Strutted Z-Beam

OpenSCAD port of RepRap Ltd "3D printed beam that is as stiff as steel" [python / FreeCAD code]. This is a parametric beam that is suitable for 3D printing, taking advantage of the fact that complexity is more or less free with this technology and that infinite-Z belt printers can print many overhanging shapes without support material. The result is a beam that is about as stiff as a steel beam of the same weight.

There are some (hopefully cosmetic) adjustments that I have made:

* Diagonal struts are from corner to corner
* Diagonal strut direction is customisable (Z shape, tetrahedron, octahedron)
* Using 61 segments for large holes, and 13 segments for small holes
* Added ridge customisation (X/Y/Z) - ridges make bridging easier

For more details about the original beam, see the github page about the Z-Beam:

https://github.com/RepRapLtd/Infinite-Z-Beam