How to create a complex roof?

[R Muller]

I am trying to create a roof for a chapel that was built in 1965. The roof is extruded from a constant bowstring truss that is tilted progressively steeper, swept along a circular arc in plan, and rotated from one truss to the next (but not around a common radius point).

I have created a model with meshes enveloping the trusses, but it was very difficult to make the ceiling mesh and the end result is fairly lumpy, even after the addition of numerous ridges. Is there a better way to do this? Some ArchiCAD add-on that could create this roof?

[Erika Epstein]

R wrote:
Dwight, your comment has validity, but this building is already built, in 1965, presumably without the use of any computer to help figure out the geometry. They must have used some kind of physical model to figure it out.

I think they probably placed the trusses and then worked out the smoothness in the field.

Did you take a look at the drawings for Ronchamp?

[Ralph Wessel]

R wrote:
Not strictly accurate, but not bad. It looks like you extended the roof as far as you needed to to cover everything, then trimmed it, so the section at the right end isn't strictly the same profile.

I drew the roof using the following process:

• 1. I used the gridline as the reference for the roof profile (because it's a continuous arc), and drew lines from the roof extents perpendicular to the arc to determine the required profile length.
  2. I used OBJECTiVE to create a profiled object and placed one across the required roof extents. Note that I offset the profile anchor to the position of the gridline so the profile path remained aligned to the grid as it was bent/twisted.
  3. Used OBJECTiVE to bend the profile to the curve.
  4. Set the required twist angle to the profile end.
  5. Used OBJECTiVE to cut the roof ends to the correct shape.
  6. The finished result in 2D, 3D, and elevation. Note the major flaw with this approach in elevation when the truss profile is overlaid on it - our profile rises much higher than is required.
  7. An alternative approach is to use OBJECTiVE to model just the roof edges, because these are the hardest part to model with meshes. By making two separate profiles, each can be a much closer match to the actual roof. The remainder can be bridged with the mesh tool (this part should be relatively easy). Note that it is much closer to the correct shape in elevation.

R wrote:
Does it matter in what order you do the bends and twists in Objective?

Not really, although changing the order of operations might be easier or harder to visualise (depending on the situation). The profiles are very dynamic and pliable, so you can continue to bend, twist, cut, and rotate as required. At no stage does it become a 'dumb', static shape.

[Ralph Wessel]

R wrote:
I couldn't figure out from your instruction manual what the offset operation does. Is it of any use in this situation?

The Offset tool can be used to move a profile end position without changing the rotation of the profile in cross-section. I've attached an example where this is used to draw a curved ramp:

• 1. I've drawn the cross-section of the ramp, made a profiled object with OBJECTiVE, and bent it to the required ramp in plan.
  2. Viewing it in elevation, I could raise one end of the ramp by rotating it. However, this tilts the ramp surface too.
  3. If I use Offset, I can raise the ramp end whilst keeping the profile tilt angle constant

This tool is applicable to your roof if you use the second method I described (making 2 different profiles for the roof edges rather than a single profile for the whole roof). I used the 'offset' tool to raise the inner profile to the pinnacle of the roof.