# Creating a parabolic-shaped arch more automatically

I'm trying to create an arch in the shape of a parabola, like the arch in JK Bridge (Brasília-Brazil).

I managed to "model" it manually, using some reference images, but it didn't turn out perfect.

Its base is a trapezoid, and its middle is an inverted trapezoid (but that's just a detail).

Is there a way to use modifiers or even Geometry Nodes to make the topology cleaner and more correct?

I've tried using Proportional Editing, and while it creates a good parabola (inverse square), the base isn't aligned with the ground.

Other methods like Simple Deform and Edge Bridge didn't give me the parabolic effect I'm looking for.

Is there a more automatic way to achieve this?

PS: I'm sending a file to perhaps make it clearer. The mesh "arc model" would be the example I made manually (and uneven).

• Hello maybe show some pictures of what you are trying to achieve? Nov 29, 2023 at 13:28

Here is the simplest example that I can imagine. This setup use X coordinate of grid, set it to power of 2 and moves Z coordinate at this number.

Rotate and scale it and it should be good.

• That's just a flat strip, how is this an answer to what he's trying to achieve? Nov 29, 2023 at 15:12
• @GordonBrinkmann there was no such clarification when I wrote this answer. Nov 30, 2023 at 20:14

Here is a more "artistic" solution:

You could of course also solve this purely mathematically, but in this case it seemed more flexible and easier for me to realize it with a simple construction.

Here, I first create two curves that give the basic shape for the upper and lower shapes. I convert these into a mesh with Curve to Mesh and define a corresponding radius beforehand with Set Curve Radius to create the changing trapezoidal shape.

I then extrude the lower mesh and move its extruded points to the positions of the upper mesh, merge the shapes and mark the edges as Sharp Edges.

(Blender 4.0+)