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?

[R Muller]

Here is my attempt with meshes.

[Erika Epstein]

This looks like a job for Ralph Wessel.
Have you tried Objective?

Objective

[R Muller]

Thanks, Erika. I looked at their website, and it looks like it might work. I'll give it a try.

[R Muller]

I downloaded a demo version of Objective and tried to create the shape. It looks like there are not the tools to do it. Using the section type of object, I can bend in plan view to get one radius, but unless I am missing something I cannot distort the shape to match the plan. The sides remain radial based on the curve of the bend, where I need them to splay to cover the floor plan.

See the attached screen shot of the roof plan with the objective roof superimposed. The heavy straight lines represent the roof trusses, which all have the same profile, but do not converge to the same center point as the curve of the roof edge.

Objective folks- is there a way to distort the angles of the ends of my shape to match my plan?

[Ralph Wessel]

R wrote:
I downloaded a demo version of Objective and tried to create the shape. It looks like there are not the tools to do it. Using the section type of object, I can bend in plan view to get one radius, but unless I am missing something I cannot distort the shape to match the plan. The sides remain radial based on the curve of the bend, where I need them to splay to cover the floor plan.

I think your assessment is correct. The bending/twisting profiles in OBJECTiVE assume a constant cross-section perpendicular to the travel path, but the trusses in this roof do not follow any consistent pattern. While you could easily set out the trusses with OBJECTiVE, I'm not sure about the connecting roof - it's quite an irregular shape. I can make something that looks similar - see attached image - but it wouldn't be strictly accurate.

[Dwight]

And, you know, when something is so arbitrary that Archicad's best math coding guy hasn't solved it, a rational architect might rethink his layout.... just saying…

[R Muller]

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. Yours certainly looks a lot smoother than my three meshes (one for the roof, one for the soffit, and a third to finish the soffit where it curves back on itself). This may be a reasonable trade-off for my more accurate, but lumpy, mesh solution.

I have the original construction drawings, which give the vertical and horizontal angle and height of each truss, so I first placed the trusses as profiled beams, then made meshes to cover them up. The meshes were frustrating and time-consuming to produce, since there are no horizontal contour lines to base them on. I had to put in lots of extra vertices, and adjust each individual vertex in 3D to cover the trusses.

Does it matter in what order you do the bends and twists in Objective? Also, I couldn't figure out from your instruction manual what the offset operation does. Is it of any use in this situation?

[R Muller]

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.

[Dwight]

It is interesting that in reality, the roof skin can be used to smoothly span non-geometric irregularity that we can't define mathematically.