split a mesh with an arc

[Tom Krowka]

Am trying to split a mesh with an arc. Won't work and don't know why. Works fine with a line, but not an arc. GRRRRRRRR. Two elements are on the the same layer, unlocked.

Thanks Tom

[Anonymous]

Splitting to an arc does not work for any element (although trim and adjust work). Are you able to split to a straight line segment then use the pet palette to arc the edge? Or does your geometry not allow this? Screenshot would be handy.

[Laszlo Nagy]

Try to use the Subtract from polygon command of the Pet Palette.
Duplicate the Mesh so there are two instances in the exact same location.
Select one and Subtract from polygon command of the Pet Palette. Then SPACE-click into either of the two sides of the Mesh separated by the arc.
Then select the other instance and do the same with the other side.

This is of course a general solution that works on all polygonal type elements like Fill, Slab, Roof, etc.

[Tom Krowka]

See the pic below for a quote directly from the help menu. Unless I'm reading this wrong, I should be able to split a mesh directly along an arc. Maybe it's an ArchiCad bug

[Laszlo Nagy]

There is no bug. There are two cases:

1. Splitting a linear element. It can be done by a Line, Arc or element edge. This can be achieved because it is easy to find the intersection point of the linear element and the splitting line, arc or element edge.
2. Splitting a polygonal element. This can only be done by a straight line or edge. If you want to split by an arc you will have to use a workaround like the one I have given you above.

On that Helpcenter page you can read:

You can split many selected elements (Walls, Beams, Lines, Slabs, Roofs, Meshes, Fill and Zone Polygons, Lines, Arcs, Polylines and Splines) along a line segment, arc or element edge.
The Split command is available in the Floor Plan and 3D Window, and – for drawing elements only – in Section/Elevation/IE and 3D Document windows, and Details and Worksheets.
Note: You cannot split a polygon – such as a Roof element – with an arc.

[Tom Krowka]

Looks like it's saying two different things. Is there any time a mesh or a slab not a polygon?

[Laszlo Nagy]

No, there is not.
A Mesh or a Slab is always a polygon - and it cannot be split with an arc, as the text says.

[adamsb]

It would be nice though, to be able to split an element by an arc, or a polyline. I've definitely used the method Lazlo explained above for years, but it certainly more time consuming than being able to simply create two from one along any arc or polyline.

[Laszlo Nagy]

Yes, I agree, that would be nice.

[Ralph Wessel]

adamsb wrote:
It would be nice though, to be able to split an element by an arc, or a polyline. I've definitely used the method Lazlo explained above for years, but it certainly more time consuming than being able to simply create two from one along any arc or polyline.

I think the difficulty is that some circumstances would be ambiguous. If you split a polygon with a line where the line doesn't cross the entire polygon, it's easy to extrapolate the line direction as far as you like. This isn't true of an arc or poly-line. If either don't clearly define 2 sides across the entire polygon, how do you handle the ends? What if the poly-line is self-intersecting? That's why you need to form a complete polygon to split one against the other – your intentions are unambiguous.