Opinion thestructuralengineer.org Notre-Dame fi re Viewpoint

Following the recent fi re at Notre-Dame de Paris, Th e fi re at Professor Jacques Heyman discusses the construction Notre-Dame of the cathedral and considers some of the questions that will arise in agreeing an approach to repair. cathedral

The double roof system of the typical Gothic great – a stone vault surmounted by a timber roof – is both decorative and functional. The steep external roof provides the necessary weatherproofi ng dictated by northern European climates (shallow pitches were used for Greek temples); indeed the stone vault, perhaps cracked and in any case not waterproof, itself needs the protection of the outer roof (in Cyprus, the crusader churches, e.g. Famagusta, hardly need this cover). However, timber burns well, and one function of the Figure 1 stone vault is to provide a Burning timbers were largely fi re-resistant barrier between caught by masonry vaults the outer roof and the church. GETTY There is thus a symbiotic action between the two coverings If, however, there are side of the church; the timber roof "ONE FUNCTION OF THE STONE VAULT IS aisles to the nave and choir, protects the stone vault and TO PROVIDE A FIRE-RESISTANT BARRIER then such buttresses would the church from the weather, obstruct those aisles. Hence and the stone vault protects the BETWEEN THE OUTER ROOF AND THE the thrusts from the high stone church from the potential fi re CHURCH" vaults are collected by the fl ying hazard of the timber roof. buttresses, and ‘fl own’ over the This dual action was very aisles to heavy masonry placed evident in the fi re of 15 April nave, and the (roughly) square the masonry rib vault and the outside the church. 2019 at Notre-Dame de Paris. vault at the crossing (Figure fl ying buttress, and both are Severe damage was done, 2). They all appear to result insistently evident at Notre- Notre-Dame over the years but this would have been very from the spectacular collapse Dame – again, one cannot exist Building of Notre-Dame started much worse if the ‘fi reproofi ng’ of sections of the 500t lead- without the other. about the middle of the 12th had not been eff ective. Videos covered oak (the fl èche) A stone vault is in the century, a little later than of the confl agration show the over the crossing (Figure 3). engineering sense a sort of Saint-Denis. But Notre-Dame whole of the roofs on fi re, but three-dimensional arch, and is unusual in that it has two in fact the falling timbers were Gothic construction all arches thrust outward at side aisles on either side of largely caught by the masonry Gothic construction is dated their abutments. If there are the nave and choir, and the vaults (Figure 1). conventionally as starting in no side aisles, as at King’s original design had a two-span It seems that collapse of the 1140, with the building of the College Chapel in Cambridge, fl ying buttress system, with vaults occurred in only three abbey church of Saint-Denis, then massive buttresses may intermediate piers between the locations – the last bay of the Paris. The two outstanding be built, bay by bay, against the two aisles. This is not today’s north , one bay in the characteristics of Gothic are walls of the church. construction; it was modifi ed

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repaired. Cathedrals in England all have fi re breaks down their lengths between the timber and stone roofs, and major confl agrations such as that at Notre-Dame (which appears to have had no such breaks) have been rare. However, there has been fairly recent experience after the 1984 destruction of the timber roof of the south transept of . This

WIKPAN / WIKIMEDIA COMMONS [CC BY-SA 4.0] BY-SA [CC COMMONS WIKIMEDIA / WIKPAN cathedral is perhaps unique in England in having no stone vaults, and it has been subject Figure 2 Collapse to successive confl agrations, "IT SEEMS THAT COLLAPSE OF THE of vaults occurred of which the 1984 fi re was the VAULTS OCCURRED IN ONLY THREE in three latest. locations LOCATIONS" (red) There was fi erce argument about the replacement of the roof of the south transept – the a century later by the removal taken down in 1792. In any case, Viollet-le-Duc. He had already ‘engineers’ thought that a steel of the intermediate piers, and the Revolution had occurred in restored the nearby Sainte- truss should be used (hidden replacement by the gigantic 1789, and the new regime was Chapelle, and it was he who from the outside by the roof 15m span fl iers which are such a actively anti-clerical, and wished designed the new lead-covered covering and from the inside feature of the present external to secularise the cathedral; timber spire which collapsed by the reconstructed timber appearance of the cathedral. little was done to maintain the over the crossing on 15 April. ceiling), while the ‘traditionalists’ There was little structural integrity of the building. The It may be conjectured that the insisted that oak should be change to the cathedral in the ruinous state is described in main roofs which burned on used. (Cologne Cathedral ensuing centuries, although Victor Hugo’s novel of 1831, that date had been subject in Germany, when it was successive kings of France Notre-Dame de Paris. to restoration and renewal by completed in the 19th century made signifi cant alterations Public awareness was Viollet. after a long intermission in to the internal ordering of aroused, and in 1844 a massive building, made extensive use the church. But the fabric restoration programme was Approaches to repair of iron.) deteriorated slowly – the initiated, which took 20 years to This is the history – discussion The traditionalists won the 13th century fl èche over the complete; the work was mainly has already started about argument at York. In the event, crossing, for example, had to be under the direction of Eugène how the cathedral should be no oak rafters could be found having suffi cient length, so that the restored roof has a

Figure 3 triangulated oak cap, and is not Collapse of a faithful reproduction of an spire caused puncturing of original medieval design. stone vaults There is some fi re damage to the masonry of Notre-Dame (Figure 4), and experienced stone masons will be needed. France will certainly be able to supply these, in contrast to recent experience in Australia. The cathedral in Brisbane, to the design of John Loughborough Pearson, was completed just at the start of the present century after 100 years of construction; it was necessary to involve masons trained in England. However, the Australian experience shows ways in

GETTY which modern techniques can

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markedly reduce the number might be innovation lies in the of skilled masons needed. crossing. Something other For example, the dimensions than a 500t fl èche could be of a block of stone required designed, and there have for a masonry column were already been suggestions. sent electronically to the It would probably have to be quarry 50 miles from Brisbane; something unexpected, like these numbers formed the I.M. Pei’s pyramid at the Louvre instructions for a numerically – which is outrageous, but it controlled machine, so that works. the stone could be delivered to site ready cut (actually a millimetre or so oversize so that Professor Jacques Heyman it could be hand-dressed on FICE, FREng installation). Jacques Heyman is the author of The (Such prefabrication in the Stone Skeleton (structural engineering quarry is not new. The Caen of masonry architecture), published by stone for the rebuilding of Cambridge University Press, 1995. in the 12th century, after the fi re, was HAVE YOUR SAY shipped ready-sized across the English Channel.) To comment on this article: It is likely that a traditional Eemail Verulam at Figure 4 [email protected] Fire damage solution will be adopted for to masonry Etweet @IStructE is limited in the restoration of Notre-Dame, #TheStructuralEngineer extent

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