Notre-Dame: Thank You, BIM!

If there’s one key player in the restoration of Notre-Dame de Paris that was overlooked at the Cathedral’s reopening ceremony, it’s digital technology. That’s why I asked Autodesk’s Emmanuel di Giacomo to explain how digital technologies, and BIM in particular, can make a vital contribution to the successful and timely completion of this restoration project.

While the firefighters, stonemasons, carpenters, roofers and numerous restorers of works of art were rightly warmly thanked at the reopening ceremony for their contribution to saving and restoring Notre-Dame Cathedral in Paris, which was severely damaged by fire on April 15, 2019, the contribution of digital technologies was virtually ignored, even though they were at the heart of the works for 5 years and will now continue to monitor the building on a daily basis.

That’s why I asked Emmanuel di Giacomo, in charge of BIM ecosystem development and AI at Autodesk for Europe, to explain why and how digital technologies have also made a major contribution to the resurrection of this heritage.

“Our CEO, Andrew Anagnost, and our AEC Vice President, Nicolas Mangon, who is French, were extremely touched, like many people around the world, by this fire. They immediately said to themselves: how could Autodesk help speed up the rebuilding of Notre-Dame, by going beyond a financial donation? That’s why they immediately proposed to the French Ministry of Culture to make available a range of software dedicated to the world of construction, with a BIM (Building Information Management) approach based on a 3D model of the cathedral for different use cases“, explains Emmanuel di Giacomo.

But at the time, the Etablissement Public Administratif responsible for the conservation and restoration of Notre-Dame de Paris cathedral did not yet exist, since this EPA, provided for by the law of July 29, 2019, was created on December 1, 2019. As it was necessary to move quickly, a partnership was quickly forged with Art Graphique et Patrimoine (AGP), a French company founded in 1994 by Gaël Hamon, a stonemason specializing in the restoration of historic monuments, who wished to combine a passion for traditional trades with a desire to put digital technologies at the service of art and culture.

Creating a BIM model in a hurry

“This was all the more logical given that AGP has been working regularly on Notre-Dame for over 25 years. In particular, they had carried out laser scans with English-born Belgian architectural historian Andrew Tallon, while he was doing a comparison of cathedrals in Europe. We then had the opportunity to have access to a point cloud of Notre-Dame, which was actually quite recent, and we asked them to build a 3D BIM model of Notre-Dame with Revit at a relatively high level of detail, an LOD 350. (See box) We thus obtained a precise structural model of Notre-Dame, but without all the small modenatures. The idea behind this BIM model was to be able to quickly extract the quantities of stone, wood or lead needed for reconstruction, but also to be able, for example, to wedge a digital 3D framework onto it.”

But this BIM model reflected the cathedral’s pre-fire condition, albeit without the framework – the famous forest – which, although laser scanned by AGP, had not been remodeled in 3D. The absence of a 3D model of the forest posed no problem for the reconstruction, since Rémi Fromont, the chief architect of historic monuments, who was commissioned to carry out this work, had a few years earlier carried out all the hand-drawn surveys of the framework, including the most minute details of all the joints. The carpenters then used these drawings to recreate all the parts identically.

doc : Public Institution in charge of the conservation and restoration of Notre-Dame de Paris Cathedral and Art Graphique & Patrimoine

In order to obtain a post-fire assessment, AGP was quickly commissioned by the French Ministry of Culture, and more specifically by the Direction Régionale des Affaires Culturelles (DRAC), and by the Préfecture de Police de Paris to carry out a post-fire laser scan, in order to better assess the damage and see any possible movements of the structure by superimposing the two scans.

Towards technological patronage

Finally, a technological sponsorship was set up as soon as the EPA was created, under the leadership of General Jean-Louis Georgelin and his right-hand man, Engineer General of Armaments Philippe Jost. This sponsorship covered the provision of a 3D BIM model of Notre-Dame created with AGP, software licenses from Collections AEC (Revit, Autocad for modeling; 3ds Max, Maya for visualization; ReCap to transform physical data into a digital model; Civil 3D to reconstitute the entire urban environment around Notre-Dame; InfraWorks to integrate projects into a real-life context), as well as a collaborative platform so that all those involved in the project on site (250 companies) can access the model, exchange and extract information from it, and indeed all the documents that ultimately made up Notre-Dame. Depending on requirements, between 50 and 100 people accessed the 3D model on a daily basis.

“But the 3D BIM model derived from the laser scans was also used extensively to create a whole series of 2D documents. Because you have to remember that Notre-Dame had no 2D documentation. There were no plans, sections or façades. When Eugène Viollet-le-Duc restored the building in the mid-19th century, he didn’t have access to today’s technologies for creating exact plans of the existing structure. As a result, the Beaux-Arts-style plans of the time are unfortunately not accurate, and cannot be used as a basis for restoration work. These beautiful plans are “as-designed”, but unfortunately not “as-built”.

A weakness of which he was apparently aware, since he commissioned numerous plaster casts of the most remarkable buildings, notably churches (tympanums, facades, special features, statues…) which are stored in warehouses in the Paris region or at the Cité de l’Architecture in the galerie des moulages.

Different use cases

Setec Opency used the BIM model right from the start, managing all site logistics. From setting up offices and lead decontamination areas in Algeco buildings, to positioning cranes to pick up materials arriving by truck and ship, with verification of gauges and clearances. We also checked the gauges to ensure that delivery trucks did not damage the listed trees in Jean XXIII Square, and phased the erection of the numerous indoor and outdoor scaffolding structures. The BIM model thus becomes 5D, as it incorporates both temporal and quantitative aspects.

The BIM model was also used by the architectural firm Patriarche, which was in charge of the technical aspects of the project, including supplies, water, electricity, etc., as well as the organization of the work of the teams on site for safety reasons.

“The BIM model has also been used in a number of simulation-related use cases, notably by CSTB to carry out wind resistance studies on the roof and the future spire. So they recovered the BIM model, on the one hand, to use it in their proprietary fluid mechanics tools and, on the other, to 3D print a small-scale physical model, to carry out wind tunnel tests, in order to validate that the results they were obtaining digitally were consistent.”

The BIM model was also used to simulate lighting that would highlight the interior of the renovated monument, and make it forget its previous very dark appearance. Contemporary artist Patrick Rimoux and lighting architect Shantidas Riedacker used the BIM model to study, position and develop the artificial lighting.

The BIM model was also a great help in studying the positioning and routing of the cathedral’s many new systems (fire safety, sprinkler system, multiple security systems, real-time monitoring of the position of people inside for emergency evacuation, lighting, sound systems, etc.). The BIM model, for example, enabled maximum use to be made of the existing channels between the collateral vaults and the floor of the grandstands to accommodate these systems.

Waiting for the digital twin

Restructuring the surroundings too

In addition to the identical reconstruction of the cathedral, the Notre-Dame project also includes the redevelopment of the surrounding area, led by the City of Paris. “For this purpose, we produced a digital terrain model of the eastern part of the Ile de la Cité, covering 5 hectares, with Notre-Dame of course, taken from the BIM model, but also the Préfecture de Police, the Hôtel-Dieu Hospital, all the buildings and bridges around it, the Jean XXIII and Ile-de-France squares with the Memorial to the Martyrs of the Deportation, etc. We also modeled the presbytery, the sacristy and the Cathedral. We also modeled the presbytery, the sacristy, the entire underground parking lot and crypt on the Notre-Dame parvis, and the sewers just below, to ensure that the future project could be carried out correctly. This digital terrain model was made available to the 4 teams taking part in this international competition, so that they could integrate their project into it. Of course, interoperability is extremely important. So we have also made available a model in IFC format, particularly for participants in the international competition who don’t use our software. We also provided the jury with a virtual reality and augmented reality model, so that they could get an idea based on the same digital terrain model. This enabled the jury to choose the solution they felt was most appropriate. In this case, that of Belgian landscape designer Bas Smets.

The entire Notre-Dame project mobilized between 10 and 20 people at Autodesk part-time during the 5 years of reconstruction (assistance in technical project management for all logistical aspects on-site with the development of connectors for certain 4D planning tools, etc.) and about 4 people at AGP in 2 phases. First, for the creation of the pre-fire BIM model, then for the creation of the post-fire BIM model and the comparison of the two models, and finally for the creation of 2D documentation, which involved about 4 full-time people for 2 years.

BIM and AI essential for heritage preservation

“In fact, BIM is a major challenge and an almost absolute necessity for heritage preservation. Every historic monument should have a 3D scan to keep track of what could be destroyed at any given moment. A few months after the fire at Notre-Dame de Paris, the cathedral of Saint-Pierre-et-Saint-Paul in Nantes fell victim to arson, destroying the organ on the tympanum. Fortunately, AGP had been commissioned by the diocese to make a scan of the cathedral, and they were able to remake documents that will enable the cathedral’s organ to be rebuilt, so this proves that it’s a necessity.”

In the longer term, Artificial Intelligence could be of real interest in heritage preservation. “For example, by automatically analyzing architectural elements in a point cloud, AI can provide automatic recognition of the typology of the elements, classify them, for example capitals or column shafts, openings… and account for them. It can also help analyze the level of deformation or pathology in a building, thanks to specific algorithms capable of detecting cracks or distortions in buildings equipped with sensors. It could also automate a number of tasks, such as project documentation or the reconstitution of documentation that doesn’t exist. Finally, we have researchers working on the reconstitution of parts of buildings that have disappeared, using AI to analyze the logic behind the construction of these buildings based on the surviving parts.”

Other digital projects

Digital technologies therefore have an ideal playground in the rescue of architectural heritage. For example, architect François Chatillon’s restoration of the Grand Palais as part of the Paris 2024 Olympic Games is in fact an intelligent and subtly balanced use of a hybrid approach to a point cloud, with DWGs that existed but were no longer necessarily up-to-date, and scanned paper documents from the period that explained the construction methods. Another example is the reconstruction of the spire of the Saint-Denis basilica, which was dismantled in 1846 after a hurricane. This spire was entirely remodeled by AGP using Inventor, and the 20,000 stones required will be cut on CNC milling machines from the digital model of each stone.

The 6 Levels of Development (LOD)

The American Institute of Architects (AIA) and the Associated General Contractors of America (AGC) have established a commonly used LOD (Level of Development) framework, which breaks down the construction model into specific levels:

• LOD 100 – conceptual design: at this stage, the model represents the basic shape and size of the elements without detailed information. It is used to convey the overall design intent.
• LOD 200 – schematic design: the model is refined, incorporating approximate quantities, sizes, shapes and locations of elements. It helps to analyze spatial relationships and initial design concepts.
• LOD 300 – detailed design: in this phase, the model includes geometric information, specific sizes, shapes and detailed object components. It is used to produce construction documents and coordinate different disciplines.
• LOD 350 – construction documentation: the model includes detailed assemblies and manufacturing or construction information. It is used to generate construction documents and shop drawings.
• LOD 400 – manufacturing and assembly: this level involves the creation of detailed models with specific assemblies and connections, suitable for manufacturing and assembly.
• LOD 500 – as-built or facility management: at this stage, the model includes information on the building’s installed and operational elements, reflecting actual maintenance and facility management conditions.

Jean-François Prevéraud

To learn more : https://rebatirnotredamedeparis.fr/en/l-etablissement-public