered rigid; this is actually not exact since the floors are supported by a timber structure. This fact has also to be allowed for in the next stages of calculations. Finally, the masonry state of decay and the impact of its creep are also relevant parameters regarding the seismic vulnerability of an edifice. The assessment of the seismic vulnerability of the transepts shows that, thanks to the tie-rods that link their front side and lateral walls, they are not highly vulnerable in the out-of plane direction. However, the resistance from the masonry connection between perpendicular walls are not taken into account and the anchorages quality must be checked and defined. The in-plane structural behaviour must be also verified. Moreover, the dynamic structural behaviour of the nave, then the vaults- pillars system must be analysed in order to assess its seismic vulnerability. As it might be deficient during a seismic event, the dynamic response of the apse has also to be studied. Finally, all these studies about different parts of the structure will lead towards the understanding of the general dynamic response of the edifice.

6 Conclusion

As it was stated above, though with quite simplified methods of calculation, the tower is seismically vulnerable. This result tallies the need of further more accurate investigations. Concerning the transepts, it seems that they do not present high seismic vulnerability; nevertheless, their seismic resistance is not enough, regarding the prevalent seism determined in the Swiss codes, without tie-rods. A more detailed study is then, in this case, also required. Further analyses will be made through more complex and detailed methods of calculations, like FEM or the application of the Limit equilibrium analyses in an



advanced way. Field tests will be also carried out in order to define mechanical characteristics of masonry and also to calibrate the models. Finally, the results obtained through to the study of the seismic vulnerability of the cathedral of Sion will be, to some extent, used for developing a methodology allowing us to assess the seismic vulnerability of other edifices in Switzerland.

References

[1] Laupper H. et al., Expert report: earthquakes and cultural property, Federal Office for Civil Protection, 2004, Bern.

[2] Forum, Protection of cultural property, Forum N.4, Federal Office for Civil Protection, 2004, Bern.

[3] Speich K., Schläpfer H. R., Swiss churches and monasteries, Ex-Libris, 1979, Zürich.

[4] Weidmann M., Earthquakes in Switzerland, Desertina, 2002, Chur. [5] Fäh D. et al., Earthquake Catalogue of Switzerland (ECOS) and the

related macroseismic data base, Eclogae geol. Helv. 96, 2003, Basel. [6] Schmid V., Culture guide of geography, history, economy, language and

culture of Brig-Glis, Naters and Ried-Brig, Wir Walser, 2001, Brig. [7] Ribordy Evéquoz V., Lugon A., The cathedral of Sion, Ribordy Evéquoz

V., Lugon A., 1995, Sion. [8] SIA 261, building codes: Actions on structures, Swiss Society of

Architects and Engineers, 2004, Zürich. [9] Lagomarsino S. et al., Mechanical models for the seismic vulnerability

assessment of churches, Structural Analysis of Historical constructions, eds. Modena, Lourenço & Roca, 2005, London.

[10] Lagomarsino S. et al., Seismic response of historical churches, 12th European Conference on Earthquake Engineering (paper Ref. 471), Elsevier Science Ltd., 2002.



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