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16 April 2019
Notre Dame caught fire on 15 AprilMichel Euler/AP/REX/Shutterstock
By Sam WongPolice and fire services will spend the next 48 hours assessing the security and safety of Notre Dame cathedral after a huge fire was extinguished in the Paris landmark.
It may take months for an official investigation to determine what caused the fire, but French media have reported that renovation work going on in the cathedral could have been a possible source.
The roof of the cathedral consisted of a stone vault protected by a timber structure above. That made it more resilient than cathedrals with timber ceilings such as York Minster in the UK, which suffered a fire in 1984.
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“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,” Robert Bowles from the Institution of Structural Engineers said in a statement.
Layers of protection
Fire engineers generally think about six layers of protection from fires: prevention, detection, evacuation, compartmentation, suppression and stability.
At Notre Dame, the only layer was prevention: keeping ignition sources – such as electrical work – away, says Guillermo Rein, a fire engineer at Imperial College London.
Unfortunately, fires in historic buildings often start during renovation work, when such precautions may be breached.
Once the fire at Notre Dame had started, there was little to stop the fire from tearing through the roof. “Firefighters were facing probably one of the most difficult fires they have ever faced,” says Rein.
Although the stone walls, vault and flying buttresses are not flammable, they may have suffered structural damage from the heat. “The masonry may have heated up and expanded, which it can do without collapsing. As the masonry cools down and shrinks it may deform,” said Bowles.
Pledges to rebuild
Assessing which parts of the building may be unstable will be the first priority. Unstable parts may be temporarily propped up or dismantled.
Then the charred timbers on top of the vaults will need to be removed, which will probably require cranes. A temporary roof will be needed to protect the masonry from the elements.
President Emmanuel Macron has promised to rebuild the cathedral. French companies and wealthy families have already pledged over 600 million euros to the reconstruction effort, according to the AFP news agency.
When the roof is rebuilt, it should have more layers of protection built in, says Rein. “We do this with iconic buildings all around the world.”
Sprinklers are a possibility, but they are very expensive.