Notre Dame Fire: what we know about

Fig. 1 – Simplified scheme of the Cathedral. Proportions and shapes are approximated.

The Notre Dame Cathedral fire is an iconic event and deserves to be studied appropriately. In the following text will be presented some considerations based on the information available on the internet.

Following at least 30 years of tragic fires that have destroyed several important cultural resources all over the world, such an important renovation site should have been followed with the utmost care. That does not mean that fire could not start but that a different outcome could have been reasonably expected if a fire happened. So, in order to better understand what really happened and, more important, why it happened (the site does not have access to direct information), in the following sections, articles and posts containing information and news about the fire will be quoted, together with the available information about the context, and the fire extinguishing operations carried out by the Paris Fire Brigade.

The Structure

The construction scheme of the building (fig. 1) has a relevant part of what has happened during the fire. Like most of the gothic cathedrals, the slender stone structure is covered by a wooden (oak) structure. As widely known by firefighters, a fire that occurs in a wood poses serious problems during extinguishing operations, both to their safety and to the building integrity. In simple terms, the problem is related to the impossibility of fighting the flames throwing water from above, since one of the main functions of a roof is avoiding that water gets inside the building. That implies that a roof fire should be fought from the inside.

In the cathedral fire, the stone vault below the wooden roof would have prevented to extinguish the fire from the ground level, even if adequate appliances capable of pumping water to 43 m/141 ft would have been available in the early phases of the fire. But normally, as happened in Notre Dame fire, the roof structure is not designed to let smoke and heat flowing outside, keeping temperature and visibility within a certain range that allows first responders to do their job in relatively safe conditions. On the contrary, the limited volume, in case of fire, makes temperature grow fastly, compromising also the stability of the structural elements. This is the condition that has been described for the 15th April 2019 fire, with one more specific problem. According to the website of the Paris Fire Brigade, the vehicles available to the Brigade that can be used in this kind operation can reach at most 32 m /104 ft with a remarkable 3.000 l/min water flow rate. Since the roof is located at more than 40 m /131 ft above the ground level, it’s reasonable to think that the operations were not easy for the firefighters (fig. 2) working from the outside, while the operations form the inside must have been dangerous (because of the high temperatures, the smoke and the risk of structural collapse) and extremely challenging, due to the need of climbing to the roof level carrying the needed equipments.

Fig. 2 – Max height of firefighting vehicles vs building height. Proportions and shapes are approximated.

Damages and saved artifacts

In order to describe the damages suffered by the Cathedral, some information about the importance of every single element on the overall static behavior of the building must be described. In particular, as in every vaulted space, we know that the horizontal thrust of an arch or a vault has to be balanced by a corresponding thrust or by an adequate mass. These masses or thrusts serve to make the lateral components of the thrust horizontal, preventing the structures to which the arches and ribs rest are overturned (Fig. 3).

Fig. 3 – Gothic cathedral static balance.

In Gothic cathedrals, this need is particularly felt due to the extremely daring project. The arches and ribs of the vaults, in fact, are very thin and concentrate their thrust on limited surfaces. So, wooden roofs and pinnacles have an important static function together with their first intended use of protecting from rain (roofs) and adding an aesthetic element to the building. In the case of Notre Dame, the destruction of the wooden roof, that was protected by some 200 metric tons of lead, may pose an important threat to the stability of the building.

The second extensive damage that the fire caused to the cathedral, which must be mentioned due to the profound effect on the stability of the building, is the degradation caused by the high temperatures developed within the building during the blaze to the mechanical characteristics of the limestone elements, which compose pillars, walls, arches and buttresses of the building.

Pillars, arches, and walls that must support the weight of the structures above and the horizontal forces that the complex static system generates could be much weaker than is necessary. In order to understand the effect of fire we can consider what happened in similar past events, like the Gran Teatro della Fenice fire (1996, Venice). In this case, the tests carried out on the bearing walls after the fire showed that the load-bearing walls had not suffered any particular damage due to the fire, thanks to their considerable thickness, to the reduced state of stress per unit area (maximum compression strength between 20 and 16 kg / cm2 and to the low thermal conductivity of the stone, while all the other leaner and more stressed elements required replacement or strong reinforcement interventions [Nicola Berlucchi]. In The Notre Dame fire, thickness of ribbons and arches could be a vulnerability.

In order to figure out the vastity of the work that is waiting to be carried out to recover Notre Dame cathedral, after the fire that damaged in 1997 severely the much smaller baroque Cappella Guariniana (also known as Cappella della Sindone – Holy Shroud Chapel – Turin), the recovery interventions involved the entire building, and over 1.400 marble elements had to be replaced, together with the consolidation and integration of some 4.000 components. After the fire, new stainless steel chains had to be inserted to support the historic ones, consolidating the masonry, reconstructing the roofs and windows and the inserting new lighting and security systems. To carry out this work, among other things, it became necessary to reopen the ancient quarry of Frabosa Soprana (Cuneo), the only way to acquire the stone needed to replace irrecoverable materials.

Among the other damages, we must consider the nineteenth-century pinnacle called “the Spire”, the historic timber (perhaps the centuries old) of the so-called forest (inner structure of the roof), part of the vault in the central section, windows, and some artifacts kept within the cathedral.

The cause of the fire

To be continued…

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