Ravaged by flames on April 15, 2019, Notre-Dame de Paris Cathedral will reopen its doors to the public on December 8, at the end of an imposing project. To avoid a new tragedy, the fire safety system envisaged for its restoration had to be exemplary. It was the responsibility of the National Institute of the Industrial Environment and Risks (Ineris) to verify its effectiveness. The organization was selected as part of a consultation carried out by the public establishment Rebâtir Notre-Dame de Paris (RNDP), which works directly with the chief architects of historic monuments. “The main challenge was to find the means to test the technical solutions to fight a fire at the heart of the framework,” recalls Alexandre Pernin, deputy director of operations at RNDP.
“We had to adapt the safety devices to the particular configuration and architecture of the wooden frame, which measures 100 m long, by studying the many parameters on which the spread of the fire depends,” explains Benjamin Truchot, project manager. “integrated approach from observation to simulation” at the fire-dispersion-explosion department of Ineris. The nature of the source of the flames, the arrangement of the wood and its quality, the ventilation conditions, the meteorology and the strength of the winds or even the natural evacuation of smoke were all factors to be taken into account.
During its mission, which lasted eighteen months, the Institute simultaneously carried out digital tests and physical tests. The first involved configuring the Fire Dynamics Simulator (FDS) software to study the spread of flames as well as the safety measures intended to counter them. “Generally, the attic of a building must be ventilated. Here, this is not the case because the modeling shows that evacuating the smoke in this way would encourage the spread of flames,” explains Rémi Fromont, chief architect of historic monuments.
Complexity of heat exchanges. Passive measures such as compartmentalization have therefore been planned. “The integration of fire-rated spans into the structure serves to limit the spread of flames and modify the flow of smoke. This device is accompanied by an increase in the section of the battens supporting the lead, which reduces the rate of wood combustion,” explains Benjamin Truchot.
These virtual tests were also an opportunity to reproduce the complexity of thermal exchanges between flames and materials. “In order to analyze the decomposition of wood under the effect of heat, we must reproduce both convective exchanges, that is to say the presence of hot gases near the material, and radiation. This involves making several complex computer codes work together,” explains the project manager.
A water mist extinguishing system was also able to be tested digitally. Guillaume Leroy, engineer in the dispersion, fire, experimentation and modeling unit (Diem) at Ineris, recalls on this subject that “the interaction between flames and droplets from fog is still a matter of research”. This is why, in order to validate the hypotheses, Ineris joined forces with Lemta, a research laboratory specializing in fire modeling.
Large-scale testing. At the same time, Ineris carried out several types of physical tests. “The first, in the laboratory, were used to qualify different wood samples. We then worked from simplified models with a pine frame to support our digital simulations,” recalls Jean-Pierre Bertrand, technician from the Diem unit at Ineris.
Final validation stage, part of the oak frame was reproduced at a scale of 4/10 and installed in the 1,000 m3 chamber of Ineris. An experiment of this magnitude, where even the presence of lead was reproduced, was possible thanks to the available volume since the platform can accommodate a brazier with a power of 15 MW with a smoke extraction rate of 100,000 m3/h . The Verneuil-en-Halatte site (Oise) also made it possible to test the effects of water mist. “This is an expensive installation, where error was not an option. The fire had to spread as we had planned and the automatic extinguishing had to be triggered at the right time. And we succeeded,” says Jean-Pierre Bertrand.
These devices, fire protection spans and water mist, now constitute the main response to all types of fire outbreaks within the framework of the cathedral. On the eve of the reopening of the monument, they are now in place.
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