f. escrig, j.p. valcarcep school of architecture, seville, spain ... · pdf filef. escrig,...

Influence of constructive systems in the

structural performance of ancient domes

F. Escrig*, J.P. ValcarceP

"School of Architecture, Seville, Spain

School of Architecture, La Coruna, Spain

Abstract

Specialization of professionals in each one of the branchs that merge at thearchitectural work, promotes that when it comes to understanding a complexbuilding, structural, constructive, formal and historical approachs havedisconnected viewpoints and that it is not understood that all of them interrelatefor a correct interpretation of the whole. This is specially necessary inhistorical buildings that we preserve not only due to their functional usefulnessbut also to much more profound values.

Briefly, it is done an analysis of different typologies from which domes aresolved, paying attention simultaneously to the constructive technology and tothe structural effectiveness.

1 Introduction

The history of architecture is joined to the history of constructive systems andall the formal advances bear a relation to the used structural type.

Some critics of architecture have led this dependence to the point of makingforms conditioned by the constructive system or by structure in '\ completelydeterministic way. Viollet le Due in his interpretation of Gothic or Choisy inhis interpretation almost all of the classical architecture are the paradigms ofthis approach.

However, despite the strictness they expound their thesis with, they havedescriptive approach s that also leave room for other likewise valid hypothesis.

With regard to the constructive justification, this deterministic approach hasinfluenced the interpretation of Architecture till nowadays in spite of its manycontradictions. But it happens the same with regard to structuralinterpretations. Criterions of analytical rationality are applied to phenomena

Transactions on the Built Environment vol 15, © 1995 WIT Press, www.witpress.com, ISSN 1743-3509

180 Dynamics, Repairs & Restoration

that need another approach, almost all because the state of science determinesresults and criterions that we apply nowadays and that were absolutelyunknown in former centuries.

Also because our analytical resources serve for more uniform and moreresistant materials arid also with more quality than ancient ones.

Also because ancient constructions have more massive elements and bigsections, are a discontinuous work and are very fissured and damaged becauseof time.

And also because the constructive system has influenced the loads historymaking that the distribution of stresses is not the expected in a perfectly elasticmaterial.

Nowadays, we analyse the great historical buildings as if they were newly-made compact pieces liable to theoretical actions that act uniformly. This leadsus to unsuitable solutions of reinforcement or restoration that should beimproved with a better knowledge of the structural performance.

Here, we want to propose an approach that bears in mind the constructiveprocess for the structural interpretation at least to encourage a profound studyin this respect.

We unfold some concret examples.

2 False domes

In all the texts of architecture they are considered as example of earlystructures and with a little effective performance. It consists on courses ofstone that advance one upon the other, closing the span by each time morereduced rings.

However, they are systems of a great structural logic and with someadvantages with regard to other solutions.

The fact is that they represent the first try of covering v/ith reducedelements. The first rooms are covered with lintels, big stones that go from asupport to another, what it required an excessive effort.

When we handle stones shorter than the gap that has to be covered, theonly thing is to use the vaulted system.

In relation to the false domes, efectiveness is very related to somedimensional parameters. Syrmakezis* has developed very interesting analyticalapproach s.

One of the great advantages of these constructions is that they can risewithout centering and therefore with very easy constructive systems.

If we analyse some of the great structures built with this system and theduration they had, we will confirm these observations, and also the fact thatin its time they had more success that the true domes.

The most ancient known example is El Romeral Dome (Fig. 1) with 5.20m. in diameter and 3.9 m. high. A very important example is the Naveta desTudons in Minorca where inclined walls close forming an entresol inwards(Fig. 2). Maybe the most important dome of antiquity till Roman constructions


Dynamics, Repairs & Restoration 181

Figure 5 Figure 6



is the Atreo Treasure one, which is 14.6 in. in diametre and 13.5 in. high(Fig. 3). This one was made of sculpted stone and a perfect finish.

All through the history this kind of constructions has not reached the pointof being replaced at all and it reappears constantly in proposals of folkarchitecture.

In the 18th century, the Gallarus Oratory in Dirgle (Ireland) uses the sameformula as Minorcan naveta does (Fig. 4). In the 12th century and thefollowings, Italian Trulli recurs with a great vividness (Fig. 5). Still nowadays,in Mediterranean countries it is mantained the tradition of what Garcfa Lison^calls "Taste for stone" and there are masons that build false domes (Fig. 6).

3 Brick domes

In places where stone was scarce, the use of brick was developed, but due toits resistance and size it was not a suitable material to solve false domes. Thatcaused the adoption of other constructive techniques that were going to havedifferent structural performance. Now it is a question of sloping the differentcourses inwards the span in such a way that the more they ascend the more thisincline increases. With that it results that upper courses press lower ones tostabilize them.

Different kinds of domes are shown in the Fig. 7, where "a" and "b"correspond to false ones and the remaining ones are an evolution that havetheir perfect shape in "e" and their constructive shape without centering in "f".

The big problem of brick domes is that they need a total "plank mould"and although the invention is from Mesopotamia, this imported system fruitsin Rome.

The first big dome of this kind that for the first time surpasses AtreoTreasure one is the Domus A urea in Rome with 16.10 m. 0 (Fig. 8) andoctagonal base ground.

The necessity of making the centering cheaper, develops systems that allowto originate strength lines inside the mass that afterwards become acomplement with a continuous material. The Pantheon is built in this way (Fig.9) where an initial reticulated system "a" is being completed by successivepeels "b", "c" and "d" that do not have to be supported by the plank mould.This causes that we can not address this structure as continuous but as ribbed.

Ribs are an alternative to the mesh. They diminish the needed plank mouldeven more. In the Minerva Medica Temple (Fig. 10) the whole is made stifferwith only eight ribs, and afterwards the whole can be built resting on them.

The limit of this approach is that one where the plaster filling among ribsis not made with a continuous surface but close with independent sheets. Piazzad'Oro's Hall in Hadrian's Villa (Fig. 11) is an example. Here, ribs support allthe weight of the covering, making the way to Islamic and Gothic ribbeddomes.

One way to avoid the centering almost totally is to make domes verypointed. In this sense, Romanic Italian baptisteries are a clear example with an



Figure 10 Figure 1



octagonal or circular base ground: Florence (Fig. 12), Pisa (Fig. 13), Cremona(Fig. 14). The advantage of this constructive system is'that all the material isutilized for work arid do not need costly form works.

The thin brick vaults made with plaster or lime and plaster mortar are avery used solution in eastern architecture and from Mediterranean lands.Catalonian vaults are specifically an example of structural optimization:minimum weight, elimination of plank moulds and speediness of performance.The Spanish Gustavino and Moya^ are the greater representatives of this kindof construction (Fig. 15).

Another fortunated solution that mix with former ones is the double-layerdomes as Saint Mary of Flowers in Florence (Fig. 16). In this case, thepointed shape allows the elimination of plank moulds, span between the twosheets lighten the weight of the dome, inner ribs make the whole stiffer andspan among them allows ladders to be placed there during the construction.

If we also add to this that the concret example was a record of coveredsurface, then we understand the good combination between constructive systemand structural solution*.

4 Ribbed domes

The next step would be to concentrate resistant elements of the dome in a gridwith thin ribs with a spatial configuration in a way that, constructively, it isonly necessary to plank these ribs and not even it has to be donesimultaneously. The filling of gaps could be done with light elements that donot need support during the assembly.

Due to its early realization more than to its size, caliphal ribbed domesstand out in Cordova (Fig. 17) and the Gothic ones, which gave spectacularsolutions (Fig. 18) almost all to cover transepts and the main chapels of the bigcathedrals. However, their potential was not ever well pro fitted and theyseldom surpassed the 20 m. in diametre, dimension that Turkish andMongolian domes with a continuous design often surpassed.

The baroque was also rich in ribbed domes with novel designs. Borromini(Fig. 19) and Guarini (Fig. 20) were masters in these geometries. Recently,Luis Moya^ has projected in Spain very spectacular proposals (Fig. 21). Butas a whole, ribs have only been successful in big sizes in metallic structures,something rather inexplicable as they should have been the most productiveway to solve domes, at least with stony ribs of good quality but not brick ones.

5 Stony domes

Although the true dome was born in order to be able to use low-qualitymaterials as bricks, mortar or lime concrete and as alternative to false stonydomes, its use in some Romanic constructions opened the door to itsgeneralization.

Romans used stony cylindrical vaults profusely. But they did not apply this



Figure 15 Figure 16

Figure 17



technique to domes. Nor Byzantine Empire, in spite of its great domedtradition. Due to this it is surprising enough the fact that the school ofAquitaine in France developed with so much intensity the construction of stonydomes in the 12th century. The type could have been imported due to thecultural exchanges that there were during the Crusades but not the constructivetechnique, more complex than any other used before*. It is necessary to standout that all these domes are continuous and without ribs and that needed a greatknowledge of spherical geometry for their sculpting.

In fact, this technology disappeared in fifty years and do not revive againtill the Renaissance with some of the most beautiful domes ever built. Fig. 22shows the Church of Consolation in Todi.

The building of stony domes during the Renaissance is always performedwith a continuous material, without ribs, although they are visible in thedetailing and with a total centering for the construction. They are not optimizedsystems either in cost or in structure efectiveness because of being heavystructures and very sensitive to movements of supports. It IMS not beenachieved important spans with them either. Nevertheless, their merit is offormal kind and once the first trend to classicism has passed, they have notbeen used again. Andres de Vandelvira* wrote one of the most magnificentessays for the design and detailing of these coverings and Palacios. In a recentpublication^ the manuscript has been made understandable. Some of the domesbuilt under guidelines that are in this essay has never been surpassed. Fig. 23shows the sacristy in Seville Cathedral.

Conclusions

From this fast and unfinished summary we have wanted to deduce someguidelines of structural kind, valid for the comprehensiveness of thefunctioning of domes and in any case for the approach of their restoration andreinforcement.

1. There is not any optimal proposal if it is not linked to the technology ofits time and in fashion with the dominant style.

2. The best results are not always linked to the most effective typologies.3. The highest quality materials have not favoured bigger spans.4. The constructive system is more determinant in the construction than the

structural effectiveness.We could extract some generalizations more but the aim of this paper

would be satisfied if the static approach in the research into these coveringswere complementary of the constructive and formal one.



Figure 18

Figure 22 Figure 23



References

1. Syrmakezis, C. "Ta MT&TCL r^ Kp^c" MOYEEIO KPHTIKHEEGNOAOPIAE KENTPO EPEYNftN, BQPOI, 1988.

2. Garcfa Li son, M. "Arquitectura rural primitiva en Seca" TemescTEtnograffa Valenciana. Institut Alfons el Magnanim. Valencia, 1983.

3. Moya Blanco, L. "Bovedas tabicadas" Direccion General deArquitectura. Madrid, 1947.

4. Escrig, F. "The Dome and The Tower" Fundacion Centro de Fomentode Actividades Arquitectonicas. Sevilla, 1994.

5. Conant, K.J. "Arquitectura Carolingia y Romanica 800-1200. Gatedra.Madrid, 1982.

6. Vandelvira, A. "Libro de Traqas de cortes de piedra..." Library of theSchool of Architecture of Madrid, 1575.

7. Palacios, J.C. "Trazas y Cortes de canterfa en el Renacimiento Espanol"Ministry of Culture. Madrid, 1990.


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