Following the devastating fire at Notre Dame Cathedral, four experts offer initial thoughts and analysis

Elec

On April 15, 2019, shortly before 6.50pm, a fire broke out in the roof of the Notre Dame Cathedral in Paris, causing severe damage to the building. The fire was extinguished the following day, 15 hours later, but fire crews remained to identify and extinguish residual fires.

The cathedral’s spire and roof collapsed, and considerable damage was caused to the interior, upper walls, and windows, as well as to numerous works of art. The stone vaulted ceiling beneath the roof prevented most of the fire from spreading to the interior of the cathedral, saving the building from worse damage.

An investigation has begun into the fire that swept through the medieval cathedral, destroying its roof and spire.

Much of roof covered in scaffolding


Much of the roof was covered in scaffolding as part of a big renovation programme, which is being investigated as a possible cause of the blaze.

About 400 firefighters and 30 fire engines were deployed, but they were unable to save the spire, replaced in the 19th century during a previous refurbishment and made of oak covered with lead. The spire collapsed just before 8pm.

The roof was made up of a ‘forest’ of wooden latticework, much of which dated back to the 13th century.

Underneath the roof, the vaulted stone ceiling appears to have survived mostly intact and this will have protected the inside of the building from the worst ravages of the fire. But it is still unclear how much of the interior will have suffered water damage.

Expert reaction and initial analysis


The UK’s Science Media Centre has published the following four experts’ reaction to the fire that damaged the Notre Dame Cathedral.

1.) Andy Dark, former London firefighter, fire health and safety expert and Fire Brigades Union (FBU) assistant general secretary:
“The Notre Dame fire is just the latest example of the fire risk to heritage buildings across the world. These buildings contain a lot of wood in their structure which will be tinder-dry due to age and often have no fire protection.

“Stone buildings are particularly hazardous for firefighters. Stone structures such as pillars and staircases can crack and fail catastrophically without warning when exposed to extremes of heat.

“The risk of collapse means that firefighters are rarely deployed into such buildings where fires have developed. Firefighters on the ground would weigh the risks against the benefits and a major factor would of course be whether there are people in need of rescue.

Before the fire, during, and after. Leighton Kille, CC BY

“The construction of the building, its sheer size, particularly the height and the likely remoteness of stairwells would have severely hampered the ability to make meaningful progress fighting the fire from inside.

Fire started high up the building


“Watching the footage, it looks like the fire started high up the building near or in the roof. It clearly developed quickly and it appears that the fire had access to open air from an early stage which would mean that much of the heat and smoke would have escaped to atmosphere rather than being contained and pressing downwards or laterally.

“At this time any consideration can only be speculative, but it seems clear the firefighters did a great job in reducing the spread of the fire to parts of the building which were unlikely to be salvageable.”

2.) Professor Guillermo Rein, professor of fire science, Imperial College London:
“This fire was enormous because it burned many tonnes of timber for hours, and it was a peculiar fire, because all the flammable content were on the roof, and not in any other place of the large cathedral. What burned was the wood of the timber structure of the roof.

“Small pieces of wood (panels and thin timber) would have burned first because they are more flammable and burn out quickly. Larger piece of wood (thick timber) would have started burning later but lasted much longer.

“This was the main focus of the fire brigades. Images show that there were few flammable contents on the ground floor (albeit it included invaluable pieces of art).

Vulnerable structure of stone vaults and walls


“The fire brigade faced a peculiar fire because they had to be aggressive fighting the big roof fire with the aerial ladders (designed for high-rise buildings) but at the same time be gentle with the vulnerable structure of the stone vaults and walls. They did a fine job, and how they tackled this fire will probably be studied in the years ahead.

“Firefighters on the ground floor and inside the cathedral were seen rescuing pieces of art while the fire was still raining over their heads. The roof fire would have produced a rain of burning embers dropping on the floor and threatening the pieces of art.

“Fire brigades are trained to rescue people and last night the Paris Pompiers used their skills to save universal and invaluable art.

The spire of Notre-Dame de Paris in flames. Guillaume Levrier,     CC BY

Fire engineers will consider combination of layers of protection


“I think that the reconstruction of Notre Dame will surely involve fire engineering to protect the iconic building from any future fire. Fire engineers will then consider a combination of layers of protection (prevention, detection, evacuation, compartmentation, suppression and endurance).

“Note that the previous timber roof that burnt last night was relying only on one single layer: prevention. The roof was known to be flammable, but they avoided the arrival of ignition sources.

“Unfortunately, it seems that the renovation works might have brought the ignition source which might have caused the accidental initial fire. I think the roof space had no detection, difficult evacuation, no compartmentation, no suppression, and no endurance.”

3.) Kealy, fire consultant, fellow of the Institution of Fire Engineers, CIBSE member and managing director of MKA Fire:
“Sadly, fire is all too common in buildings under construction or during renovation and is likely to be caused or exacerbated by the renovation works. The work may involve disarming or removing some of the normal fire protection measures and the space will be full of equipment and material that isn’t usually there – scaffolding, timber, building materials etc.

“These buildings are very fragile structures, built nearly a thousand years ago and not to modern engineering standards. Even the robust stone walls – which look so resilient – will only stay in place when supported by the structure of the complete building.

“This is a very intense fire and the damage may be even more severe than it appears now. We will not know for some time what the impact on the remaining structure will be.

Modern building regulations and fire safety standards


“It should be noted that modern building regulations and fire safety standards are designed to protect people and not buildings. The fact that no visitors or passers-by were injured in such an extreme incident is something to be thankful for.

“We could also speculate on the fact that, had we been having this conversation 500 years ago, we would have been speculating about the future of the city as a whole. Now we can view it is as a tragedy of a single building.

“Donald Trump’s advice was certainly attention-grabbing. While dumping huge quantities of water may be an effective way to fight huge forest fires it is unlikely to be an effective tactic on a city-centre building where even the firefighters immediately adjacent to the building were having difficulty directing water to flames under the superstructure.

“There is guidance to mitigate risk during building renovations (CIBSE Guide E and RICS guidance) but that is really all you can do. At the end of the day, any work that interferes with the normal operation, structure and use of a building creates a risk, and all too many historic buildings have fallen foul of it.”

4.) Robert Bowles, Institution of Structural Engineers:
How the fire spread:
“The masonry walls and flying buttresses of a building like Notre Dame support a stone vault, whose underside one sees as a ceiling from inside the building.

“Vaults are often substantial structures which are protected from the elements by a separate timber structure that supports the lead or copper waterproofing. This creates an attic space between the top of the vault and the boarding on which the lead is laid. Once a fire is established in such a space, it tends to spread laterally.

“The basic structure of these timber structures is normally a series of timber trusses, supporting secondary timber members which span between the trusses.

“Some cathedrals (like York Minster) have timber ceilings or vaults, and if the roof catches fire the ceiling also catches fire and is destroyed. During a fire, burning timbers will punch through the vault or ceiling and fall all the way to the floor – and will no longer fuel the blaze at roof level.

However, at Notre Dame the stone vault was so strong that the collapsing trusses did not (generally) punch through it. This meant that the collapsed timber members remained on top of the vault, fuelling the fire.”

How might the structure be affected by the blaze:
“In a structure like Notre Dame the stone masonry walls, flying buttresses and vault act together to create a stable structure which is entirely incombustible.

“Historically fires leading to the complete loss of cathedral roofs were quite common – usually triggered by lightning strikes, and those with stone vaults often survived remarkably well.

“The effects on the surviving structure are therefore likely to be secondary.

The Coronation of Napoleon, by Jacques-Louis David, 1808. Louvre Museum. Wikipedia

“The vault will have had burning timbers sitting on it for some hours. The masonry may have heated up and expanded, which it can do without collapsing. As the masonry cools down and shrinks it may deform.

“In addition, much water will have been introduced into a structure that was intended to be protected and kept dry.”

Assessing safety:
“The first priority will be to identify which parts of the surviving structure are potentially unstable, and then devise means of providing safe access for engineers to assess how unstable they actually are. From this will come any proposals for temporary propping etc. or careful dismantling of unstable parts.

Involve work from cranes


“The next priority will be to assess the risk of problems as the masonry cools down, and devise means of removing the charred timbers sitting on the top of the vaults so that the stone itself can be inspected. That will probably involve work from cranes – so the workers are not standing on something that might give way.

“Without the timber roof the masonry will be exposed to the elements, which is not a good thing. Someone will be rapidly working out how to erect temporary roofs, so the process of drying out can start – which is particularly important for the surviving fixtures and fittings.

“There is a substantial amount of scaffolding that has survived. That will require its own assessment to see if it is an asset or a liability.”

http://www.engineersjournal.ie/wp-content/uploads/2019/04/a1-23.jpghttp://www.engineersjournal.ie/wp-content/uploads/2019/04/a1-23-300x300.jpgDavid O'RiordanElecfire,ICE UK,structures and construction
On April 15, 2019, shortly before 6.50pm, a fire broke out in the roof of the Notre Dame Cathedral in Paris, causing severe damage to the building. The fire was extinguished the following day, 15 hours later, but fire crews remained to identify and extinguish residual fires. The cathedral's spire...