topic Re: How to create a complex roof? in Modeling

How to create a complex roof?

R Muller — Tue, 23 May 2023 12:59:30 GMT

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?

Re: How to create a complex roof?

R Muller — Tue, 24 Nov 2009 21:32:45 GMT

Here is my attempt with meshes.

Re: How to create a complex roof?

Erika Epstein — Wed, 25 Nov 2009 00:20:23 GMT

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

Objective

Re: How to create a complex roof?

R Muller — Thu, 03 Dec 2009 01:43:17 GMT

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

Re: How to create a complex roof?

R Muller — Thu, 03 Dec 2009 18:22:20 GMT

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?

Re: How to create a complex roof?

Ralph Wessel — Thu, 03 Dec 2009 22:34:36 GMT

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.

Re: How to create a complex roof?

Dwight — Thu, 03 Dec 2009 23:45:05 GMT

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…

Re: How to create a complex roof?

R Muller — Thu, 03 Dec 2009 23:48:01 GMT

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?

Re: How to create a complex roof?

R Muller — Thu, 03 Dec 2009 23:52:11 GMT

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.

Re: How to create a complex roof?

Dwight — Thu, 03 Dec 2009 23:55:21 GMT

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

Re: How to create a complex roof?

Erika Epstein — Fri, 04 Dec 2009 06:02:51 GMT

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?

Re: How to create a complex roof?

Ralph Wessel — Fri, 04 Dec 2009 12:28:15 GMT

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.

Re: How to create a complex roof?

Ralph Wessel — Fri, 04 Dec 2009 12:53:28 GMT

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.