Sunday, October 22, 2023


 Why is the layout of Notre Dame out of square?

I tend to think it was cumulative error, perhaps exacerbated by building around older buildings that had not been fully demolished when the foundations were set out. That would have made the traditional methods of sight lines, tight string and pythagoras, difficult to implement.

A second problem for us was inadequate and inconsistent source data. There are hand drawn plans and a very low resolution horizontal slice through Andrew Tallon's point cloud. We tried to get higher resolution point cloud data but it's a high profile government project, so a bunch of BIM enthusiasts didn't carry much clout.

Revit despises small angular deviations. In any case it makes dimensions difficult and you face the prospect of custom making the arches and vaults for every single bay.

The compromise we made was to make almost everything orthogonal with repetitive bay sizes, but to make the north aisles of the nave wide than the south, and the north tower, wider still. That reflects reality. I think Ryan was the first to notice that there are more statues on the left side of the gallery of kings than the right, when viewed from the plaza in front of the west end.


The other half of the grid setting out sheet for Project Notre Dame.

2d drafting that I call a "skew" grid, trying to follow the point cloud footprint, allowing deviations from 90 degrees, but aiming for a modicum of regularity. I took this seriously enough to work it through, but on reflection it didn't make any sense for our purposes.

You might call our model a "didactic" project. We accepted a level of abstraction and simplification so as to better see the big picture issues. The regularities of the model allow us to pose questions, explore possibilities, seek understanding.

It was a fascinating struggle to understand by drawing, modelling, gathering material, questioning.

It. The global cooperation called Project Notre Dame.


Two old photos found on the Internet and marked up on one of the sheets in our model of Notre Dame de Paris.

19th Century renovations were very extensive and included things like a new sacristy building between the chancel and the river and linked to the main building by a pair of corridors.

Viollet-le-Duc was not afraid to add new features in the Gothic style whereas today we would make a clear distinction between what has been preserved/reconstructed and new work in a "modern" style.

Who was right? I don't know. Maybe both. Received wisdom will probably change again. I'm certainly glad that the roof and spire are being restored back to how they were before the fire.


Drafting views to show the nave as originally built, and as modified about a century later. Early Gothic featured relatively small windows, similar to the preceding Romanesque, but with pointed arches

Gradually builders became bolder. Larger windows, structural drama, flying buttresses. At Notre Dame, the six middle bays of the nave were opened up, but the end bays were not disturbed, presumably for fear of affecting the stability of the spire and the bell towers.

This seems like a simple story, but figuring it out took several weeks of poring over the source material. For me, these kinds of "Sherlock Holmes" moments are fundamental to the work. They justify the time spent building a Revit model "just for fun".

Learning by doing. Learning through play. Lifelong learning. What could be better?







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