Sunday, June 23, 2019


I’ve been so busy with my day job, that my usual write-ups of Notre Dame have fallen behind.  So I’m going to fast forward to more recent work. 

I have embarked on a second pass of the bell towers: starting with my first tentative roughing out and taking it to the next level. This involves a lot of estimation and intelligent guesswork.  We don’t have access to a point cloud or to measured drawings.  Personally I find this a good thing.  The experience is enjoyable, a game of discovery, and I am honing important skills, that tend to be neglected in the BIM world: judging proportions by eye, “winging it”, taking imaginative leaps into the unknown.

These are the kinds of skills you employ when sketching from life, or engaging in the early stages of design.  You have to trust your instincts, make bold decisions, employ simplifying assumptions. And it’s a physical process, training hand, eye and brain to work together fluently.  This was unavoidable when we drew by hand, but sadly neglected in digital workflows.  It need not be so.  After all, the touch interface of smartphones has taken us sharply back into intuitive, directly physical “ways of thinking”.  I long for the day when drawing with a stylus will be an integral part of BIM processes, as respectable as “computational design” or “reality capture”.

The towers are rectangular with two buttresses at each corner. During the first pass these were represented by tall wall segments, edited in profile to capture the gradual stepping-back towards the top.  Essentially there are four apparent storeys, the top one carrying the bells.  I decided to gradually chop layers off the existing buttresses using the edit-profile feature, and substitute loadable families.

First of all a bit of sketching over the west elevation to generate dimensions.  Then use these to build a wall-hosted architectural column. Quite simple at first but it needs an offset from the wall to create space for an in-place extrusion that defines an octagonal extension to each corner. 

I’m agonizing over the fact that the towers are not square.  Doesn’t seem to match the photos.  But it seems unavoidable that the sides of the rectangle at ground floor level are significantly unequal.  Maybe there’s a way to make the adjustment on the way up.  But for the moment I focus on keeping things centred.

The two towers are instances of a group, but the mouldings that wrap around the whole West Front are modelled outside the group for the sake of continuity. The upper one of these two sweeps is an open skywalk that was packed with tourists before the fire. Was this area always open to the general public? Or was it part of the extensive network of maintenance access that we are just starting to understand.

It’s very satisfying to see this project becoming a genuine team exercise.  Since I was last in the model, Marcel has been busy, first of all mapping out the timber trusses above the stone vaults.  You can also see Paul Aubin’s work on the flying buttresses here.  Most of you will be aware of Alfredo’s modelling of the vaults around the apse, and you may have seen his Rose Window.  Can’t wait for that to show up in the model.

Between those two sweeps, a transparent screen of slender columns runs around the building.  I had mimicked this with a family that simply pierces holes through the thickness of the wall.  That was OK as a “quick & dirty” placeholder, but the time has come to express the layered reality: a delicate screen defining a passage with a wall behind.  My approach here, as so often, is to start with a simple screen family that captures the scale and proportions required, then gradually increase the “Level of Detail” until it matches the current stage of development of the surrounding context. 

 The passage behind the screen is practical as an access system, but also introduces an interesting transparency, a change in texture defined by a filigree with a shadow behind.  The wall at the back has windows of different shapes, much more to figure out about these, but we’re making progress.  One thing I like about using a BIM approach is the way you are drawn into considering multiple factors that affect the form of a building: aesthetics, functional relationships, structural alignments, the practicalities of construction.

This layered colonnade, echoes the row of statues running across the façade lower down.  Here again we have deep shadow, a repetitive rhythm and an overall horizontal emphasis.  The other thing that strikes me is how deceptive the scale of the building is.  You need to keep looking at the tiny human figures on the skywalk to remind yourself how huge the structure actually is.  At the moment the interior of the bell towers looks very empty.  In reality there is a network of timbers supporting the bells and the wheels on which they are mounted.

The model has reached a stage where it is really interesting to adjust the section box and assess what is revealed about the relationships between spaces, structural elements, aesthetic balance, daylighting etc.  In parallel with this we keep stumbling across additional reference material which gets uploaded to BIM360.  It’s a collective exercise and we often have discussions on Slack about historical details that pop up: Victor Hugo, Viollet le Duc, Crockets, 

We are using Revit 2020 which has the long awaited “free perspective view”.  It seems that “old habits die hard”, and I haven’t been using this feature.  A screenshot doesn’t do it justice of course.  We’ve been able to do camera views “forever”.  The extra value comes from being able to “uncrop” the view and move around freely, to get inside the model in a way that isn’t possible with a parallel projection.

I guess the fact that I have Enscape also makes it less important for me to use free-perspective in a Revit shaded view, because I can do it with all the added richness of a real-time render.  On the other hand, the shaded view directs my attention towards modelling issues, wall joins etc. in a way that a rendered view doesn’t.  So, I must make the effort to switch my default 3d into perspective mode more often.

Let’s finish off with a couple of shots from Enscape.  There is an access route running all along the eaves, corbelled out from the wall.  Just starting to show this, in simplified form.  I like the shot of the skywalk looking back over the ridge, with Marcel’s first pass of the Spire in the background.  This is quite misleading though because the tall narrow “windows” are missing the metal louvres/baffles that would block out that splendid sunburst effect.  A bit of poetic license is allowable in my view.

And finally the roof, from above and from inside.  We are making steady progress.  Our model doesn’t have the richness of detail of some “mesh-based” models out there, but there are advantages to taking a BIM approach.  You are forced to think about how the building is built, how it functions, where the staircases go and how the doors connect into real spaces.  It’s exciting to add layers of detail, to gradually enrich the model, but for me the questions we ask along the way are probably more important than the “wow” images we can sometimes generate.  That’s it for now.  More to follow.


  1. Fascinating article! The model is getting better and better every week, and we are learning more and more about it. Thanks for sharing your work with the world.

  2. Hi Andrew,
    I have been following your experimentations in revit and historical buildings for quite a bit at the beginning of project Soane and never got a chance to help out.
    Here are some links that can give you a bit of missing information and documentation regarding the Notre Dame de Paris building

    The first one is from the french national archives

    The second from the franch national library.

    The third from the art history library

    This one is interesting if you are looking for details of the "chapiteaux" or columns head;

    In the 8th book of Viollet Le Duc Dictionnary there are a lot of mentions and details of Notre-Dame de Paris

    also in case you missed it:
    and an english version of the construction history

    I hope this help.
    If you need some french speaking skills, let me know.
    cheers and keep on.
    Jean-Marc Couffin

  3. thank for the wonderful post , lots of information gained , visit us Revit Modeling in India

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